As a general rule, the average adult needs around 8 hours of sleep a night, but in practice actually gets little more than 7-7.5 hours. However, it is very difficult to pin down what is optimal for any particular person, as individual sleep needs can vary quite significantly. Some people are just naturally “long sleepers” or “short sleepers”, and this does not constitute a sleep disorder of any sort, merely a genetic predilection, and daytime functioning in such individuals may be normal and healthy.
For example, some 6-10% of the adult population appears to need substantially more sleep than the average (9 or 10 hours or more a night), while about 5% can get by quite well on less than 6 hours a day. In a few extremely rare cases, as little as 3 hours may be enough for certain individuals to function without excessive daytime sleepiness or impaired performance. It should be noted that, although high achievers like Napoleon, Florence Nightingale, Louis XIV, Edison, Churchill and Margaret Thatcher are famous for sleeping short hours, others, like Einstein, for example, slept for 10 or even 12 hours a day (singer Mariah Carey claims to need 15 hours a day).
It is now clear that genetics regulates at least some aspects an individual’s sleep behaviour and requirements. The gene DEC2 is one gene that has been identified as specifically affecting sleep duration, and some people with a mutation of this gene may regularly sleep two hours less than the average. Some people with this mutation may need as little as 3 or 4 hours a night and still wake up feeling refreshed and alert. The gene ABCC9 is another gene that has been identified as affecting the duration of sleep, although more work remains to be done in this area. Because it is genetically programmed, there is therefore little we can do to “train” ourselves to need more or less hours of sleep.
Experts differ on whether, in this caffeine-fuelled world of electric light and 24-hour supermarkets, home entertainment and Internet access, we are actually getting less sleep than we used to. But it does appear that adults, at least in Western societies, are sleeping at least an hour less on average than a hundred years ago, and possibly up to two or three hours less than prior to the availability of electric light. By some estimates, 80% of the world's population now needs an alarm clock to wake up each morning. There is, however, a significant amount of variation between different countries and societies (see the section on Sleep In Different Cultures).
The change in the sleep patterns of children and teens may be even more pronounced. They are certainly getting substantially less sleep than historically, although precise figures are difficult to come by. What is even more clear is that teens in particular are getting substantially less sleep than they actually need, and an estimated three-quarters of teens are sleep deprived.