The effects of chronotype and social jetlag on health, academic achievement, and career

The effects of chronotype and social jetlag on health, academic achievement, and career

E.M. van Andel, BSc

Supervisor: Camilla Kring

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Laymen’s summary All organisms, even the most primitive ones, have an internal biological clock that follows a rhythm of approximately 24 hours. This circadian rhythm (derived from Latin, meaning 'about one day') is driven by a small part of the brain and regulates the rhythm of other biological and psychological processes, such as body temperature, hormone levels, mood and cognition. It is genetically determined, but light exposure can change it slightly, albeit within genetic boundaries.

It has long been recognised that there are individual differences in the timing of this clock. In humans, these differences have been called chronotypes. They lie along a continuum from early chronotypes to late chronotypes. Early types wake up early, are active during the morning and early afternoon, and go to bed early. Late types, on the other hand, often wake up later and prefer being active later in the afternoon and in the evening, and go to bed later. Late types are more common in the population than early types, and during adolescence people generally become very late types. In adulthood, they gradually become earlier again.

Many people have to get up early to go to school or work, but for the majority this means waking up during their internal night. As a result, most people suffer from sleep deprivation for which they compensate on the weekends, reduced sleep quality, and tiredness during the day. This condition is called social jetlag, because the mismatch between internal time and ‘social’ time causes symptoms similar to jetlag, for example insomnia despite being exhausted. The difference between social and regular jetlag is that social jetlag is chronic and it can therefore have long-lasting negative consequences.

Social jetlag has been associated with mental health problems and with depressive symptoms in particular. Physical health is also affected by social jetlag: people who suffer from social jetlag are more likely to start smoking, and every hour of social jetlag increases the chance that someone becomes overweight with 30%. Social jetlag also negatively affects academic achievement and job performance. The resulting sleep deprivation and tiredness make it difficult for late types to perform well at school, university, and at work. Start time delays are therefore advised, and have already shown promising outcomes. There are still many objections to school start time delays, but the positive effects should outweigh any costs in terms of money or effort. In terms of work, flexible hours are becoming more and more common and offer a solution. However, there is still a ‘morning bias’: people who start their working day late are generally perceived as poorer performers than early starters.

Social jetlag is thus a condition affecting many people. It has widespread negative consequences, for example on health, academic achievement, and career. People should be made more aware of the existence, effects, and importance of biological rhythm and social jetlag. This way, people may be motivated to make healthy changes in their lifestyle to reduce the negative effects of social jetlag.

Preferably, school and work times should be delayed to reduce social jetlag directly.

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Table of Contents

Abstract ............................................................................................................................................................. 5 Introduction ...................................................................................................................................................... 6 Chronotype .................................................................................................................................................... 6 Social jetlag.................................................................................................................................................... 7 Assessment of chronotype ............................................................................................................................. 8 Current review and hypothesis ...................................................................................................................... 8 Mental and physical health .............................................................................................................................. 8 Depressive symptoms .................................................................................................................................... 9 Smoking ......................................................................................................................................................... 9 Obesity ......................................................................................................................................................... 10 Conclusions .................................................................................................................................................. 10 Academic achievement................................................................................................................................... 11 Social jetlag and academic achievement..................................................................................................... 11 Delaying start times..................................................................................................................................... 11 Objections to delaying start times ............................................................................................................... 12 Conclusions .................................................................................................................................................. 12 Career .............................................................................................................................................................. 12 Career choice ............................................................................................................................................... 13 Job performance .......................................................................................................................................... 13 Work times .................................................................................................................................................. 14 Conclusions .................................................................................................................................................. 14 Discussion ........................................................................................................................................................ 14 References....................................................................................................................................................... 17

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Abstract Chronotype refers to individual differences in biological rhythm. It is genetically determined and forms a continuum from early to late types. Late types have often been associated with health problems and poorer academic achievement and career success. In this review it is hypothesised that these relationships are not direct, but rather mediated by social jetlag. Social jetlag is caused by misalignment between internal and external time, and characterised by sleep deprivation, reduced sleep quality, and daytime sleepiness. Since most people are not early types, but society is morning-oriented, the majority of people suffer from social jetlag to some degree. This can have widespread negative consequences. Adolescents seem particularly vulnerable to the effects of social jetlag, partly as a result of a shift towards lateness during this period. This review shows that the literature indeed suggests a central role for social jetlag in the relationships between chronotype and mental and physical health, academic achievement, and career.

Educating people on the existence, effects, and importance of biological rhythm is advised, so people may make changes in their lifestyle. It is recommended to delay school and work start times to minimise social jetlag. Such adjustments are believed to be advantageous to all chronotypes.

