The answer is different for everyone, and depends on factors such as age, how much sleep the person has had recently, overall health and other factors.
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 nine hours on average. For most adults, seven to eight hours a night appears to be the best amount of sleep, although some people may need as few as five hours or as many as 10 hours of sleep each day. Women in the first three 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 five 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 norm.
Many studies make it clear that sleep deprivation is dangerous. Sleep-deprived people who are tested by using a driving simulator or by performing a hand-eye coordination task perform as badly as or worse than those who are intoxicated. Sleep deprivation also magnifies alcohol's effects on the body, so a fatigued person who drinks will become much more impaired than someone who is well-rested. Driver fatigue is responsible for an estimated 10,000 motor vehicle accidents and 750 deaths each year (In Canada). Since drowsiness is the brain's last step before falling asleep, driving while drowsy can - and often does - lead to disaster. Caffeine and other stimulants cannot overcome the effects of severe sleep deprivation.