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. Women tended to sleep longer than men, but only by 14 minutes.
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. We recently identified the first gene associated with sleep duration — if you have one variant of this gene, you need more sleep than if you have another.
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 researchers call our optimal “sleep window.” With the widespread use of 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, and then we try to compensate by sleeping in on free days. Many of us sleep more than an hour longer on weekends than we do 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. Studies show that teenagers who sleep later and start school later exhibit improved academic performance, higher motivation, decreased absenteeism and better eating habits.