It is often touted that the average person’s sleep needs are 8 hours of sleep each night. In fact, research suggests that 8 hours and 10 minutes of sleep are required, on average, to avoid the effects of sleep deprivation.
When examining a large population, however, considerable variation exists. There are some who only need 6 hours of sleep, and more time spent in bed will result in insomnia. Others, who require nine or even 10 hours of sleep, will be sleep deprived by consistently getting 8 hours per night.
Excerpted from conditions.about.com