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Usually sleepers pass through five stages: 1, 2, 3, 4 and REM (rapid eye movement) sleep. These stages progress cyclically from 1 through REM then begin again with stage 1. A complete sleep cycle takes an average of 90 to 110 minutes, with each stage lasting between 5 to 15 minutes. The first sleep cycles each night have relatively short REM sleeps and long periods of deep sleep but later in the night, REM periods lengthen and deep sleep time decreases.
There are five stages of sleep. Stages 1-4 are non-REM sleep, followed by REM sleep.
Stage 1 is light sleep where you drift in and out of sleep and can be awakened easily. In this stage, the eyes move slowly and muscle activity slows. During this stage, many people experience sudden muscle contractions preceded by a sensation of falling.
In stage 2, eye movement stops and brain waves become slower with only an occasional burst of rapid brain waves. The body begins to prepare for deep sleep, as the body temperature begins to drop and the heart rates slows.
When a person enters stage 3, extremely slow brain waves called delta waves are interspersed with smaller, faster waves. This is deep sleep. It is during this stage that a person may experience sleepwalking, night terrors, talking during one’s sleep, and bedwetting. These behaviors are known as parasomnias, and tend to occur during the transitions between non-REM and REM sleep.
In stage 4, deep sleep continues as the brain produces delta waves almost exclusively. People roused from this state feel disoriented for a few minutes.
During REM (rapid eye movement) sleep, brain waves mimic activity during the waking state. The eyes remain closed but move rapidly from side-to-side, perhaps related to the intense dream and brain activity that occurs during this stage.
|Waking||REM Sleep||NREM Sleep|
|Stage 0||Stage R|
|Eyes open, responsive to external stimuli, can hold intelligible conversation||Brain waves similar to waking. Most vivid dreams happen in this stage. Body does not move.|
|16 to 18 hours per day||90 to 120 min/night||4 to 7 hours per night|
A sleep cycle refers to the period of time it takes for an individual to progress through the stages of sleep outlined above. One does not go straight from deep sleep to REM sleep, however. Rather, a sleep cycle progress through the stages of non-REM sleep from light to deep sleep, then reverse back from deep sleep to light sleep, ending with time in REM sleep before starting over in light sleep again. For example, the order looks something like this:
Stage 1 (light sleep) – Stage 2 (light sleep) – Stage 3 (deep sleep) – Stage 2 (light sleep) – Stage 1 (light sleep) – REM Sleep
After REM sleep, the individual returns to stage 1 of light sleep and begins a new cycle. As the night progresses, individuals spend increasingly more time in REM sleep and correspondingly less time in deep sleep.
How long is a sleep cycle? The first sleep cycle takes about 90 minutes. After that, they average between 100 to 120 minutes. Typically, an individual will go through four to five sleep cycles a night.
Stages 3 and 4 are referred to as deep sleep, slow wave sleep, or delta sleep. It is very difficult to wake someone from them. Children are nearly impossible to wake up from this stage, and may be prone to bedwetting, sleepwalking or night terrors. In deep sleep, there is no eye movement or muscle activity.
Deep sleep reduces your sleep drive, and provides the most restorative sleep of all the sleep stages. This is why if you take a short nap during the day, you’re still able to fall asleep at night. But if you take a nap long enough to fall into deep sleep, you have more difficulty falling asleep at night because you reduced your need for sleep.
During deep sleep, human growth hormone is released and restores your body and muscles from the stresses of the day. Your immune system restores itself. Much less is known about deep sleep than REM sleep. It may be during this stage that the brain also refreshes itself for new learning the following day.
In 2008 the sleep profession in the US eliminated the use of stage 4. Stages 3 and 4 are now considered stage 3 or N3.
Slow wave sleep comes mostly in the first half of the night, REM in the second half. REM sleep typically begins about 90 minutes after you first fall asleep, with the first REM cycle lasting about 10 minutes. Each successive REM cycle last longer, with the final REM stage lasting up to 1 hour. Most people experience three to five intervals of REM sleep each night.
Waking may occur after REM. If the waking period is long enough, the person may remember it the next morning. Short awakenings may disappear with amnesia.
In the REM period, breathing becomes more rapid, irregular and shallow, eyes jerk rapidly and limb muscles are temporarily paralyzed. Brain waves during this stage increase to levels experienced when a person is awake. Also, heart rate increases, blood pressure rises, males develop erections and the body loses some of the ability to regulate its temperature.
REM sleep is the time when the most vivid dreams occur, because the brain is so active during this stage. If awoken during REM sleep, a person can remember the dreams.
Muscle paralysis often accompanies REM sleep. Scientists believe this may be to help prevent us from injury while trying to act out our dreams.
A person may dream 4 to 6 times each night. A French study found that all people do in fact dream, whether they remember their dreams or not.
Graphic courtesy of National Institutes of Health
As sleep research is still a relatively young field, scientists did not discover REM sleep until 1953 when new machines were developed to monitor brain activity. Before this discovery it was believed that most brain activity ceased during sleep. Since then, scientists have also disproved the idea that deprivation of REM sleep can lead to insanity and have found that lack of REM sleep can alleviate clinical depression although they do not know why. Recent theories link REM sleep to learning and memory.
|Stage||Frequency (Hz)||Amplitude (micro Volts)||Waveform type|
|3||2-4||100-150||spindle waves and slow waves|
|4||0.5-2||100-200||slow waves and delta waves|
The different cycles of sleep last for different amounts of time during the night. Non-REM sleep dominates the first half of the night, while the amount of time spent in REM stage sleep increases during the second half.
The amount of time you spend in each stage also depends on your age.
Infants spend almost 50% of their time in REM sleep. Adults spend nearly half of sleep time in stage 2, about 20% in REM and the other 30% is divided between the other three stages. Older adults spend progressively less time in REM sleep.
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