Keywords: chronotype, social jetlag, health, academic achievement, career, adolescents

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Introduction All organisms, even the most primitive such as single cell organisms, possess an internal biological clock with a rhythm of approximately 24 hours independent of external cues (e.g. Baron & Reid, 2014; Roenneberg, 2012). This circadian rhythm (derived from Latin, meaning 'about one day') is partially genetically determined and driven by the suprachiasmatic nucleus (SCN). In combination with the homeostasis of sleep pressure that builds up during wakefulness, this central clock regulates the sleep/wake cycle in combination with the homeostasis of sleep pressure that builds up during wakefulness (Borbély, 1982; Roenneberg, 2012). Many physiological and behavioural rhythms are peripherally regulated by the central clock, including hormone levels, body temperature, mood, and cognition (e.g. Baron & Reid, 2014).

Individual differences in biological timing have long been recognised, leading to the determination of different chronotypes (from ‘chronos’, Greek for time). Chronotypes lie on a continuum with early types and late types at opposite ends. Chronotypes are normally distributed among the population, albeit with slightly more late than early types (Figure 1; Roenneberg, 2012).

Figure 1. Distribution of chronotypes among the population in Central Europe, based on information of around 100,000 people. Chronotype is represented by midpoint of sleep on free days. The distribution is almost perfectly normal, but extreme late types are more common than extreme early types. Adapted from Roenneberg, 2012.

Chronotype Chronotype is not only determined by genes that directly affect the central clock, but also by age. During adolescence, a shift towards a later chronotype occurs with its peak around 20 years of age after which people gradually become earlier again. Sex also plays a role: men are generally later types than women (Figure 2; Roenneberg, 2012; Roenneberg et al., 2004).

Figure 2. Chronotype differs between the sexes and changes with age. It is displayed as midpoint of sleep on free days, corrected for weekend oversleep (MSFSC). During adolescence, a shift towards eveningness occurs, after which people become earlier again. Men are generally later types than women. Adapted from Roenneberg, 2012.

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The internal rhythm of most people is not exactly 24 hours and therefore needs environmental signals called zeitgebers to entrain to the 24-hour rhythm. The strongest zeitgeber is the light-dark cycle caused by the sun. A specialised light receptor in the retina, melanopsin (Freedman et al., 1999), can impact the central clock and compress or expand internal day length, depending on when in the internal day light exposure occurs. Light exposure during internal dawn (the first half of the internal day) compresses internal day length, whereas light during internal dusk expands it (black lines in Figure 3). The internal day of most

Figure 3. Entrainment of the internal clock to a 24-hour rhythm under the influence of daylight. Light exposure during internal dawn compresses internal day length, exposure during internal dusk expands it (black lines). A: people with an internal day longer than 24 hours need light exposure around their internal dawn to compress internal day length. They therefore delay their internal time and become late types. B: people with an internal day shorter than 24 hours need light exposure around their internal dusk to expand internal day length. They therefore advance their internal time and become early types.

Note that internal and external time differ in scale. Adapted from Roenneberg, 2012.

people is slightly longer than 24 hours and must thus be compressed, which can be done by increasing light exposure during internal dawn. Normally, it would still be dark during the first part of internal dawn. To increase light exposure during this period, internal time must be delayed relative to external time. This way, a larger portion of internal dawn is exposed to light and the internal day length is compressed (Figure 3A).

Consequently, these people become late types: 7 a.m. on the social clock may actually be 6 a.m. on their internal clock. The opposite happens for people whose internal day is shorter than 24 hours: they need to expose more of their internal dusk to sunlight to expand their internal day length. This leads to an advance of their internal time (Figure 3B). As a result, 8 a.m. on the social clock may feel like 9 a.m. to these early types (Roenneberg, 2012). The continuum of chronotypes thus develops as a result of people with varying internal day lengths entraining to a 24-hour rhythm under the influence of zeitgebers.

Social jetlag Until relatively recently, the sun was the main zeitgeber and most people's rhythms were entrained to a 24hour day. Currently, though, people spend most of their time indoors and are exposed to artificial light, which interferes with their biological rhythm. Especially the use of technology at night can make people become later types, as their internal time is delayed because they are exposed to light during their internal dusk. The population as a whole is thus shifting towards a late chronotype (e.g. Kantermann, n.d.; Roenneberg, 2012). Most people have to get up during their internal night to go to school or work.

