Repaying your sleep debt
By the faculty of Harvard Medical School for The Harvard Medical School Adviser

Q: Lately, I've been so busy with work and caring for my family that I've been sleeping only about five or six hours a night. Other than making me a little tired during the day, are there downsides to getting less than eight hours of sleep?

A: For good health, most adults should get seven to nine hours of sleep daily. But more than 60 percent of us regularly fall short of that goal. The trouble is that the more sleepdeprived you become, the harder it is to recognize it. This "sleep debt" can cause fuzzy-headedness, irritability and fatigue. But after a while, that feels normal, and you can hardly recall what it's like to be fully rested. And as your sleep debt mounts, the health consequences increase. Inadequate sleep puts some people at risk for weight gain, diabetes, heart disease, stroke and memory loss. What's more, driving while you're sleep-deprived can have deadly consequences. A 2006 review by the Institutes of Medicine of the National Academy of Sciences found that almost 20 percent of all serious car accidents and 57 percent of fatal accidents are associated with driver sleepiness.

Sleep debt sometimes results from insomnia, the inability to fall and stay asleep. But most often, it's simply caused by burning the candle at both ends -- going to bed too late and/or getting up too early.

In a landmark study of human sleep deprivation, University of Chicago researchers followed a group of student volunteers who slept only four hours for six consecutive days. The volunteers developed higher blood pressure and higher levels of the stress hormone cortisol. And they produced only half the usual number of antibodies to a flu vaccine. The sleep-deprived students also showed signs of insulin resistance -- a condition that often leads to Type 2 diabetes. All the changes were reversed when the students made up the hours of sleep they had lost. The research helps to explain why chronic sleep debt may raise the risk of disease.

Sleep loss exacts a toll on the mind as well as the body. Another study involved volunteers who were randomly assigned to three groups that slept eight, six or four hours nightly. A fourth group agreed to go without sleep for three days. Over the course of two weeks, the people who slept eight hours maintained their scores on tests of reaction time, memory and cognitive (thinking) ability. But scores for the four-hour and six-hour sleepers fell close to those of the fourth group, whose scores plummeted during their three days without sleep.

We don't fully understand how sleep keeps us healthy and alert, but brain-wave studies provide some clues. A technique called encephalography (EEG) can trace people's brain waves as they slumber. These readings have revealed several distinct phases of sleep (see illustration). There are two forms of sleep, distinguished by specific brainwave activity and the presence or absence of rapid eye movement (REM). Most Stage 3 sleep occurs during the first two to three hours of sleep. Early in Stage 3 sleep, the brain releases a hormone that stimulates tissue growth and repair. As morning approaches, REM sleep occupies an increasing share of slumber. Studies have shown that people learning a new physical task will improve their performance overnight, but only if they get enough REM sleep.

So how can you counter the effects of chronic sleep debt? Don't think of sleep as an indulgence or a luxury but rather something that's just as important to your health as diet and exercise. Try to go to bed and get up at the same time every day -- at the very least, on weekdays. If need be, use weekends to make up for lost sleep. If you can't manage to go to bed earlier or sleep a little later, consider napping. If possible, nap shortly after lunch: People who snooze later in the afternoon fall into a deeper sleep, which can keep you up at night. An ideal nap lasts no longer than an hour, and even a 15- to 20- minute nap can help you feel more alert.

For most people, paying down the mortgage is good for the budget. And for most, paying down the sleep debt is good for health.

Copyright 2007 the President and Fellows of Harvard College. Developed by Harvard Health Publications (www.health.harvard.edu). Distributed by UFS. Submit questions to harvard_adviser@hms.harv ard.edu.