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- What is sleep?
- What causes the body to sleep?
- What are the stages of sleep?
- How long does it take to get REM sleep?
- Why is REM sleep important?
- What percentage of sleep should be deep sleep?
- How much sleep does a person need?
- Does the amount of sleep we need change as we age?
- What are signs and symptoms of sleep deprivation?
- What are and what causes sleep disorders?
- How are sleep problems diagnosed?
- How are sleep problems treated?
- Sleep hygiene
- Other therapies
- Sleep aids (prescription and OTC)
- Are sleep problems and disease related?
- How can I get a good night's sleep?
Quick GuideSleep Disorders: Insomnia, Sleep Apnea, and More
What are the stages of sleep?
There are two general states of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep is further subdivided (see below).
- 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%.
- NREM (non-rapid eye movement): NREM sleep occurs 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.
Disruptions in the entire sleep cycle or in the individual phases are believed to account for the various types of sleep disorders.
How long does it take to get REM sleep?
Sleep typically occurs in cycles that range from 90 to 120 minutes in length, with 4 to 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. Typically, there is a greater portion of N sleep in the first half of the night and REM sleep in the later portion of the night.
Why is REM sleep important?
REM sleep makes up less than 25% of total sleep time, and the reason for its importance is not fully understood. Some studies have suggested that REM sleep is necessary for the brain to preserve memories and maintain appropriate neurological connections.
What percentage of sleep should be deep sleep?
Deep (N3) sleep, as defined above, only accounts for about 20% of total sleep. The largest amount of deep sleep takes place in the first half of the night.