Medical evidence suggests that for optimum health and function, the average adult should get seven to nine hours of sleep daily. But more than 60% of women regularly fall short of that goal. Although each hour of lost slumber goes into the health debit column, we don't get any monthly reminders that we've fallen in arrears. In fact, the greater the sleep debt, the less capable we are of recognizing it: Once sleep deprivation — with its fuzzy-headedness, irritability, and fatigue — has us in its sway, we can hardly recall what it's like to be fully rested. And as the sleep debt mounts, the health consequences increase, putting us at growing risk for weight gain, diabetes, heart disease, stroke, and memory loss.
In some cases, sleep debt results from insomnia or other underlying conditions that may require medical attention. But most sleep debt is due to burning the candle at both ends — consistently failing to get to bed on time and stay there until we've slept enough.
It may take some work, but you can repay even a chronic, longstanding sleep debt.
How we sleep
We need sleep, and, in a sense, we're programmed to be sure that we get it. The body summons sleep in two ways: by boosting circulating levels of the neurotransmitter adenosine and by sending signals from the circadian clock, which controls the body's daily rhythms. Together, these two systems establish an ideal bedtime for each of us.
The circadian clock regulates all body functions — not just the pattern of sleeping and waking during the 24-hour cycle, but also fluctuations in body temperature, blood pressure, and levels of digestive enzymes and various hormones. Most of us experience a major "sleepiness" peak between 12 a.m. and 6 a.m. and a minor one between 2 p.m. and 4 p.m. Of course, individuals vary.
Why we need sleep
Although sleep doesn't trump food and water in the hierarchy of physical needs, we can't live without it. Given the ethical limits on research involving human subjects, scientists have no direct evidence on how extended sleeplessness — that is, beyond a few days — affects human beings. Laboratory rats, however, have been deprived of sleep for long periods, and after a week or two, the results include loss of immune function and death from infections.
In a landmark study of human sleep deprivation, University of Chicago researchers followed a group of student volunteers who slept only four hours nightly 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 is the precursor of type 2 diabetes and metabolic slowdown. All the changes were reversed when the students made up the hours of sleep they had lost. The Chicago research helps to explain why chronic sleep debt raises the risk of obesity, heart disease, stroke, and diabetes.
The architecture of sleep
Scientists have documented the debilitating effects of sleep deprivation very well, but they still know little about how sleep accomplishes its restorative handiwork.
The studies show that there are two forms of sleep, distinguished by specific brain-wave activity and the presence or absence of rapid eye movement (REM). During non-REM sleep, brain waves become slower and more synchronized, and the eyes are still; during REM sleep, brain waves are faster and less organized, and the eyes scan back and forth under the lids.
We fall into non-REM sleep in four stages that represent a continuum of shallow to deep sleep. In Stage 1, characterized by relatively fast waves, we are perched on the brink of sleep and are readily aroused. By Stage 4, or slow-wave sleep, we are dead to the world; breathing has slowed considerably and blood pressure and heart rate have dropped by as much as 30%. The brain becomes less responsive to external stimuli, making it difficult to awaken. Although most of the body's systems are in "sleep mode" at this stage, some are at their most productive. Early in Stage 4 sleep, for example, the pituitary releases a pulse of growth hormone that stimulates tissue growth and repair.
A typical night's sleep consists of four or five REM/non-REM cycles with occasional, brief episodes of wakefulness. Most Stage 4 sleep occurs during the first two to three hours of sleep. As morning approaches, REM sleep occupies an increasing share of slumber.
Countering the effects of sleep loss
Dr. Lawrence J. Epstein, regional medical director of the Harvard-affiliated Sleep Health Centers, has been treating patients with sleep disorders since the early 1990s and has successfully counseled scores of patients out of sleep debt. In The Harvard Medical School Guide to a Good Night's Sleep, Dr. Epstein advises us to avoid regarding sleep as an indulgence or luxury. Rather, we should recognize that adequate sleep is just as important for health as diet and exercise are. To that end, he offers the following advice:
Settle short-term debt. If you missed 10 hours of sleep over the course of a week, add three to four extra sleep hours on the weekend and an extra hour or two per night the following week until you have repaid the debt fully.
Address a long-term debt. Plan a vacation with a light schedule and few obligations. Then, turn off the alarm clock and just sleep every night until you awake naturally.
Avoid backsliding into a new debt cycle. Once you've determined how much sleep you really need, factor it into your daily schedule. Try to go to bed and get up at the same time every day — at the very least, on weekdays.
August 2007 update