Sleep is essential to life. Laboratory animals deprived of sleep die. And humans don’t seem to be immune to the life-threatening effects of sleeplessness. A survey conducted by the American Cancer Society concluded that people who sleep 6 hours or less per night, or who sleep 9 hours or more, had a death rate 30 percent higher than those who regularly slept 7 to 8 hours. Even those who slept 6 hours or less who otherwise had no health problems had death rates 1.8 times higher than those who slept “normal” hours.
Our biological “clock” largely corresponds to the cycle of the day, and in fact the term “circadian” means “about a day.” The cycle of wakefulness and sleep is tied closely to core body temperature: the higher the temperature, the more alert we are; conversely, when it reaches its low point, sleepiness may be irresistible. The body’s rhythms seem based on two sleep periods each day: a long one through the night and a second short period in the afternoon, when many people nap or at least feel less alert than at other points during the day.
Researchers have found that when people are removed from any outside reminders of time (no clocks, no outside light, etc.) their “clock” seems to be set approximately for a 25-hour day. When one’s personal biological clock gets out of sync with society’s clock, sleep problems can ensue.