Consequently, more and more people are experiencing a mismatch between internal and external time, and around 85% of the population needs an alarm clock to wake up (Roenneberg, 2012). They live in a different time-zone during the week (external time) than on weekends (internal time). It is therefore not surprising that they experience symptoms similar to those of jetlag: insomnia despite being exhausted, subsequent sleep deprivation, reduced physical and cognitive skills, digestive problems, et cetera. Because the mismatch is brought on by social time, this condition is called social jetlag. While the principles and

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symptoms are similar to real jetlag, social jetlag is chronic (Roenneberg, Wirz-Justice, & Merrow, 2003; Roenneberg, 2012; Wittmann, Dinich, Merrow, & Roenneberg, 2006).

Social jetlag is generally largest in adolescents (Figure 4; Roenneberg, 2012). This is not surprising as they become very late types while still having to start school early in the morning. Adolescence is a vulnerable period because of biological and emotional changes (e.g. De Souza & Hidalgo, 2014), so social jetlag can have particularly detrimental long-term consequences for adolescents.

Figure 4. Average social jetlag (black) changes with age, with a peak around late adolescence. Males (blue) generally have larger social jetlag than females (red). Adapted from Roenneberg, Allebrandt, Merrow, & Vetter, 2012.

Assessment of chronotype Chronotype is best defined by midpoint of sleep on free days corrected for weekend oversleep (MSFSC), when sleep/wake behaviour is dictated predominantly by the internal clock rather than the social clock (Roenneberg, 2012). The most reliable way of assessing MSF is by measuring physiological rhythmicity, for example by actigraphs (Roenneberg, 2012; Thun et al., 2012). These are often worn as wristbands and can monitor sleep-wake patterns by e.g. body temperature, activity patterns, and light exposure. However, questionnaires are more commonly used. Horne & Östberg (1976) developed the Morningness-Eveningness Questionnaire (MEQ) that assesses an individual’s preference for activities during a typical day, which are strongly related to actual chronotype. The MEQ does not measure social jetlag. The Munich ChronoType Questionnaire (MCTQ; Roenneberg, 2012) assesses sleep and wake times during work and free days, making it possible to calculate social jetlag (defined as the difference of midsleep between work and free days) in addition to MSFSC (chronotype) and sleep duration. Other questionnaires are also used, but these two are the most common. They have both been validated by actigraphy and other physiological measures (Roenneberg, 2012; Thun et al., 2012) and are thus thought to reliably reflect chronotype.

Current review and hypothesis A late chronotype has often been associated with negative effects on e.g. health and school and work performance (Adan et al., 2012; Baron & Reid, 2014; Partonen, 2015). Moreover, as mentioned, social jetlag is most common among late types and can have long-lasting consequences. This review focuses on the relationships between chronotype, social jetlag, and these variables. It is hypothesised that social jetlag plays an important role in mediating the associations between late chronotypes and mental and physical health problems, poorer academic achievement, and lower career success.

Mental and physical health Chronotype has been associated with several aspects of both mental and physical health. Especially late types seem to have health problems (Adan et al., 2012; Baron & Reid, 2014; Partonen, 2015). According to the hypothesis, social jetlag rather than chronotype is thought to be related to health problems. Social

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jetlag is largest among adolescents, who are generally very late types (Roenneberg, Allebrandt, Merrow, & Vetter, 2012), making them particularly vulnerable to mental and physical health issues that can have longlasting consequences. This section will focus on three of the most studied health aspects associated with chronotype, and specifically on social jetlag in adolescents: depressive symptoms, smoking, and obesity.

Depressive symptoms An association between depressive symptoms and sleep is widely acknowledged. A common symptom of depression is insomnia, and sleep problems can in turn cause depressive feelings (Saxvig, Pallesen, Wilhelmsen-Langeland, Molde, & Bjorvatn, 2012). A growing body of evidence suggests that chronotype influences this relationship, with more depressive symptoms among evening types (e.g. Randler, 2011) and more evening types among the clinically depressed (Antypa, Vogelzangs, Meesters, Schoevers, & Penninx, 2015; Baron & Reid, 2014).

The role of social jetlag was first investigated in an adult rural population. Social jetlag was largest in late types and was associated with more depressive symptoms (Levandovski et al., 2011). A similar study showed that, in adolescents only, late types again had more depressive symptoms (De Souza & Hidalgo, 2014). However, social jetlag was not larger in late types and not correlated to depressive symptoms.

Because this was a young sample, the majority of participants were not able to choose their own wake-up times on free days, making social jetlag appear smaller than it would have been if they had been able to follow their preferred sleep times. Future investigations should therefore calculate social jetlag on the basis of school days and truly free days, i.e., days on which people choose their own wake-up time.

A recent longitudinal study showed that adolescents with late bedtimes reported significantly more emotional distress 2 years later than those with earlier bedtimes (Asarnow, McGlinchey, & Harvey, 2014).

Social jetlag was not measured, but these findings are still relevant as they indicate that having a late bedtime, which often causes social jetlag, can negatively affect mental well-being in the longer term.

Future longitudinal studies should examine this in more detail by using validated chronotype questionnaires or objective measures and considering social jetlag specifically.

It is thus not yet clear how chronotype can affect depression, although social jetlag seems to offer an explanation for the association between a late chronotype and the development of depressive symptoms. The existence of a mutual relationship between sleep and depression is nevertheless widely acknowledged and the influence of social jetlag should be studied in more detail, preferably in longitudinal studies using objective measures.

Smoking An aspect of physical health that has often been associated with chronotype, in particular late types, is smoking (Adan et al., 2012; Baron & Reid, 2014; Partonen, 2015). Wittmann and colleagues (2006) showed that social jetlag was more strongly related to smoking than chronotype alone and showed that this correlation was strongest in adolescents and young adults. Smokers were more numerous among late types, a finding later confirmed in an entirely adolescent sample (Urb谩n, Magyar贸di, & Rig贸, 2011).

Importantly, only 10 to 15% of people without social jetlag was a smoker, compared to around 60% of people who experienced social jetlag of 4 or more hours (Foster & Roenneberg, 2008; Wittmann et al., 2006). Furthermore, Saxvig et al. (2012) showed that delayed sleep phase (DSP) was associated with smoking in adolescents specifically. DSP was defined as having difficulties with falling asleep before 2 a.m. and waking up on time. DSP is also characterised by weekend oversleep and can thus be considered a form of social jetlag. Together, these findings suggest that social jetlag may be a stronger risk factor for smoking than chronotype itself and that the risk rises with increasing social jetlag.

Interestingly, quantity of smoking was not related to chronotype or social jetlag in the only study that assessed this (Wittmann et al., 2006). Chronotypes thus seem to differ in their vulnerability to start

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smoking, suggesting a behavioural factor. Smoking behaviour is often related to depressive symptoms. For example, Wittmann and co-workers (2010) found that depressive symptoms were lower in only those late types who smoked, and that social jetlag may play an important role in becoming a smoker. Late types might turn to smoking as a mechanism of coping with negative emotions resulting from social jetlag (UrbĂĄn et al., 2011; Wittmann et al., 2010). Smoking in turn can lower mental well-being (Wittmann et al., 2010) and increase social jetlag by affecting circadian rhythmicity (UrbĂĄn et al., 2011), leading to a vicious cycle.

In conclusion, social jetlag seems to increase the risk that late types start smoking, especially during adolescence. People may turn to smoking as a way of dealing with depressive symptoms that may be caused by social jetlag (see above). Longitudinal research is necessary to investigate how exactly social jetlag can lead to smoking.

Obesity Obesity is currently one of the most prominent health problems, especially among adolescents (Arora & Taheri, 2014). A growing body of evidence proposes a bidirectional relationship between sleep an obesity, with most studies showing that shorter sleep duration is associated with obesity (Arora & Taheri, 2014; Randler, Haun, & Schaal, 2013). Because social jetlag is largest during adolescence (Roenneberg et al., 2004), it is possible that short sleep duration is caused by social jetlag and that this is actually the most important sleep variable in relation to obesity.

Roenneberg and colleagues (2012) specifically examined the relationship between BMI and social jetlag in addition to sleep duration in over 65,000 participants between 10 and 80 years old. Controlling for age and gender effects (Figure 4), longer sleep duration decreased the possibility of being overweight (BMI ≼ 25), agreeing with aforementioned findings. More specifically, the chance of belonging to the overweight group rose with increasing social jetlag: every hour of social jetlag increased the chance of being overweight with 30% (Roenneberg et al., 2015, 2012). Within the overweight group, social jetlag was a stronger predictor than chronotype and sleep duration alone (Roenneberg et al., 2012). Similar findings were obtained in an entirely adolescent sample (Randler et al., 2013). These results confirm the idea that social jetlag plays an important role in obesity, rather than its effect on sleep duration.

Physical and mental health are interrelated (Wittmann et al., 2010), as are different facets of physical health. For example, social jetlag can cause depressive symptoms (see above), which can lead to emotional eating and an unhealthy diet. These can in turn negatively influence mood (Konttinen et al., 2014), creating a vicious cycle. Moreover, smoking can lower weight, which could lead to an underestimation of the effects of social jetlag on BMI (Parsons et al., 2014). Also, people with a higher BMI may turn to smoking as a result of depressive symptoms or in an attempt to lower weight. It is therefore important for future studies to consider the link between smoking and weight, especially as a late chronotype and social jetlag have been related to both.

Late types thus seem to have a higher chance of becoming overweight than early types. Shorter sleep duration on its own plays a role in this association, but social jetlag appears an even more important risk factor and is probably the cause of short sleep duration.

Conclusions Overall, social jetlag seems to be associated with mental and physical health problems. The vulnerability to experience depressive symptoms, to start smoking, or to become overweight rises with increasing social jetlag. Mental and physical health variables influence each other and may in turn also affect circadian rhythmicity, leading to vicious cycles further increasing health and sleep problems. Social jetlag is largest among adolescents (Roenneberg, 2012) and poor physical and mental health during this vulnerable period can have long-term negative consequences.

These health problems could be reduced by making people aware of the risks and educating them

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on healthier lifestyle choices. It would obviously be more effective if social jetlag, one of the causes underlying these health problems, i.e., social jetlag, could be prevented. This is discussed in more detail in the next section.

Academic achievement Social jetlag and academic achievement The largest part of adolescent life consists of school and/or university, which generally have early start times that create a mismatch between internal and external time. It is therefore not surprising that social jetlag is largest around late adolescence, coinciding with the peak in lateness (Roenneberg et al., 2004). A recent meta-analysis (Tonetti, Natale, & Randler, 2015) showed that a late chronotype has consistently been associated with poorer academic achievement. This is a particularly interesting finding, late types generally score higher on intelligence tests (Kanazawa & Perina, 2009; Roberts & Kyllonen, 1999) and intelligence is usually a reliable predictor for school performance (Preckel, Lipnevich, Schneider, & Roberts, 2011). Since late types experience the largest social jetlag, it is possible that they are unable to perform at their best as a result of social jetlag and subsequent sleep deprivation. The latter has indeed been correlated to poorer academic performance (Dewald, Meijer, Oort, Kerkhof, & Bögels, 2010). Social jetlag can also lower intrinsic motivation to learn and perform well, which also negatively affects academic achievement (Escribano & Díaz-Morales, 2016; Roeser, Schlarb, & Kübler, 2013).

One study investigated the effects of social jetlag on academic achievement in a sample of medical students (Haraszti, Ella, Gyöngyösi, Roenneberg, & Káldi, 2014). Social jetlag showed a stronger negative correlation with academic achievement than chronotype alone. A study in adolescents found no relationship between academic achievement on high school leaving exams and chronotype, assessed both subjectively and objectively (Tonetti et al., 2015). Instead, their results suggest that sleep quality is a more important factor. Social jetlag was not measured in this study, but had previously been associated with poorer sleep quality (e.g. Short et al., 2013; Wittmann et al., 2006) and may have caused the lower sleep quality.

It can be concluded that chronotype per se is not necessarily related to poorer academic achievement. Late types seem more intelligent, yet perform worse. Social jetlag and subsequent reduced sleep quality and duration likely have a negative influence on performance at school and university.

Delaying start times For this reason it has often been recommended to delay school start times. This might minimise the amount of social jetlag from which the majority of adolescents suffer. Of course, such delays should not be too large as this could put early types at a disadvantage (Roenneberg, 2012).

Several studies have reported positive effects of delaying school start times. For example, Wahlstrom (2014) studied a large sample of adolescents from different schools where start time was delayed for 30, 60, or 80 minutes. Even the smallest delay resulted in less sleep deprivation, which was in turn related to higher grades and better physical and mental well-being. A study by Perkinson-Gloor, Lemola, & Grob (2013) showed that even a 20-minute delay significantly increased sleep duration and reduced daytime sleepiness. Academic performance and motivation were however not affected, although they were related to sleep duration and sleepiness. A delay of 20 minutes is probably not sufficient to improve motivation and academic achievement.

The small effect sizes might be explained by the fact that delayed start times in previous studies were often still before 09.00, while Kelley, Lockley, Foster, & Kelley (2014) calculated that school start times for 10-year-olds should be between 08.30-09.00, between 10.00-10.30 for 16-year-olds, and 11.00-11.30 for 18-year-olds to be in synchrony with their circadian biology. However, the fact that minor changes already show significant positive effects only implies that larger delays will have stronger effects.

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Social jetlag was not considered in the aforementioned studies. However, sleep duration increased while daytime sleepiness decreased after a delay, which could be seen as evidence that social jetlag had diminished. It is still advisable for future studies to specifically assess social jetlag. Since most adolescents suffer from social jetlag, a start time delay would benefit the majority of students without disadvantaging those who are earlier types. This would have positive effects not only on academic achievement, but also on other factors associated with social jetlag, such as physical and mental health.

Objections to delaying start times Despite the growing body of evidence that start time delays benefit the majority of students, there is still a lot of controversy and criticism. Possibly the largest point of criticism is a financial argument. Restructuring school schedules can be expensive, and schedules for transportation and extracurricular activities need to be altered in concert. This is of course a valid argument, although other interventions to help disadvantaged students, such as smaller class sizes or private teaching, are often more expensive (Kelley et al., 2014). Furthermore, the positive effects of a delay on adolescents' academic achievement and health should outweigh the costs of implementing such a change (Jacob & Rockoff, 2011; Kelley et al., 2014; Kirby, Maggi, & D’Angiulli, 2011).

A more social point of criticism is that adolescents may have less time to engage in extracurricular or familial activities (e.g. Jacob & Rockoff, 2011). However, after a 30-minute school start delay, students showed more interest in such after-school activities, possibly as a result of improved mood and more energy because of reduced social jetlag (Owens, Belon, & Moss, 2010). In addition, time spent on such activities was not affected by a delay of 25 minutes (Boergers, Gable, & Owens, 2014). Larger delays may of course interfere with after-school activities, but shifting these activities to later times does not have to be problematic, especially if planned well.

A start time delay is thus recommended, although making it large enough to fully eliminate social jetlag might not be feasible. Alternatively, schools could schedule exams during the early afternoon, where the difference between early and late types disappears (van der Vinne et al., 2015). Still, evening types will then be at a disadvantage during teaching hours. Restructuring classes in such a way that the most cognitively challenging subjects (e.g. maths, science) are not taught in the morning might prove effective.

Conclusions Adolescents are among the latest types, yet have to start school very early, during their internal night.

Consequently, social jetlag is largest during adolescence and subsequent reductions in sleep duration and quality make it difficult for late types to show their true potential at school and university. Despite having higher intelligence scores, they consistently have poorer academic achievement than earlier types.

Probably the most effective way to diminish social jetlag is by delaying school start times, which has already led to positive effects on sleep duration, and subsequently on academic achievement and health. It is probably not feasible to delay start times enough to fully eliminate social jetlag. To further reduce the effects of social jetlag, classes and exams within school times should also be rescheduled. There are however still many objections to these suggestions. Rescheduling school times and after-school activities indeed requires money and thorough planning, but the positive effects should more than justify such investments.

Career Chronotype could also have an effect on career. Poorer academic achievement at both school and university may make it more difficult for late types to get good jobs. Also, they may be less motivated to put effort into their job as a result of depressive symptoms and poorer mental health in general. Even though lateness decreases after the age of 20, it still leads to social jetlag in the majority of adults (Figure 4;

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Roenneberg, 2012). Since most jobs start in the morning, the negative effects of social jetlag may persist long after school and university. This section focuses on the effects of social jetlag on career choice and job performance.

Career choice Early and late chronotypes may choose different careers. Professions where people have to perform well in the morning may favour early types simply by selection. For example, teachers have to be in command of their students from (early) in the morning and it is therefore improbable that there are many late types among teachers. Also, early types may have felt more comfortable at the environment at school and university, and may therefore be more likely to want to stay in that environment (Roenneberg, 2012).

Conversely, late types may prefer more creative jobs, which generally have more flexible schedules.

A late chronotype has been related to a more creative way of thinking (DĂ­az-Morales & Escribano, 2013; Giampietro & Cavallera, 2007). This relationship may be direct, but it is also possible that creative thinking has been more strongly developed in late types. They spent years having to perform at non-optimal times of the day, which has been associated with creative problem solving (Giampietro & Cavallera, 2007; Wieth & Zacks, 2011). It would be interesting to examine the effects of social jetlag on different styles of thinking in longitudinal studies.

Early and late types may thus choose different careers. This is all still purely speculative and any conclusions should be drawn with caution, but it would be interesting to investigate the prevalence of early and late types among different occupations. Of course there is nothing wrong with different chronotypes having different interests or aspirations in life, but in terms of equal opportunities it is important to consider the effects that chronotype and social jetlag may have on someone's options and choices.

