Definition - What does sleep stages mean?
Sleep stages is the term used to define the types of sleep a person progresses through during a period of slumber. The first four sleep stages in a sleep cycle are non-REM sleep. The last stage in the sleep cycle is Stage R, or REM sleep.
Sleep researchers have identified these stages of sleep by measuring the brain waves of individuals as they cycle through the various stages. Stages 3 and 4, the non-REM deep sleep stages, are characterized by slow or delta waves. Stage 2 sleep is represented by waves called spindles. REM sleep, on the other hand, is characterized by brain waves similar to those presented during wakefulness.
SureHire explains sleep stages
During a single sleep cycle, a normal person will pass through stages 1 and 2 before entering the deep sleep of stages 3 and 4, then cycling back through to stage 2 or 1. After this reversal of the stages, the sleeper transitions to REM sleep. REM, or rapid eye movement, sleep is the stage during which a sleeper is most active, though he/she is still asleep. During REM sleep the person may dream and experience an increased heart rate. However, the individual's muscles will remain relaxed or paralyzed during this sleep stage.
At the end of the full sleep cycle, the individual will leave the REM sleep stage and return to stage 1 or 2, the light sleep stages. Each sleep cycle may last from 90 minutes to 120 minutes. Each of the first four stages may last up to 15 minutes. REM sleep may last just 10 minutes during the first sleep cycle of a sleeping period but becomes progressively longer during each subsequent sleep cycle occurring during that same sleep period.