Since the early 20th century, human sleep has been described as a succession of five recurring stages: four non-REM stages and the REM stage. A sixth stage, waking, is often included. Waking, in this context, is actually the phase during which a person falls asleep. Rapid eye movement (REM) sleep is marked by extensive physiological changes, such as accelerated respiration, increased brain activity, eye movement, and muscle relaxation. People dream during REM sleep, perhaps as a result of excited brain activity and the paralysis of major voluntary muscles.
Sleep quality changes with transition from one sleep stage into another. Although the signals for transition between the five (or six) stages of sleep are mysterious, it is important to remember that these stages are, in fact, discretely independent of one another, each marked by subtle changes in bodily function and each part of a predictable cycle whose intervals are observable. Sleep stages are monitored and examined clinically with polysomnography, which provides data regarding electrical and muscular states during sleep.
The waking stage is referred to as relaxed wakefulness, because this is the stage in which the body prepares for sleep. All people fall asleep with tense muscles, their eyes moving erratically. Then, normally, as a person becomes sleepier, the body begins to slow down. Muscles begin to relax, and eye movement slows to a roll.
Stage 1 Sleep
Stage 1 sleep, or drowsiness, is often described as first in the sequence, especially in models where waking is not included. Polysomnography shows a 50% reduction in activity between wakefulness and stage 1 sleep. The eyes are closed during Stage 1 sleep, but if aroused from it, a person may feel as if he or she has not slept. Stage 1 may last for five to 10 minutes.
Stage 2 Sleep
Stage 2 is a period of light sleep during which polysomnographic readings show intermittent peaks and valleys, or positive and negative waves. These waves indicate spontaneous periods of muscle tone mixed with periods of muscle relaxation. Muscle tone of this kind can be seen in other stages of sleep as a reaction to auditory stimuli. The heart rate slows, and body temperature decreases. At this point, the body prepares to enter deep sleep.
Stage 3 and Stage 4 Sleep
These are deep sleep stages, with Stage 4 being more intense than Stage 3. These stages are known as slow-wave, or delta, sleep. During slow-wave sleep, especially during Stage 4, the electromyogram records slow waves of high amplitude, indicating a pattern of deep sleep and rhythmic continuity.
The period of non-REM sleep (NREM) is comprised of Stages 1–4 and lasts from 90 to 120 minutes, each stage lasting anywhere from 5 to 15 minutes. Surprisingly, however, Stages 2 and 3 repeat backwards before REM sleep is attained. So, a normal sleep cycle has this pattern: waking, stage 1, 2, 3, 4, 3, 2, REM. Usually, REM sleep occurs 90 minutes after sleep onset.
REM sleep is distinguishable from NREM sleep by changes in physiological states, including its characteristic rapid eye movements. However, polysomnograms show wave patterns in REM to be similar to Stage 1 sleep. In normal sleep (in people without disorders of sleep-wake patterns or REM behavior disorder), heart rate and respiration speed up and become erratic, while the face, fingers, and legs may twitch. Intense dreaming occurs during REM sleep as a result of heightened cerebral activity, but paralysis occurs simultaneously in the major voluntary muscle groups, including the submental muscles (muscles of the chin and neck).
Because REM is a mixture of encephalic (brain) states of excitement and muscular immobility, it is sometimes called paradoxical sleep. It is generally thought that REM-associated muscle paralysis is meant to keep the body from acting out the dreams that occur during this intensely cerebral stage. The first period of REM typically lasts 10 minutes, with each recurring REM stage lengthening, and the final one lasting an hour.
The five stages of sleep, including their repetition, occur cyclically. The first cycle, which ends after the completion of the first REM stage, usually lasts for 100 minutes. Each subsequent cycle lasts longer, as its respective REM stage extends. So a person may complete five cycles in a typical night's sleep.
The sleep cycle is variable, influenced by several agents. Sleep cycles subsequent to the first one in a night's sleep typically feature less slow-wave sleep, as Stages 3 and 4 shorten. Slow-wave, deep sleep is longest early in a night's sleep. Generally, sleep disorders affect the quality, duration, and onset of sleep.
Sleep deprivation, frequently changing sleep schedule, stress, and environment all affect the progression of the sleep cycle. Rapid eye movement latency (the time it takes a person to achieve REM sleep) may be affected by a sleep disorder like narcolepsy.
Psychological conditions like depression shorten the duration of rapid eye movement. Also, treatment for psychiatric conditions often affects sleep, typically inducing a change in sleep habits. For example, antidepressants like Prozac® may cause trouble sleeping and insomnia and can inhibit REM sleep stages.
The percentage of REM sleep is highest during infancy and early childhood, drops off during adolescence and young adulthood, and decreases further in older age. Of course, infants require the greatest amount of sleep. As parents know, total sleep time typically becomes shorter during childhood and may become longer again in adolescence.
The stage-respective dimensions of sleep change relative to age. Stages 3 and 4 in the first sleep cycle shorten even more dramatically in older people than they do during a typical night for everyone else, so older people get less total deep sleep than younger people do. Also with age comes the lengthening of the first REM stage. Older people commonly enter REM sleep quicker and stay there longer.