We spend between a quarter and a third of our lives asleep, but that doesn't make us experts on how much is too much, how little is too little, or how many hours of rest the kids need to be sharp in school. Let's tackle some myths:
1 You need 8 hours of sleep per night.
The ideal amount of sleep is different for everyone and depends on many factors, including age and genetic makeup. In the past 10 years, my research team has surveyed sleep behavior in more than 150,000 people. About 11 percent slept six hours or less, while only 27 percent clocked eight hours or more. The majority fell in between. Women tended to sleep longer than men, but only by 14 minutes.
Bigger differences are seen when comparing age groups. Ten-year-olds needed about nine hours, while adults older than 30, including senior citizens, averaged about seven. Although it's common to hear warnings about getting too much sleep — and 80 percent of the world uses an alarm clock to wake up on work days — it's not difficult to figure out how much sleep we need. We sometimes overeat, but we generally cannot oversleep. When we wake up unprompted, feeling refreshed, we have slept enough.
In our industrial and urban society, we sleep about two hours less per night than 50 years ago. Like alcohol, this sleep deprivation significantly decreases our work performance and compromises our health and memory.
2 Early to bed and early to rise makes a man healthy, wealthy and wise.
Benjamin Franklin's proverbial praise of early risers made sense in the 18th century, when his peers were exposed to much more daylight and to very dark nights. Their body clocks were tightly synchronized to this cycle. This changed as work gradually moved indoors, performed under the far weaker intensity of artificial light during the day and, if desired, all night long.
The timing of sleep — earlier or later — is controlled by our internal clocks, which determine what researches call our optimal "sleep window." With electric light, our body clocks have shifted later while the workday has essentially remained the same. We fall asleep according to our (late) body clock, and are awakened early for work by the alarm clock. We therefore suffer from chronic sleep deprivation, which we try to compensate for by sleeping in on free days. Many of us sleep more than an hour longer on weekends than on workdays.
My team calls this discrepancy between what our body clocks want and what our social clocks want "social jet lag." This is most obvious in teenagers. Their tendency to sleep longer is biological, not because they're lazy, and it reaches its peak around age 20.
3 Exercise helps you sleep.
Exercising may contribute to falling asleep earlier, and it certainly helps us sleep soundly through the night. But it's light, not physical activity, that proves the German proverb "Fresh air makes you tired." Exercise often means being outside and getting more light — on average, 1,000 times more than indoor levels. Exposure to sunlight synchronizes our body clocks with daylight.
Sleep is not only regulated by the body clock, but also by how long we were awake (also known as the buildup of "sleep pressure"). But not all waking hours are equal. We'll get more tired skiing, for example, than sitting at a desk sending email. This is one reason we sometimes lie awake at the end of a long day at the office despite utter exhaustion.
4 Sleep is just a matter of discipline.
Most parents and teachers think that if teenagers are zombies in the morning, they just lack the discipline to go to bed early. Think of teenagers as early shift-workers who suffer the most social jet lag. They go to school at their biological equivalent of midnight with profound consequences for learning and memory. They suffer from sleep deprivation during the school week and certainly should be allowed to catch up on weekends. However, they should sleep with daylight coming into their bedrooms and should refrain from using light-emitting devices after 10 p.m.
Till Roenneberg, a professor of chronobiology and medical psychology at the Ludwig-Maximilians University in Munich, is the author of "Internal Time: Chronotypes, Social Jet Lag, and Why You're So Tired."
© 2012 Washington Post