Sleep Stages and Sleep Cycles

SLEEP STAGES AND SLEEP CYCLES

We’ve talked a little bit about the different sleep stages. There are about four separate sleep stages. There are three stages of non-REM sleep called 1, 2, and 3, along with REM or rapid eye movement sleep. These from a complete sleep cycle that lasts about 90 minutes each, with each stage lasting about five to fifteen minutes in total length. The first few cycles have short REM sleeps but later on in the night, the REM sleep periods lengthen and the deep sleep non-REM sleep shortens.

Stage 1 is the lightest stage of Non-REM sleep. There are slow eye movements and easy arousability. Muscle tone decreases and brain waves slow. This is when hypnic jerks or muscle spasms are most experienced. The sensation of falling happens in this phase.

Stage 2 is the first defined stage of non-REM sleep. Slow eye roles stop and brain waves slow. There are brief bursts of brain activity called sleep spindles that are associated with K complexes. These together keep you from waking up. Body temperature drops and heart rate slows.

Stage 3 is the deepest stage of non-REM sleep. This is when you get more restorative sleep and you can get slow waves or delta waves. It is hard to awaken the person from this stage of sleep and is when sleep talking, sleep walking, and night terrors usually most occur.

REM sleep is rapid eye movement sleep, when most people do a great deal of dreaming. There are rapid eye movements and more active brain wave patterns. It is easier to awaken a person while in REM sleep but it can leave a person feeling groggy when awakening in this stage of sleep.

One sleep cycle is the progression through the various non-REM sleep states through REM sleep and finally back again. This lasts about 90 minutes. You do not go through deep sleep directly to REM sleep. It goes from light to deep sleep and back to light sleep before going into REM sleep and starting again. There are no abrupt changes between the stages from deep to light or vice versa.

Stage 2 lasts the longest time—about 40 to 60 percent of total sleep time. Restorative stage 3 lasts only five to fifteen percent of the time. It lasts longer in adolescents and children. REM sleep starts 90 minutes after sleep onset and is relatively short.

Deep sleep or stage 3 sleep involves the slow delta waves. It is least likely to be affected by external stimuli and is hard to wake up from. After sleep deprivation, this is the time of sleep most people sleep in in order to restore lost sleep. Bedwetting an occur in this phase of sleep because of how deep it is. It has the capacity to reduce the sleep drive the most of all of the stages. Human growth hormone is released during this phase and the immune system recovers.

A person will have about 3 to 5 REM sleep periods per night with the longest time being just before waking up. If awakening from REM sleep too early, the person will be sleepy and have sleep inertia for minutes to hours after waking up. Breathing is rapid, shallow, and irregular with jerking of the eyes. The limbs are paralyzed with brain waves similar to those when awake. Erections occur and temperature regulation diminishes. Figure 2 shows the different brain waves in the different times of sleep.

Dreams can happen in any stage but are most vivid in REM sleep. Brain waves in rem sleep are low amplitude with mixed frequencies. Individuals dream about 4 to 6 times each night, even if they do not remember their dreams. People usually remember their dreams when they awaken in the middle of REM sleep. Obstructive sleep apnea is worse during REM sleep because of the paralysis they have.

Interestingly, deprivation of REM sleep does not lead to insanity but a lack of REM sleep can help individuals who have clinical depression. The way in which REM sleep deprivation helps depression is not completely clear. REM sleep may be linked to both learning and memory, according to more recent theories on sleep.

Sleep will change throughout life. As newborns, there is a lot of REM sleep or “active” sleep that allows for frequent awakening for feeding. Infants between four months and a year sleep 10 to 13 hours a day with standard sleep stages. Toddlers spend about 25 percent of the time in stage 3 sleep and about the same amount of time in REM sleep. Children aged 3 to 6 will sleep 9 to 10 hours of sleep per night. Older children spend more time in restorative sleep—good for growth and development.