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Medical Author:

Melissa Conrad Stöppler, MD

Melissa Conrad Stöppler, MD

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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Medical Editor:

Charles Patrick Davis, MD, PhD

Charles Patrick Davis, MD, PhD

Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

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What is sleep?

There are over 20 definitions of "sleep" in several dictionaries. The first, a verb, seems most appropriate:

to take the rest afforded by a suspension of voluntary bodily functions and the natural suspension, complete or partial, of consciousness; cease being awake.

Physiologically, sleep is a complex process of restoration and renewal for the body. Scientists still do not have a definitive explanation for why humans have a need for sleep. We do know that sleep is not a passive process or "switching off" of body functions; sleep is believed to be important in many physiologic processes including the processing of experiences and the consolidation of memories. It is also clear that sleep is essential, not only for humans but for almost all animals.

The importance of sleep is underscored by the symptoms experienced by those suffering from sleep problems. People suffering from sleep disorders do not get adequate or restorative sleep, and sleep deprivation is associated with a number of both physical and emotional disturbances. In addition, sleep is influenced by the circadian rhythms (regular body changes in mental and physical characteristics that occur in the course of about 24 hours). These are controlled by brain neurons that respond to light, temperature and hormones and other signals and comprise the body's biological clock. This clock helps regulate the "normal" awake and sleep cycles. Disruption of these cycles can make people sleepy at times people want to be awake. For example, travelers experience "jet lag" when they cross time zones. When a New Yorker arrives in Paris at midnight Paris time, his or her body continues to operate (their biological clock) on New York time. It may take some time (about 1-3 days) to reset a person's biologic clock, depending on how much it has been altered by the time change.

There is evidence that some aspects of sleep are under genetic influence; a gene termed DEC2 is being investigated as causing people that possess it to require only about 6 hours of sleep. Researchers have only begun to examine the genetics involved in 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.

How much sleep does a person need?

Individuals vary greatly in their need for sleep; there are no established criteria to determine exactly how much sleep a person needs. Eight hours or more may be necessary for some people, while others may consider this to be too much sleep.

The National Institutes of Health (NIH) suggests that most average adults need about 7 to 9 hours of sleep each night. Newborn babies, by contrast, sleep from 16 to 18 hours a day. Preschool-aged children typically sleep between 10 and 12 hours a day. Older, school-aged children and teens need at least 9 hours of sleep a night. Women in the first trimester of pregnancy have been observed to need a few more hours' sleep than is usual for them.

Does the amount of sleep we need change as we age?

Changes in the sleep cycle do occur with aging. Deep or slow wave sleep (Stage N3) sleep declines as we age, while light sleep (Stage N1) increases with age, so that older adults may spend less time in the more restorative stages of sleep and more time in lighter sleep. Older people are also more easily aroused from sleep. While some people believe that older adults need less sleep as they get older, there is no scientific evidence that older people need less sleep than younger adults.