Note: This is a “classic” Interesting Thing of the Day article from over 10 years ago. It has not been edited recently, so it may contain broken links, outdated information, or other infelicities. We plan to eventually update or retire most classic articles, as time permits.

My life is full of contradictions, as is true for many of us. For example, if you asked me what my top five favorite things in life are, sleep would certainly be high on that list. I love to sleep—it’s not merely a necessity, it’s a joy. Circumstances permitting, I’d sleep 12 hours a day if I were physically able to. On the other hand, my actions don’t bear out this enthusiasm for sleep. I drink outrageous amounts of caffeinated beverages. I’m usually still awake and working at 2 or 3 a.m. And frankly, I prefer a lifestyle that’s at least partly nocturnal—stay up late, wake up late. This in itself doesn’t result in a contradiction; if I went to bed every morning at 3 and woke up at noon, I could enjoy a nice long stretch of sleep and still maintain my desired schedule. But it generally doesn’t work that way. There are too many things to do—deadlines to meet, appointments to keep—and the rest of the world doesn’t conform to my schedule. So I end up getting out of bed after only six or seven hours of sleep (which is far too little for me) and feeling tired most of the day.

I recognize that this is a problem. When I’m sleepy most of the time, I can’t think clearly, and I am much less effective at my work. I don’t like this situation, and I sense that it may be taking some toll on my physical and mental health. So my New Year’s resolution this year was to get plenty of sleep. I think I kept it for about a week, but hope springs eternal: maybe I’ll sleep next month or, if not, the one after that. Sooner or later, though, something’s got to give, because the effects of too little sleep are cumulative—what sleep researchers refer to as sleep debt.

Racking Up Debt

Sleep debt is defined as the difference between the amount of sleep you need and the amount you actually get. So if you need 8 hours of sleep per night and get 7, you accumulate 1 hour of sleep debt for that night. And if you get 7 hours of sleep every night for a week, you’ve accumulated 7 hours of sleep debt. Of course, the amount of sleep required per night varies from person to person—and for a particular person, it changes with age. Some people, like me, operate at peak efficiency with 9 or more hours of sleep; others function perfectly well with 5 or 6. But whatever that amount is, sleep debt accrues when you get too little.

Casually speaking, say experts, if you feel drowsy during the day, you probably have some sleep debt. This is more than a mere annoyance; besides making you grumpy, drowsiness can negatively affect your productivity, reduce reaction time, increase the risk of traffic accidents, and even contribute to weight gain. Various studies have suggested that anywhere from 50% to 90% of Americans experience, and suffer the consequences of, sleep debt. One major cause, according to some people at least, is the alarm clock—or, rather, rigidly defined schedules that demand its use. When you awaken every day before your body says it’s ready, you add to your sleep debt.

At Least There’s No Interest

Until recently, some sleep researchers claimed that sleep debt could be accumulated indefinitely—that, like financial debt, it simply never goes away until it is repaid. If this were the case, I would probably be in the red for about a year’s worth of sleep. Current research suggests that this notion is a mistake, and that the body’s maximum sleep debt is under 20 hours, no matter how many consecutive nights you’ve had too little sleep.

Be that as it may, the only way to “repay” sleep debt, so the experts say, is to sleep more—and there are limits, both practical and physiological, to how much one can sleep. If I had, say, a month off with absolutely no obligations or distractions whatsoever, a perfectly dark, quiet bedroom, a great deal of motivation to repay my debt, and some really boring reading materials—even then, with the optimal conditions, my body simply wouldn’t stay asleep 24 hours a day, or even 12. On those lazy weekends when I’ve had every hope of repaying a week’s worth of sleep debt, I still couldn’t stay asleep more than 10 or 11 hours at a time. So to the extent that sleep debt really is like a financial debt, my body appears to disallow anything other than a minimum payment of an hour or two at a time.

Of course, unlike financial debt, sleep debt isn’t rigorously tracked in a database somewhere. Sleep specialists can perform tests to determine one’s propensity to fall asleep, which gives a rough indication of sleep debt level. But there is no test that will tell you exactly how many hours or days of unpaid sleep debt you have, so the claim that it’s strictly cumulative is ultimately just an educated guess. Still, the claim that it exists is generally accepted, and certainly borne out by my own experience. In fact…(yawn)…I think I need to go make a deposit. —Joe Kissell

More Information

Other articles discussing sleep debt include: