Sleep Duration, Health, and Performance

by Dr. Gregg D. Jacobs

One of the biggest current debates amongst sleep specialists involves two related issues: How much sleep do we need to stay healthy?; and, what are the effects of sleep deprivation on our daytime functioning? This focus article examines each of these questions in an attempt to develop a more accurate, complete understanding of how sleep can affect health, well-being, and performance.

Sleep Duration and Health

One widely disseminated recommendation made by some sleep specialists is that we need at least eight hours of sleep per night to stay healthy. This recommendation is based in large part on a study conducted by Dr. Thomas Wehr at the National Institutes of Health. Wehr allowed young, healthy individuals to spend up to twelve hours per day in bed for almost a month. He found that, after a few days, his subjects' sleep duration stabilized at about eight and one quarter hours per night.

Some sleep researchers have interpreted these findings to mean that we are biologically programmed to sleep over eight hours per night. Other support for this notion comes from examinations of biographical data of people's sleep in the past century, which indicated that they typically slept about eight and one-half hours per night compared to today's average of seven hours.

However, not all sleep researchers agree with the conclusion that we biologically designed to sleep over eight hours per night.

First, the Wehr study involved young, healthy people who may need more sleep than middle aged or older adults. Sleep need appears to decline with age, which means that Wehr's findings may not apply to everyone.

Second, Wehr's subjects spent twelve hours a day in bed. Some sleep researchers argue that, under these conditions, we sleep more than usual due to the extreme boredom of the experimental environment.

Third, Wehr's subjects began to have problems falling and staying asleep when they slept over eight hours per night. Specifically, they tended to sleep in two four-hour blocks separated by two hours of wakefulness in the middle of the night. This means that sleeping over eight hours per night can actually cause sleep problems by reducing the drive for sleep and making sleep less efficient.

Fourth, Dr. Jim Horne, a European sleep researcher, points out that Europeans hundreds of years ago only slept six hours per night. This contradicts the notion that we are programmed by nature to sleep more than eight hours per night. Finally, many people know all too well that they could not sleep eight hours per night even if they tried.

Some scientist have suggested that sleeping less than eight hours per night is a significant risk factor for increased sickness and death- as important a risk as lack of physical activity! The primary reason for this claim comes from research by Dr. Eve Van Cauter at the University of Chicago. She compared physiological functioning in healthy young male sleepers who were allowed to sleep just under four hours a night, just over seven hours, and just over nine hours over several weeks.

She found that, compared to nine hours of sleep, four hours of sleep was associated with an impairment in the body's ability to process sugar (meaning that people could become more susceptible to diabetes or obesity). Other studies have found the same changes when eight hour sleepers were compared to six and one-half hour sleepers. Van Cauter interprets these findings to indicate that sleep loss may be as dangerous to health as lack of exercise!

However, these generalizations appear premature. First of all, Van Cauter only found changes in physiological activity when four hours of sleep was compared to nine hours of sleep. This may be an unrealistic comparison since very few people can, or do, sleep nine hours per night. Furthermore, there were no changes in physiological activity when four hours of sleep was compared to a more realistic seven hours of sleep.

In all of these studies, sample sizes were small, young healthy sleepers were used (so that the results may only apply to young people), and the same physiological changes have been observed in response to daily life stress. Also, the physiological effects from sleep loss may not be clinically meaningful, and insomniacs, who sleep less than six hours per night on average, do not have a greater incidence of diabetes or obesity. Finally, other well-done studies have shown that sleep deprivation did not result in significant changes in metabolism or physiological functioning.

Other sleep deprivation studies have shown that, when eight hour sleepers are limited to four hours of sleep, their immune systems are compromised.

However, these studies involve a severe reduction of sleep (50% loss of normal sleep) in longer sleepers (eight hour sleepers). What about smaller reductions that we typically experience in daily life and what about sleep loss in short sleepers? Also, these findings on immune suppression may not be clinically meaningful since no one has shown that they actually cause disease; if they did, we would expect to see a greater incidence of these kinds of health problems in insomniacs but we don't. Finally, stress has been shown to have the same effects on immune functioning. In fact, it is very likely that the stress of the laboratory situation may be responsible for these effects instead of sleep loss itself.

Several recent studies have cast significant doubt on the belief that we need to sleep eight hours per night to stay healthy. For example, in a study of sleep and health-related quality of well-being, researchers at the University of California at San Diego measured the duration of sleep of almost 400 people in their home environment, and then assessed their health-related quality of life. There was no relationship between longer sleep and better health-related quality of life. In fact, people who slept six hours per night were just as healthy as those who slept eight.

In another large-scale study, Dr. Daniel Kripke and his colleagues at the University of California at San Diego studied over a million adults, ages 30-100, to determine the relationship between sleep duration and mortality. This study involved the largest group of people ever employed in a sleep study and the widest age range (most studies involve only young, healthy college students). Kripke found that people who slept seven hours per night had the lowest death rates over a six year period, while people who slept eight or more hours had a greater risk of dying over the same period.

In fact, the greater the sleep duration beyond eight hours per night, the greater the death rate; and, people who only slept five hours per night lived longer than those who slept eight or more. These findings suggest that seven hours of sleep may be the ideal sleep length in terms of longevity. The study also found a wide variation in usual sleep duration in the population: 20 % of the sample reported sleeping six or fewer hours per night. The Kripke study suggests that sleep may be like food: too little or too much is unhealthy, but the optimal amount may help us to live longer.

