Last updated: March 11, 2017
The human body requires adequate sleep to ensure proper function and mental and physical health. Sleep is an active yet restorative process, as is clearly shown by electroencephalograms (EEGs) that record brain activity during sleep. Scientists now know that sleep occurs in carefully regulated cycles; when the stages of sleep cycle are disrupted, it can create serious health consequences.
The Basics of Sleep
Sleep includes two main types of sleep. The first is rapid eye movement or REM sleep. The second is non-REM sleep. REM sleep is considered the active phase. During REM sleep, an EEG will show characteristic low amplitude (small) brain waves that occur with high frequency in a pattern called alpha rhythm. The patient will also have the characteristic rapid eye movements that give this stage its name.
Experts think the eye movement may be related to dreaming. When people are awakened during REM sleep, they often report being in the middle of vivid and sometimes bizarre dreams. People who are awakened during non-REM sleep are much less likely to report they were dreaming. Research has also shown that the muscles in the arms and legs are temporarily paralyzed during REM sleep, perhaps to prevent people from acting out their dreams and possibly becoming injured or hurting someone else.
Non-REM and REM sleep alternate throughout the night in what are called sleep cycles. However, the stages of sleep cycle are not exactly the same throughout the night. Non-REM sleep occurs primarily in the first part of the night, while REM sleep episodes gradually become longer through the night.
The stages of the sleep cycle may vary a bit from one person to another, but all healthy individuals go through all the cycles of sleep. Non-REM sleep has three distinct stages – N1, N2 and N3 – each with a characteristic EEG pattern. People cycle through REM and non-REM sleep throughout the night. When a person falls asleep, the transition from wakefulness to sleep may last only a few seconds to minutes.
~ The first stage of sleep is N1, which may last only one to seven minutes.~ Stage N2 is next and usually lasts about 10 to 24 minutes.During the N2 stage, an EEG will gradually begin to show the “slow wave” activity that occurs in stage N3.
~ Stage N3 is considered the deepest stage of non-REM sleep and is sometimes called deep or slow wave sleep because of its EEG pattern. N3 usually lasts 20 to 40 minutes.
As a person moves more deeply into sleep, the brain becomes less responsive to outside stimuli and it is more difficult to waken the person from sleep. At this point, the person “ascends” from N3 to a lighter stage of non-REM sleep, usually a period of about five to 10 minutes of N2 sleep, before moving fully into REM sleep.
For most healthy adults REM sleep is about 20 to 25 percent of their total sleep time. The first episodes of REM sleep may last from one to five minutes; by the end of the night a REM episode is usually considerably longer. N3 sleep tends to become shorter throughout the night and may even disappear entirely, while the body cycles from N2 to REM and back again. In the first non-REM/REM cycle of the night, the cycle is usually between 70 to 100 minutes. The second and succeeding cycles are usually between 90 and 120 minutes. No one knows why humans have this particular sleep pattern.
Sleep patterns differ with age. For example, newborns go through the same pattern as adults, but the total length of the cycle is about 50 to 60 minutes. By the age of two to six months, infants begin to develop more typical sleep and EEG patterns. The kind of slow wave sleep seen in the N2 stage is greatest in young children and gradually decreases with age, although the total duration of sleep may not change.
Adults spend about 50 percent of their total sleep time in N2 sleep, approximately 20 percent in REM sleep and 30 percent in the other stages of sleep or in transition. Elderly people may have less or even no REM sleep. Scientists don't know if this is a normal part of aging or if it is related to medications or medical conditions.
REM sleep is vitally important for survival, according to animal studies. Rats repeatedly deprived of REM sleep live about five weeks rather than the normal two- or three-year lifespan. If REM sleep is disrupted for any reason, the next night people are more likely to go directly into REM and remain there for longer periods, as if the body is “catching up.”
