Throughout wakefulness and sleep, the electrical activity of the brain changes and creates different types of brain waves, characterizing the various stages of sleep.
There are two broad stages of sleep: rapid eye movement (REM) and non-rapid eye movement (NREM), which is further broken up into three stages. Each of these sleep stages is described in more detail below.
Stage 1 NREM Sleep
Stage 1 NREM sleep is the initial stage when an individual drifts in and out of light sleep and can easily be awakened. The brain activity exhibits alpha and theta waves, the activity of the voluntary muscles reduces and the eyes move slowly.
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. This stage usually lasts for approximately 20 minutes before the individual progresses to the next stage of sleep.
Stage 3 NREM Sleep
Stage 3 NREM sleep, also known as deep sleep or delta sleep, is marked by very slow delta brainwaves. There is no eye movement or voluntary muscle movement and it is difficult to wake an individual in this stage of 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: 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 that is awake.
Other changes include an increase in heart rate and blood pressure, and 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.
Cycling through Sleep Stages
Throughout the night, sleep begins with stage 1 NREM sleep and progresses through stage 2 and stage 3 to reach REM sleep approximately 90 minutes after falling asleep. Once REM sleep stage concludes, the brain activity slips back to stage 2 sleep, 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 spend approximately 50% of the time that they are asleep in REM sleep. Adults spend about half of sleep time in stage 2, 20% in REM and 30% stage 1 or stage 3. As individuals age, less time is spent in REM sleep and more time in other stages of sleep.