How Much Sleep Is Enough?

Animal studies suggest that sleep is as vital as food for survival.

Rats, for example, normally live 2 - 3 years, but they live only 5 weeks if they are deprived of REM sleep and only 2 - 3 weeks if they are deprived of all sleep stages - a timeframe similar to death due to starvation. But how much sleep do humans need? To help answer that question, scientists look at how much people sleep when unrestricted, the average amount of sleep among various age groups, and the amount of sleep that studies reveal is necessary to function at your best.

When healthy adults are given unlimited opportunity to sleep, they sleep on average between 8 and 8.5 hours a night. But sleep needs vary from person to person. Some people appear to need only about 7 hours to avoid problem sleepiness whereas others need 9 or more hours of sleep. Sleep needs also change throughout the lifecycle. Newborns sleep between 16 and 18 hours a day, and children in preschool sleep between 10 and 12 hours a day. Schoolaged children and adolescents need at least 9 hours of sleep a night.

The hormonal influences of puberty tend to shift adolescents' biological clocks. As a result, teenagers are more likely to go to bed later than younger children and adults, and they tend to want to sleep later in the morning. This sleep - wake rhythm is contrary to the early-morning start times of many high schools and helps explain why most teenagers get an average of only 7 - 7.5 hours of sleep a night.

As people get older, the pattern of sleep also changes - especially the amount of time spent in the deep sleep stages. Children spend more time than adults in these sleep stages. This explains why children can sleep through loud noises and why they might not wake up when they are moved from the car to their beds. During adolescence, a big drop occurs in the amount of time spent in deep sleep, which is replaced by lighter, stage 2 sleep. Between young adulthood and midlife, the percentage of deep sleep falls again - from less than 20 percent to less than 5 percent, one study suggests - and is replaced with lighter sleep (stages 1 and 2). From midlife through late life, people's sleep has more interruptions by wakefulness during the night. This disruption causes older persons to lose more and more of stages 1 and 2 non-REM sleep as well as REM sleep.

Many older people complain of difficulty falling asleep, early morning awakenings, frequent and long awakenings during the night, daytime sleepiness, and a lack of refreshing sleep. Many sleep problems, however, are not a natural aspect of sleep in the elderly.

Because older people are more likely to have many illnesses that can disrupt sleep, their sleep complaints often may be due, in part, to illnesses or the medications used to treat them. In fact, one study found that the prevalence of sleep problems is very low in healthy older adults. Other causes of some of older adults' sleep complaints are sleep apnea, restless legs syndrome, and other sleep disorders that become more common with age. Also, older people are more likely to have their sleep disrupted by the need to urinate during the night.