What Happens During Sleep
People in N1 sleep may drift in and out of sleep and can usually be easily awakened. The eyes move slowly and muscle activity also slows; when awakened, people often remember fragmented images. Some people report experiencing sudden muscle contractions (hypnic myoclonia), preceded by a feeling of starting to fall. Hypnic myoclonia is similar to the “jump” people give when startled. In N2 sleep, eye movements stop and the brain waves become slower, although occasional bursts of rapid waves called sleep spindles appear on the EEG. Children in REM sleep may have night terrors or nightmares, wet the bed or sleepwalk. During REM sleep, breathing becomes more shallow, rapid and irregular, and the eyes jerk rapidly in various directions. The heart rate and blood pressure rise, and men may have an erection. Growth hormone is released in children and young adults during the REM sleep phase and body cells increase the production of proteins. There is some evidence that memories are processed and encoded during REM sleep. People who learned something new and then were deprived of REM sleep had trouble remembering what they'd learned. Most people spend about two hours a night dreaming, nearly always during REM sleep.
External Factors That Affect Sleep
External factors can affect the sleep cycle. People who work rotating shifts, such as nurses, police officers or firefighters, often have irregular sleep patterns. New parents may be disturbed frequently during the night. Doctors may miss an entire night's sleep repeatedly during residency. A redistribution of the sleep cycle often occurs as a result of sleep cycle disruption. For example, slow-wave non-REM sleep may become more prolonged or deeper.
Illegal substances, some medications and alcohol all affect sleep stages. Alcohol tends to suppress REM sleep during the early part of the night. Once the alcohol is metabolized, REM sleep rebounds. However, the individual may also be more likely to awaken repeatedly later in the night, or to wake much earlier than usual and not be able to go back to sleep. Anti-depressant medications can suppress REM sleep. Caffeine stimulates the brain and may affect sleep cycles as well as sleep duration. People who smoke heavily may sleep lightly and not have as much REM sleep as non-smokers.
The body has more difficulty regulating temperature during REM sleep; a very cold or hot environment can disrupt REM sleep.Sleep apnea and restless leg syndrome can also affect the sleep cycle.
Naps and Sleep
In addition to sleeping at night, in some countries or cultures it is also common for people to take naps in the afternoon. The custom of siesta is well-established in some countries, to the point that businesses and even government offices may close for an hour or two in the afternoon.
There are biological reasons to nap. The stages of sleeping and waking are driven by various biological processes such as hormone secretion. The drive to stay awake increases throughout the day, but there is a brief period in mid-afternoon when the drive to sleep cab become uppermost. A heavy meal and a warm day often increase the desire to nap. For most people, an afternoon nap lasts 30 to 60 minutes. Shorter naps are usually N2 sleep, while a nap that approaches or exceeds an hour is more likely to result in REM sleep. Once the sleep drive has dissipated people who nap often report getting their “second wind”, which can keep them up later so their total night-time sleep is less than that of people who don't nap.
Napping may or may not help people who don't sleep well at night. In some cases, it helps make up for a sleep deficit, while in others it seems to exacerbate insomnia.
The Moon and Sleep
The moon may affect the cycles of sleep, although the premise for the moon's effect is currently based on a single study reported in Current Biology. The researchers weren't actually studying the moon's effects on sleep; the data was a byproduct of another study. The study participants were in a sleep lab, so they had neither moonlight nor other external cues to tell them when the moon was full. The researchers found that for a few days around the full moon, people took about five minutes longer to fall asleep. The total sleep duration was reduced by an average of 20 minutes and study participants did not sleep as deeply. Brain activity related to deep sleep (the REM cycle) dropped by 30 percent. Study participants also showed a decrease in melatonin, a light-sensitive hormone that is one of the regulatory mechanisms for sleep-wake cycles.
Promoting Healthy Sleep Cycles
Because sleep is so important to mental and physical health, people should try to promote healthy sleep cycles. Here are some effective strategies:
~ A regular routine helps promote sleep, such as going to bed and getting up at the same time every day.
~ Exercise for 20 to 30 minutes every day usually improves sleep patterns; about five or six hours before bedtime (or earlier) is best. Exercise too close to bedtime, however, can make it more difficult to get to sleep.
~ People should limit or avoid caffeine, nicotine and alcohol, especially if they have sleep problems.
~ A soothing bedtime routine – a warm bath, reading (paper, not books electronics), relaxing music, meditation or something similar - can make it easier to fall asleep.
~ It's best for people to wake up with the sun; if that's not possible the us of bright lights first thing in the morning can help reset the internal clock.
Healthy sleep means a healthy person. If people's sleep cycles are disrupted or they are having trouble sleeping and the sleep-inducing strategies above don't help, a talk with the family doctor is in order. Some people may need to see a doctor who specializes in sleep disorders.