The Kripke group's findings were recently replicated and extended by a group of researchers at Harvard Medical School led by Dr. S.R. Patel. This study, which as reported recently at the 2003 APSS national sleep meetings in Chicago, used the Nurses' Health Study cohort to assess whether mortality risk attributed to sleep time could be explained by confounding variables such as depression, alcohol use, sleep disordered breathing , or shift work. Patel's group studied a sample of 82, 975 female nurses (one of the largest samples ever employed in a prospective study on sleep) who answered questions about how much they slept and then were followed for 12 years for causes of death.

Nurses who slept 7 hours per day had the fewest deaths over the twelve year follow-up period, which replicated the findings of the Kripke group. For nurses who slept 5 hours per day (short sleepers), there was no increased death risk compared to 7 hour sleepers when controlling for age, smoking, weight, health problems such as diabetes and hypertension, alcohol use, depression, shift work, and snoring (sleep disordered breathing).

People who slept 8, 9, or 10 or more hours per day had a significantly increased risk of death compared to 7 hour sleepers even when controlling for age, smoking, health problems, alcohol use, etc. For example, 8 hour sleepers had a 13% increases risk of death, 9 hour sleepers had a 40% increased risk of death, and 10 or more hour sleepers had a 70% increased risk of death compared to 7 hour sleepers.

The Patel group's findings confirm and extend the Kripke group's findings by again demonstrating that people who sleep 7 hours per night have the lowest death rates and those who sleep 8 or more hours per night have progressively increasing rates of death.

Finally, there is little evidence in the animal world that sleep duration is linked to longevity. In fact, there is a negative correlation between these two variables. Specifically, animals, (including mammals) who exhibit longer sleep durations have shorter life spans, while animals who exhibit shorter daily sleep durations have longer life spans.

Sleep Loss and Daytime Performance

A related theme that has been disseminated in the media is that sleep deprivation can have serious effects on our daytime functioning. Exactly what are the effects of sleep deprivation and how serious are they?

First of all, sleep deprivation can have significant effects on performance if sleep loss is significant and occurs for several days. For example, many studies show that if eight hour sleepers are restricted to four hours of sleep for as little as a few days, their performance deteriorates on tasks like problem solving, reaction time, and memory.

However, the effects on memory are much less consistent and speed on the performance of various tasks is more affected than accuracy. The main effects of sleep loss are primarily mood impairment and, secondarily, sleepiness, both of which impair performance particularly on boring. monotonous, or sedentary tasks.

The effects of sleep loss on performance depend upon the person, how much sleep is lost, motivation, and the circumstances under which sleep loss take place. Some individuals show a remarkable tolerance for sleep loss, particularly if the person is motivated to cope with sleep loss (examples would be dealing with a crisis, caring for a newborn, etc.) or if the sleep loss occurs under positive circumstances (excitement, a vacation, social event, etc.). Sleep deprivation researchers have learned that there are substantial individual differences in sensitivity to the effects of sleep deprivation on performance- some people are severely affected while others show minimal effects.

Also, the effects of sleep loss are more pronounced when people lose half of their normal sleep but less pronounced when they only lose one or two hours. Therefore, an eight hour sleeper will show little or only moderate impairment in daytime functioning if restricted to six hours of sleep but will show more significant impairment when only allowed to sleep only four hours. Furthermore, for every study that reveals impaired performance after sleep loss, other studies find minimal or no effects. These findings suggest that the effects of sleep loss are not consistent or robust.

Researchers are beginning to realizes that the effects of sleep deprivation studies are often confounded by the effects of stress, since it may the stress of the experiment that produces impairment after sleep loss. (Many sleep deprivation studies are very challenging and involve highly stressful conditions that are far from routine). Similarly, real-life sleep loss typically occurs as a result of stress so we don't know if the performance decrements after sleep loss are due to sleep loss itself or the stress that causes the sleep loss.

And then there are the positive effects of sleep deprivation that are not reported by the media or acknowledged by the sleep specialists who warn us of the dangers of sleep deprivation. For example, sleep deprivation is therapeutic for depression. In fact, sleep deprivation produces a faster improvement in depression than antidepressant medication and is considered by some researchers to be the most potent treatment for depression! Similarly, a well-controlled study demonstrated that deprivation of dream sleep can make us more alert during the day.

There is also research to suggest that individuals who need to maintain performance under challenging circumstances such as rescue workers, armed services personnel, physicians, and solo yacht racers can actually maintain or improve their performance if they sleep in multiple short bouts throughout the day that total less than six hours. For example, solo yacht racers who adopt this schedule and reduce sleep needs to about 5-6 hours per day actually perform the best and win races.

Another study showed that the less students slept before an exam, the higher were their grades. Finally, many studies have shown that maintaining a small sleep debt actually helps us to sleep efficiently each night for, when individuals are allowed to "sleep out", they eliminate all of their sleep debt and have a harder time falling and staying asleep. Indeed, this is what happened in Dr. Wehr's study: when subjects were allowed to sleep as long as possible for several weeks (just over eight hours per night), they eventually had a harder time sleeping and spent more time lying awake during the night! By depriving ourselves of a little bit of sleep each night, we sleep better due to a stronger sleep drive.

A final issue concerning the effects of sleep deprivation is whether we build up a long-term sleep debt when we are chronically sleep deprived. Presently, we the only thing we know for certain is that sleep debt accumulates for up to two weeks. However, no has shown that sleep debt builds up over months or years.

If it did, then virtually all parents would experience the effects of long-term sleep debt when their sleep is disrupted for months after a baby is born. We also know that, if sleep debt accumulates, it does not need to be paid back hour for hour. Why? Because the brain compensates for sleep loss by producing recovery sleep, which contains increased percentages of the most crucial stages of sleep (deep sleep and dream sleep).

So if you are an eight hour sleeper and you go sleepless one night, you do not have to sleep your usual eight plus another eight hours, or sixteen hours, the following night. This makes sense since most people could not possibly sleep sixteen hours at one time.