The idea that it's necessary to get eight hours of sleep each night partly comes from studies that ask people how much time they normally spend sleeping. For example, in a survey conducted by researchers at Queen's University in Kingston, Ont., more than 17,000 students in 24 countries were asked how much sleep they got each night. Sixty-three percent of participants said they slept from seven to eight hours.

"[Eight hours] is the norm for the amount of time that most people sleep," says Helen Driver, a Kingston, Ont.-based sleep researcher and president of the Canadian Sleep Society. "On an individual basis, it's the amount of sleep that allows you to wake up feeling refreshed and able to stay awake during the day."