Job performance In recent years, work schedules have become more flexible, enabled by globalisation and more sophisticated technology (Yam, Fehr, & Barnes, 2014). Such flexible work practices (FWPs; Kelly & Moen, 2007) usually have positive effects, e.g. on productivity and job satisfaction, but could still negatively affect career outcomes and perceived job performance. Especially start times of employees seem to have a large impact on how supervisors and managers rate their performance, which in turn influences their future career success (Yam et al., 2014).

Yam and colleagues (2014) showed that the performance of employees who start their working day late was rated lower by their superiors. There thus appears to be a morning bias; people who seem to start their day late receive lower evaluations. Late starters were perceived as less conscientious, which seemed to be the most important factor in lower performance evaluation after controlling for other factors such as total work hours. It should be noted that higher conscientiousness may lead to lower evaluations in creative jobs rather than higher, as this personality trait has been negatively associated with creativity (Feist, 1998). In the study by Yam and co-workers (2014), conscientiousness was however related to better performance ratings. Since the occupation of participants in their study was not known, this idea can be neither confirmed nor contradicted by the results. It might be interesting to investigate whether a morning bias also exists in creative careers.

The authors also found that employees with a later start time were only perceived as less conscientious and lower performers by supervisors who were early types. In other words, the morning bias was absent in supervisors who showed an evening preference. This might be explained by social identity theory (see Yam et al., 2014), which would suggest that late type supervisors consider later starters as belonging to the same social group. As a result of such in-group favouritism, they would be more likely to evaluate late starters more positively. Interestingly, early starters were not rated more negatively by late

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type supervisors. Thus, it seems that early type supervisors exhibit a morning bias, but a similar but opposite 'evening bias' does not seem to exist among late type supervisors.

Flexible work practices may thus allow late types to better live in synchrony with their inner biological rhythm, but both employees and supervisors should be aware of the morning bias in the workplace. Research into chronotype and career is relatively recent, so more studies are necessary to answer currently outstanding questions, for example regarding career opportunities and evaluations in different occupations. As many factors are involved in job performance, future investigations should attempt to take these into account.

Work times A recent intervention study introduced chronotype-adjusted (CTA) schedules in a factory. Schedules consisted of morning, evening, and night shifts. Late types no longer worked morning shifts, whereas night shifts were removed for early types. Intermediate types had no changes in their schedules and served as control group. After implementation of the CTA schedule, social jetlag was reduced by 1 hour and subsequent sleep duration and quality were improved, as well as subjective well-being (Vetter, Fischer, Matera, & Roenneberg, 2015). Even though the sample was small and the intervention was probably too short (5 months) to examine long-term effects, the results show that a personalised working schedule can reduce circadian misalignment and have positive health effects.

Conclusions Social jetlag is largest in late adolescence and young adulthood, which may influence career choice. This is not necessarily a bad thing, but it might be interesting to investigate the impact of social jetlag on career options and choices. Social jetlag persists far into adulthood, with all the aforementioned negative consequences. Flexible working hours, especially when adapted to someone's internal time, may offer a solution. There is however still a pervasive morning bias that people should be aware of, which can negatively influence evaluation of job performance and subsequent career success.

Discussion In line with the hypothesis, social jetlag appears to play a central role in mediating the associations between chronotype and health, academic achievement, and career. Chronotype is determined by biological factors and entrained to a 24-hour rhythm by light, the main zeitgeber. Social jetlag occurs when people are not able to live in synchrony with their internal clock. Most people suffer from social jetlag to some degree as they have to get up during their internal night to go to work or school. Social jetlag has negative effects on mental and physical health. It has been associated with depressive symptoms and with vulnerability to start smoking and to becoming overweight. All of these factors may be interrelated as well and can further increase social jetlag, creating vicious cycles worsening the problems. These risks rise with increasing social jetlag and may be the result of unhealthy mechanisms of dealing with circadian misalignment. In addition, social jetlag negatively influences academic achievement. It is largest in adolescents, who subsequently may not be able to perform at their best. Late types in particular are put at a disadvantage by early school start times. They generally have higher intelligence scores, yet perform worse at school and university, which might be caused by the negative effects of social jetlag. Furthermore, social jetlag can affect career choice and job performance, which are also influenced by academic achievement. Taken together, it becomes evident that social jetlag can have widespread negative consequences for the majority of people, with adolescents forming a particularly vulnerable group, and is a problem that needs to be addressed.

It should be noted that social jetlag interacts with many more variables than those mentioned in this review. For example, some studies have suggested that personality traits such as conscientiousness and

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novelty-seeking are related to different chronotypes and seem to be involved in the effects of social jetlag (Adan et al., 2012; Tsaousis, 2010). Differences in behaviour, which can also arise as a result of social jetlag, also seem to play a role. Although a thorough discussion of the interaction with personality and behaviour is beyond the scope of this review, it is important to keep in mind that they may impact the relationships discussed here.

