Many people feel exhausted during the holiday season. We spend between a quarter and a third of our lives asleep, but that doesn't make us experts on it. Let's tackle some popular myths about sleep:

1. You need eight hours of sleep per night.

That's the cliche. Napoleon, for one, didn't believe it. His prescription went: "Six hours for a man, seven for a woman and eight for a fool."

But his formula wasn't right, either. The ideal amount of sleep differs 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 of sleep, while adults older than 30, including senior citizens, averaged about seven hours.

It is 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 society, we sleep about two hours less per night than people did 50 years ago. Sleep deprivation 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.

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 and it reaches its peak around age 20. Teenagers who sleep later and start school later exhibit improved academic performance, higher motivation, decreased absenteeism and better eating habits.

3. Exercise helps you sleep.

Exercising may contribute to falling asleep earlier, and may help us sleep soundly. 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. Exposure to sunlight synchronizes our body clocks with daylight.

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.

Teenagers go to school at their biological equivalent of midnight with profound consequences for learning and memory. They 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 very different sleep habits.

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 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, 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." He wrote this for The Washington Post.