The sleep cycle
Not all sleep is the same. In fact, there are distinct stages and types of sleep, all of which have their own characteristics. The two main types of sleep are non-rapid eye movement (NREM), which is split into three stages, and rapid eye movement (REM).
Sleep usually begins in Stage 1 of NREM, then moves to Stage 2 then 3 then back to 2. We then move into REM sleep and back to Stage 2 again. The first period of REM usually happens about 90 minutes after falling asleep, with REM periods becoming longer as sleep goes on.
Each cycle lasts around 90 minutes for adults (less for infants) and a good night’s sleep requires four to six cycles. All four stages need to be experienced for a person to wake up feeling rested. The amount of sleep needed depends on factors such as age, sex and health.
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