Circadian Rhythms

There are many different factors that influence sleep. These can include the presence of recent sleep or wakefulness, the patient’s age, the time of day, and other factors, like stress, environmental conditions, exercise, and certain chemicals. As an infant, sleep starts in REM sleep. Babies have a shorter sleep cycle, alternative between REM and non-REM sleep every 50 minutes, instead of every 90 minutes in adults.

The recent sleep history will impact the sleep pattern of a given night’s sleep. An irregular sleep schedule or missing a night’s sleep will affect the pattern of sleep so there is more non-REM sleep. Drugs like alcohol will suppress REM sleep but this rebounds as the night progresses. The more REM sleep a person has, the more frequently the person will awaken.

While it is common to get sleep in one big block, this isn’t the only possible sleep pattern. Many cultures, particularly in tropical areas, will have an afternoon nap among adults. Stores and buildings close so that people can get a one to two-hour nap. Napping follows the largest meal of the day and occurs during the hottest time of the day. Naps in most cases are between thirty minutes and sixty minutes. It is a good way for people who do not sleep well to catch up during the day.

CIRCADIAN RHYTHMS

It is generally common for adults to get 16 hours of wakefulness with 8 hours of sleep. There is a small bundle of nerve cells that promote or initiate sleep. There are areas in the brainstem and hypothalamus that are involved in wakefulness. The tuberomammillary nucleus or TMN releases histamine to cause wakefulness. This is why anti-histamines promote sleep. Other neurons will make orexin or hypocretin that stimulates the arousal centers in the cerebral cortex.

The hypothalamic nucleus called VLPO or ventrolateral preoptic nucleus connects to the arousal centers, promoting sleep. There is mutual inhibition between the sleeppromoting neurons and the wake-promoting neurons that cause stable wakefulness or sleep. The VLPO and the TNM operate opposite to one another in order to have a normal sleep pattern.

There are relatively rapid switches between sleep and wakefulness and between REM and non-REM sleep. It takes several minutes to relax enough to fall asleep and about 20 minutes or more to get into a deep sleep. Sleep onset, however, can occur abruptly, such as when falling asleep at the wheel. Waking up can also be abrupt, such as when an alarm goes off.

There are internal factors (like circadian rhythms) and external factors (like noise) that play a role in getting to sleep and waking up. Adenosine builds up with prolonged wakefulness, ultimately inducing sleep by blocking wake-promoting neurons. Caffeine inhibits adenosine to keep us awake.

The internal biological clock in the brain is the suprachiasmatic nucleus or SCN. This receives light signals from the eye in order to reset the sleep clock. It regulates many of the factors associated with sleep, promoting wakefulness that offsets the sleep drive. It also maintains sleep in the second half of the night, when the sleep drive has left.

The sleep drive is the natural drive to sleep when you haven’t gotten enough. It is a homeostatic mechanism that regulates the amount of sleep we get. Every waking hour we have strengthens the homeostatic sleep drive, which builds up until we absolutely have to go to sleep. The only way to overcome it is to actually sleep.

The reason we are able to stay awake for sixteen hours at a time is because of the circadian rhythm. It is controlled by a small number of neurons in the brain and is synchronized to the sleep/wake cycle. It opposes the sleep drive that normally increases with every hour that we’re awake. Only when the circadian pattern drops off does the sleep drive kick in so we can sleep.

The sleep drive is high during the first half of sleep and the alert signal is low. This means it is easy to maintain sleep. After four hours of uninterrupted sleep, the sleep drive is lowered and the alerting signal begins to kick in. When these things are coordinated, they interact in order to have a prolonged period of sleep and alertness. We are tired after a midday meal because the alerting signal diminishes slightly. It is difficult to fall asleep between 8 and 9 pm at night, because the alerting signal is still too high.

There are changes in the structure and function of the brain during development that affect sleep. The amount of sleep needed and the fragmentation experienced as we age changes. Stress and many different medical conditions can affect the amount of sleep we can get in real life. Medications and the food we eat also affect sleep. Light, of course, affects the suprachiasmatic nucleus and its regulation of sleep. In fact, light is one of the most important factors affecting sleep. Exposure to light in the late evening will affect the ability to sleep during that time.

Things like shift work and jet lag will change the circadian rhythm over the course of the day. This interferes with when we sleep and which sleep stages can be achieved during sleep. Jet lag and shift work will lead to insomnia and excessive sleepiness at the wrong times.

Pain, certain medical conditions, anxiety, and depression make it harder to fall asleep. There will be lighter sleep and more REM sleep with less non-REM sleep. Stress will make restful sleep much harder to achieve by affecting the arousal response. Pain will limit the deepest stages of sleep. Medications that will impact sleep include alcohol, caffeine, antihistamines, beta blockers, nicotine, antidepressants, and alpha blockers. Both nicotine and caffeine will inhibit sleep.

If sleep happens after taking in caffeine, the stimulant will impact the quality of sleep. There will be decreased slow-wave sleep and REM sleep and increased awakenings. Some people are relatively intolerant to caffeine and it will have less of an effect on sleep, especially when the sleep drive is great. Alcohol will cause falling asleep more quickly but the quality of sleep will be worse. High amounts of alcohol will cause insomnia. Sleep apnea will be worse with alcohol consumption.

There are many medications that have effects on sleep. Beta blockers will decrease REM sleep and slow-wave sleep, making you tired in the daytime. The same is true of alpha blockers. Antidepressants will decrease REM sleep but some SSRIs will promote insomnia in susceptible individuals.

The environment you sleep in has an impact on sleep quantity and quality. This includes the levels of noise, light, and temperature. Too much light will negatively impact sleep so small nightlights are beneficial. Noise can relax some people if the

volume is low. There is no prescribed “good temperature” for sleep but extremes will disrupt sleep. REM sleep is more sensitive to temperature-related disruption in sleep. Cold temperatures deprive us of REM sleep.

How much sleep is necessary? In general, it is about six to nine hours of sleep a night with seven hours of actual sleep being optimal. Everyone has differences in the amount of sleep they get with genetic influences commonly seen. Age, physical exertion, mental activity, and health also influence the amount of sleep people need. The recommendations are 8 hours for people between 18 and 21 years of age and 7 hours for those over 21 years.

Babies under a year need 15 to 20 hours of sleep a day. Age 1-2 years needs 14 hours of sleep. By 13 to 19 years of age, about 9 hours of sleep are needed. Seniors still need about seven to eight hours of sleep per night. Only about 32 percent of older adults are getting the amount of sleep they probably need to get.

Sleep deprivation will affect your reaction time and judgment. You can’t actually change the amount of sleep your body needs and eventually you will have to sleep in order to catch up on the sleep you miss when you are sleep deprived. In reality, only about a third to a half of all sleep is caught up when doing this. If you fall asleep within five minutes of lying down, you probably have sleep deprivation. Microsleeps or brief sleeps are another sign of sleep deprivation.

Mortality is greatest with long sleepers and short sleepers. People with sleep debt have more weight gain and depression, with worsened memory and a poorer immune system. Those with insomnia get less deep sleep and poorer sleep quality. Insomnia is an increasing part of the aging process that may have something to do with the medications they take.

Signs of too much sleep include awakening before the alarm, taking longer than an hour to fall asleep, having low energy, having hypersomnia, and experiencing weight gain from a lack of activity. Signs of too little sleep include waking up from stress during the night, feeling worn-out, being moody, having weight gain, and being drowsy during the day. Forgetfulness can be a sign of too little sleep.