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Stages of sleep

What are the stages of sleep?

There are two general states of sleep: 1) rapid eye movement (REM) sleep, and 2) non-rapid eye movement (NREM) sleep. NREM sleep is further subdivided into stages (see below).

1. REM sleep (rapid-eye movement): REM sleep is unlike any of the other stages of sleep. It was first described in 1953 when sleep researchers noticed a unique pattern of brain waves (signals recorded on an electroencephalogram (EEG), a type of test that measures the electrical impulses within the brain). These brain waves had a fast frequency and low voltage, similar to the brain waves seen in the normal awake state. Other characteristics of REM sleep include complete inactivity of the voluntary muscles in the body, with the exception of the muscles that control eye movements. Rapid eye movements are also observed during REM sleep. People who are awakened during REM sleep often report that they were dreaming at the time. About 20% to 25% of sleep time is REM sleep; in infants it can comprise about 40%.

2. NREM (non-rapid eye movement): NREM sleep has traditionally been considered to be divided into 4 stages. However, new guidelines from the American Academy of Sleep Medicine (AASM) published in 2007 have recharacterized NREM sleep as occurring in 3 stages, according to the pattern of brain electrical activity:

Stage N1 sleep, or the transition from wakefulness to deeper sleep. This is the lightest stage of sleep, and people may not always perceive they are asleep when in this stage.

Stage N2 sleep is a true sleep state, and accounts for 40% to 50% of sleep time.

Stage N3 sleep has been called deep sleep, delta sleep, or slow wave sleep. This stage accounts for about 20% of sleep in young adults.

Sleep typically occurs in cycles that range from 90 to 120 minutes in length, with 4-5 cycles occurring during each night’s sleep. In the first half of the night, there is a transition from wakefulness into stage N1 sleep, then to stages N2, and N3. Stages N2 and N3 then reappear, followed by the first instance of REM sleep. Cycles of stage N2 and REM sleep alternate with each other for the second half of the night.

Disruptions in the entire sleep cycle or in the individual phases are believed to account for the various types of sleep disorders.



Source: http://www.medicinenet.com/sleep/article.htm

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