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(From the National Institutes of Health):
The amount of sleep each person needs depends on many factors, including age. Infants generally require about 16 hours a day, while teenagers need about 9 hours on average. For most adults, 7 to 8 hours a night appears to be the best amount of sleep, although some people may need as few as 5 hours or as many as 10 hours of sleep each day. Women in the first 3 months of pregnancy often need several more hours of sleep than usual. The amount of sleep a person needs also increases if he or she has been deprived of sleep in previous days. Getting too little sleep creates a "sleep debt," which is much like being overdrawn at a bank. Eventually, your body will demand that the debt be repaid. We don’t seem to adapt to getting less sleep than we need; while we may get used to a sleep-depriving schedule, our judgment, reaction time, and other functions are still impaired.
People tend to sleep more lightly and for shorter time spans as they get older, although they generally need about the same amount of sleep as they needed in early adulthood. About half of all people over 65 have frequent sleeping problems, such as insomnia, and deep sleep stages in many elderly people often become very short or stop completely. This change may be a normal part of aging, or it may result from medical problems that are common in elderly people and from the medications and other treatments for those problems.
Experts say that if you feel drowsy during the day, even during boring
activities, you haven’t had enough sleep. If you routinely fall
asleep within 5 minutes of lying down, you probably have severe sleep
deprivation, possibly even a sleep disorder. Microsleeps, or very brief
episodes of sleep in an otherwise awake person, are another mark of sleep
deprivation. In many cases, people are not aware that they are experiencing
microsleeps. The widespread practice of "burning the candle at both
ends" in western industrialized societies has created so much sleep
deprivation that what is really abnormal sleepiness is now almost the