Sleep normally progresses through several stages, which are repeated in sequence several times during the night. These are categorized as rapid eye movement (REM) sleep and non-REM sleep, which has four stages.
Stage 1 (light sleep) occurs in the transition between wakefulness (hypnotic state)
Stage 2 (so-called true sleep).
Stage 3 and 4 are collectively known as deep sleep and are characterized by slow brain activity.
Research is now suggesting that non-REM sleep may play a role in bolstering the immune system and may also be tied to the rhythms of the digestive tract.
It is during non-REM sleep that growth hormone is released. REM first occurs about 90 minutes after falling asleep and then about three to five times thereafter; during REM sleep muscle tone is greatly reduced, but brain waves show the same active frequency as when a person is awake. This is the dreaming phase of sleep, necessary for long-term emotional well being. Scientists now believe that REM sleep may also help bolster memory. Although the benefits of sleep are not precisely known, interrupted sleep at night can lead to daytime impairments

WASHINGTON, DC, March 27, 2001--A world that "never goes to sleep" offers many diverse activities -- even the possibility of working 24/7, but encourages unhealthy and sometimes antisocial lifestyles for America's adults.
Instead of working to live, they are living to work, a shift that has had a profound impact on their personal lives. Adults report spending less time sleeping, engaged in social and leisure activities, and having sex than they did just five years ago. Most Americans say they suffer from sleep problems and when they go to sleep, many sleep alone -- even if they are married.

These are some of the key findings in the National Sleep Foundation's (NSF) 2001 Sleep in America poll, which looks at the relationship between Americans' lifestyles, sleep habits and sleep problems. The poll was released today as part of NSF's National Sleep Awareness Week (March 26-April 1), as most of the nation prepares to lose an hour of sleep this weekend with the arrival of Daylight Saving Time Sunday morning, (April 1).

Sleep deprivation continues to be widespread in America. According to the NSF poll, a majority of American adults (63%) does not get the recommended eight hours of sleep needed for good health, safety, and optimum performance. In fact, nearly one-third (31%) report sleeping less than seven hours each week night, though many adults say they try to sleep more on weekends.

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