Let's do some sleep math. You lost two hours of sleep every night last week because of a big project due on Friday. On Saturday and Sunday, you slept in, getting four extra hours. Come Monday morning, you were feeling so bright-eyed, you only had one cup of coffee, instead of your usual two. But don't be duped by your apparent vim and vigor: You're still carrying around a heavy load of sleepiness, or what experts call "sleep debt"—in this case something like six hours, almost a full nights' sleep.



Sleep debt is the difference between the amount of sleep you should be getting and the amount you actually get. It's a deficit that grows every time we skim some extra minutes off our nightly slumber. "People accumulate sleep debt surreptitiously," says psychiatrist William C. Dement, founder of the Stanford University Sleep Clinic. Studies show that such short-term sleep deprivation leads to a foggy brain, worsened vision, impaired driving, and trouble remembering. Long-term effects include obesity, insulin resistance, and heart disease. And most Americans suffer from chronic deprivation.



A 2005 survey by the National Sleep Foundation reports that, on average, Americans sleep 6.9 hours per night—6.8 hours during the week and 7.4 hours on the weekends. Generally, experts recommend eight hours of sleep per night, although some people may require only six hours of sleep while others need ten. That means on average, we’re losing one hour of sleep each night—more than two full weeks of slumber every year.



The good news is that, like all debt, with some work, sleep debt can be repaid—though it won't happen in one extended snooze marathon. Tacking on an extra hour or two of sleep a night is the way to catch up. For the chronically sleep deprived, take it easy for a few months to get back into a natural sleep pattern, says Lawrence J. Epstein, medical director of the Harvard-affiliated Sleep HealthCenters.



Go to bed when you are tired, and allow your body to wake you in the morning (no alarm clock allowed). You may find yourself catatonic in the beginning of the recovery cycle: Expect to bank upward of ten hours shut-eye per night. As the days pass, however, the amount of time sleeping will gradually decrease.



For recovery sleep, both the hours slept and the intensity of the sleep are important. Some of your most refreshing sleep occurs during deep sleep. Although such sleep's true effects are still being studied, it is generally considered a restorative period for the brain. And when you sleep more hours, you allow your brain to spend more time in this rejuvenating period.



As you erase sleep debt, your body will come to rest at a sleep pattern that is specifically right for you. Sleep researchers believe that genes—although the precise ones have yet to be discovered—determine our individual sleeping patterns. That more than likely means you can't train yourself to be a "short sleeper"—and you're fooling yourself if you think you've done it. A 2003 study in the journal Sleep found that the more tired we get, the less tired we feel.



So earn back that lost sleep—and follow the dictates of your innate sleep needs. You’ll feel better. "When you put away sleep debt, you become superhuman," says Stanford's Dement, talking about the improved mental and physical capabilities that come with being well rested. Finally, a scientific reason to sleep in on Saturday.