Throughout wakefulness and sleep, the electrical activity of the brain changes and creates different types of brain waves. These brain waves are therefore used to characterize the various stages of sleep.
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There are two broad stages of sleep which include rapid eye movement (REM) and non-rapid eye movement (NREM). REM and NREM are further broken up into three stages, each of which is described in more detail below.
Stage 1 NREM sleep
Stage 1 NREM sleep is the initial stage of sleep during which the individual drifts in and out of light sleep and can easily be awakened. The brain activity during this sleep stage is associated with alpha and theta waves, reduced activity of the voluntary muscles, and slow eye movement.
During this stage, it is common to experience strange sensations known as hypnogogic hallucinations, such as a feeling of falling and sudden muscle contractions. This stage typically lasts between 5-10 minutes before the individual progresses to the next stages of sleep.
Stage 2 NREM sleep
The slowing of brain waves with intermittent bursts of rapid brain waves, known as sleep spindles, characterizes stage 2 NREM sleep. Additionally, the eyes stop moving, the body temperature drops, and the heart rate begins to slow down during this stage of sleep. Typically, stage 2 NREM sleep will last for approximately 20 minutes before the individual progresses to the next stage of sleep.
Stage 3 NREM sleep
Stage 3 NREM sleep, which is also known as deep sleep or delta sleep, is marked by very slow delta brainwaves. There is no eye movement or voluntary muscle movement during this stage. Furthermore, it is often difficult to wake an individual who is in stage 3 NREM sleep. This stage usually lasts for approximately 30 minutes before the individual progresses to the next stages of sleep.
Previously, this stage was split into two stages including stage 3 and stage 4. Stage 3 is integrated with more rapid brainwaves, whereas stage 4 has constant slow delta wave brain activity.
Rapid movement of the eyes, temporary paralysis of the voluntary muscles, and fast, irregular breathing characterize REM sleep. During this stage, brain waves become faster and resemble the activity of a person who is awake.
Other changes that occur during REM sleep include an increase in heart rate and blood pressure, as well as a reduced ability to regulate body temperature. Males may also experience an erection during this stage of sleep. Most dreams occur during REM sleep, although the individual will only remember the dreams if they are awoken in this stage.
Sleep (Cycle, EEG Waveforms, Pathology)
Cycling through sleep stages
Throughout the night, sleep begins with stage 1 NREM sleep and progresses through stages 2 and 3 to reach REM sleep approximately 90 minutes after falling asleep. Once the REM sleep stage concludes, brain activity returns back to stage 2 sleep and then stage 3 sleep, before returning to REM sleep. This cycle usually repeats about four to five times each night.
The proportion of different sleep stages depends on the individual. Infants, for example, will spend approximately 50% of the time that they are asleep in REM sleep. Comparatively, adults will spend about half of their sleep time in stage 2, 20% in REM, and 30% in stages 1 or 3. As individuals age, less time is spent in REM sleep and more time is instead spent in the other sleep stages.