If you are sleep deprived you may fall asleep briefly several times during the day and be unaware of it. And if you fall asleep--or if your reflexes are slow--when driving, it can kill you. The National Highway Traffic Safety Administration estimates almost 200,000 car accidents each year may be related to fatigue. "Drowsy driving" is potentially as dangerous as drunk driving, but public awareness of the problem is low, according to Dement.



How Sleep Debt Works

Your brain tallies up how much sleep is owed, like a stern taskmaster, and carries the debt over to the next day. "You can't work off a large sleep debt by getting a good night's sleep," says Dement, a professor of psychiatry and behavioral science at Stanford's medical school and co-author of The Promise of Sleep.

In other words, if you miss 2 hours of sleep one night, and then miss 1 hour for three nights in a row, and then miss 3 hours one last night--amassing a total "sleep debt" of 8 hours--you can't fool your brain into thinking sleeping late on the weekend is enough. You need a full night's sleep plus the hours you lost.

It's hard to have a 16-hour Saturday sleep binge, however. Both your internal biological clock--which tends to wake you up at about the same time, even without an alarm clock--and the demands of daily life make it difficult. But this is what your body craves.

Sleep More Stay Younger

Insufficient sleep raises cortisol (a stress hormone) and blood sugar levels but lowers thyroid hormone, contributing to a sluggish metabolism. Too much cortisol can hurt how the brain works. These changes are also found in the aging process.

Eve Van Cauter, a research professor at the University of Chicago Pritzker School of Medicine, notes that even one night of less than complete sleep can result in a higher cortisol level. Cauter published a study of men age 18-27 in the Lancet in October 1999 in which those studied were deprived of sleep for nearly a week. The good news: The young men, who slept only 4 hours a night for six nights returned to normal after sleeping 12 hours a night for seven nights.

Nevertheless, a well-known 6-year study of over 1 million people by the American Cancer Society found that people who slept less than 7 hours per night were much more apt to die--within 6 years--than those who slept 7 hours or more. Research has repeatedly confirmed the findings of this 1965 study, however, an article in the British Medical Journal [December 1998] reported that people who spend a longer-than-average amount of time in bed sleeping each night have an associated increase in mortality.

Long Commute Short Sleep

People who commute long distances to work--a total of 2 1/2 hours or more each day--get almost 3 hours of sleep per week less than short-run commuters and are almost twice as apt to have high blood pressure, according to a September 1999 study in the medical journal Sleep.

Joyce Walsleben, a director of New York University Medical Center's Sleep Disorder Center (located in Bellevue Hospital) who studied 4,715 commuters on New York's Long Island Railroad, cautions that long distance commuting takes a long-term toll on people's health. No matter how lulling the train sounds, short naps between stations can't make up for health damage.