Sleep cycles are the oscillation between the slow-wave and REM phases of sleep. There are four stages of sleep, including one for rapid eye movement (REM) sleep and three that form non-REM (NREM) sleep. The stages are determined based on an analysis of brain activity during sleep, which shows distinct patterns that characterize each stage. The stages are as follows:
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Stage 1 (N1): This is when the person is drowsy or awake to falling asleep. Brain waves and muscle activity start to decrease at this stage.
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Stage 2 (N2): This is when the person experiences a light sleep. Eye movement has stopped by this time. Brain wave frequency and muscle tonus are decreased. The heart rate and body temperature also go down.
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Stage 3 (N3) or even N4: These are the most difficult stages to be awakened. They are also known as slow-wave sleep (SWS), delta sleep, or deep sleep. The brain waves are very slow, and there is no eye movement or muscle activity.
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Rapid Eye Movement (REM) sleep: This is when the person experiences rapid eye movement, and the brain is more active. Dreams typically happen during REM sleep. The heart rate and breathing quicken, and the body becomes more relaxed.
A sleep cycle consists of three NREM stages and one REM stage, and it lasts about 90 to 120 minutes. A person will typically go through four to six sleep cycles per night. The first sleep cycle is often the shortest, ranging from 70 to 100 minutes, while later cycles tend to fall between 90 and 120 minutes. The composition of each cycle, how much time is spent in each sleep stage, changes as the night goes along. Sleep cycles can vary from person to person and from night to night based on a wide range of factors such as age, recent sleep patterns, and alcohol consumption.
