How Much Shut-Eye Is Enough?

Scientists still know very little about what sleep really is, or what it accomplishes on a molecular level. Researchers reported in June that sleep helps rid the brain of a chemical called adenosine that builds up during wakeful hours, but no one knows why adenosine must be eliminated, or what other brain chemicals may contribute to the subjective feelings of sleepiness or mental exhaustion.

How much sleep is enough? The answer varies from person to person.

“Some people can get by with 4 1/2 hours, and others really need nine,” said Ronald Dahl, director of the child and adolescent sleep laboratory at the University of Pittsburgh Medical Center.

In addition, people differ in terms of the time of day in which they function best; some have no problem getting up early in the morning, while others are inexplicably but unquestionably night people and look upon dawn with Dracula-like dread. Sleep researchers refer to them as “larks” and “owls.”

Keeping those variations in mind, experts said, the following rules are generally true:

* Full-term newborns need to sleep about two-thirds of the time, or about 18 hours a day.

* Toddlers and young children need about 12 hours a day, some of which may be taken in naps.

* Youngsters from about 8 up to the late teens need 8 1/4 to 9 1/4 hours of sleep a night, with older teens generally needing to fall asleep a little later and so needing to sleep a little longer in the morning.

* Adults given the opportunity to sleep as much as they want in a laboratory setting that includes 14 hours of darkness each day often take the full 14 hours at first, then scale back, apparently having caught up with overdue sleep. Ultimately, they reach an equilibrium that includes about 8 1/4 hours of sleep each night.

* Older adults typically get less sleep than that, perhaps because of an age-related decline in melatonin, the brain hormone that helps induce sleep. But it’s not known whether they’d feel better if they got more than that.