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 popular myths about Mr. Sandman:
1. You need eight hours of sleep per night.
That’s the cliche. Napoleon, for one, didn’t believe it. His prescription went something like this: “Six hours for a man, seven for a woman and eight for a fool.”
But Napoleon’s formula wasn’t right, either. 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.
Bigger differences are seen when comparing various age groups. Ten-year-olds needed about nine hours of sleep, while adults older than 30, including senior citizens, averaged about seven hours.
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 second half of 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 day-night 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.
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. Studies show that teenagers who sleep later and start school later exhibit improved academic performance, higher motivation, decreased absenteeism and better eating habits.
Yet, many cultures reward people who start work early, even if they’re operating on reduced sleep. As a result, many successful people are early-risers.
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. But not all waking hours are equal.
We’ll get more tired skiing, for example, than sitting at a desk sending e-mail. 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.
Although it is true that exposure to computer and television screens late at night makes for late rising, early-to-bed teenagers will still have a hard time getting up at the crack of dawn.
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.
5. Most couples have different sleep habits.
We’ve all heard stories: A woman tries to sleep while her husband is reading. Or one spouse needs to sleep in, but the other wants to start the day.
But again, this is a matter of biology and genetics, not habits and personal preference. Women generally fall asleep earlier than men, who tend toward night owlishness. Women, however, tend to control the sleep times in a partnership. Husbands of women who work late shifts at night, for example, go to bed much earlier when their wives are at home than when their wives are working late, research has found.
One finding that might be surprising, given how much time we spend in our beds: Men and women don’t seem to give any consideration to sleep patterns when choosing a mate.
TILL ROENNEBERG is a professor of chronobiology and medical psychology at the Ludwig-Maximilians University in Munich. He is a special contributor to The Washington Post.