Among adolescents, sleep-related problems are mostly recognised, but most people do not seem to take into account that they might be the result of a biologically determined circadian rhythm. It is good to see that recently there has been increasing media attention on biological rhythm rather than sleep alone.

However, reactions of scientists and laymen alike suggest that there is still a lot of unawareness on the fact that biological rhythm is dictated by genetic and biological factors and cannot simply be changed or ignored. Consequently, adolescents who experience difficulties waking up or tiredness during the day are often told to just go to bed earlier. Without knowledge on chronotypes, this sounds like a logical solution.

However, this review has hopefully already made it clear that this is simply not an option because of the underlying circadian biology and a shift towards a later chronotype during adolescence. Even if they go to bed earlier, adolescents will probably not be able to fall asleep before their body and brain are ready for that. The couple of hours before the brain promotes sleeping are called the Wake Maintenance Zone (WMZ). During the WMZ, the circadian drive to stay awake is stronger than any built-up sleep pressure, making it very difficult to fall asleep, even if sleep pressure is quite high (Kelley et al., 2014; Roenneberg, 2012). As the WMZ in adolescents is also shifted towards later in the evening, earlier bedtimes will probably not lead to earlier sleep onset times and longer sleep duration. People who suggest earlier bedtimes should thus be aware that late types are physically incapable of falling asleep at that time.

Earlier bedtimes may even have adverse effects. Lying awake without being able to fall asleep may lead to feelings of restlessness and frustration, making it even harder to eventually fall asleep. Also, people might get up to do something, keeping them awake for longer than desired. It should be noted that this is purely speculative, but nonetheless interesting to consider.

Even though I argue that chronotype is not a choice but dictated by the internal biological rhythm, I do acknowledge the existence of zeitgebers that can influence this rhythm, albeit within genetic boundaries. As already mentioned, the use of technology may delay chronotype (e.g. Kantermann, n.d.; Roenneberg, 2012). Adolescents, especially the latest types, may use technology until late in the evening which can aggravate the negative effects of social jetlag. It is therefore recommended to educate people on these effects and how to diminish them. Fortunately, the media are already advising people to turn off all screens at least an hour before going to bed. Also, some applications have recently been developed that reduce the brightness and emission of blue light, to which circadian rhythm is particularly sensitive, of laptops (e.g. f.lux速, https://justgetflux.com/) or smartphones (e.g. Twilight by Urbandroid) in synchrony with local sunset time. A zeitgeber that could be used to slightly advance chronotype is the sun (Roenneberg, 2012). Encouraging late types in particular to spend time outdoors, especially in the beginning of their day (see Figure 3), might therefore be advisable. This may also have additional benefits, for example in terms of health.

In addition to these rather biological interventions, it is essential to make people aware of the existence, effects, and importance of biological rhythm. People should be advised on alternative, healthier ways of coping with misalignment between internal and external time than maladaptive strategies that may create vicious cycles, such as smoking and unhealthy eating.

Still, all these suggestions will probably only be slightly effective in advancing rhythm and making people better able to deal with negative effects of social jetlag. It would obviously be better if social jetlag itself would be eliminated, or at least reduced. The main cause of social jetlag seems to be early school and work start time, which is not in synchrony with the internal rhythm of most people. More awareness and knowledge on chronotype and the negative consequences of social jetlag will hopefully lead to the delaying

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of start times and a further increase in work schedule flexibility. The reservations for making such changes are valid, but the benefits should outweigh the costs and justify any investments. I believe such societal changes will benefit the majority of people, who are not early types, without disadvantaging those who are.

For example, flexible working hours will likely reduce crowdedness and delays in traffic and public transport during rush hours. More importantly, decreased social jetlag will improve general well-being and quality of life for many people, thereby increasing equality between chronotypes and enabling more people to fulfil their potential. Furthermore, more knowledge on biological rhythm will lead to a better understanding of one's own functioning, emotions, and behaviour, and hopefully to more acceptance of those who are different.

In conclusion, people should be made more aware of the existence, effects, and importance of chronotypes and social jetlag. Consequently, they may be motivated to make changes in their lifestyle, for example regarding daylight exposure, use of technology, and eating and smoking behaviours to better cope with social jetlag. Preferably, social jetlag would be minimised by delaying school and work start times or by making them more flexible. Such adjustments will not only benefit late types without disadvantaging early types, but should be profitable to both early and late types.

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