Archived Columns
The Function of Sleep: Regulation or Just Plain Survival?
October 16, 2000
Today's issue of U.S. News & World Report arrived on the shelves a few days ago. It contains a story advertised in dramatically huge letters on the cover, Sleepless in America: New research links lack of sleep to health problems like obesity, diabetes, and the common cold. I would encourage everyone to read this article.
For many years, sleep researchers have known as absolute fact that sleep loss is associated with progressive impairment of alertness. On the other hand, we have had to say that we do not know the vital biological function or functions of sleep. Moreover, we are not at all certain that human beings would eventually die if they were completely prevented from sleeping.
In the past few years, however, this uncertainty has begun to diminish. As the subtitle of Sleepless in America implies, investigators are finding that sleep loss may have deleterious consequences for our immune and endocrine systems and possibly other metabolic functions. I firmly believe that the application of currently available powerful scientific technologies to the problem of sleep will provide decisive answers in the not too distant future.
In order to help our readers understand and assimilate the current and future findings of sleep research, I have dedicated this commentary to a further discussion of the most fundamental aspect of interpreting the effects of sleep deprivation. I began this discussion in my September 25, 2000 commentary, Sleep Debt I. The following excerpts are from this commentary:
Sleep HomeostasisAny consideration of the need for sleep must clearly distinguish the need to satisfy the homeostatic requirement for sleep from the still unclear vital need or needs for sleep. It is not difficult to understand that people may be somewhat confused by this. When we have lost a great deal of sleep and are feeling very sleepy and extremely miserable, it may seem that we would die if the sleep deprivation continued. In addition, the homeostatic regulation of a process ensures that a vital function is strongly protected, for example, fluid intake and food intake. Part of the regulation is to modulate a motivational response. We become hungry if we do not eat and very hungry if the eating hiatus is prolonged. Hunger is the motivation to obtain and eat food without which we would eventually die. Intense hunger in response to not eating occurs within a day, yet it would take quite some time to starve to death. Not drinking leads fairly quickly to thirst and not drinking for a day or two is associated with an intense and irreststable drive to slake one's thirst. Yet it might be several days before dehydration would lead to death. We know that food and water are vitally necessary. As indicated above, we are not absolutely certain that sleep is vitally necessary but the intense desire to sleep after only a night or two without sleep points in that direction.
When a process works continually to maintain a constant level or flow of something in our bodies, it is called "homeostatic." An example of such a process is the regulation of body temperature to maintain it close to 98.6 degrees. One powerful component of sleep regulation is sleep homeostasis. When our prior nocturnal sleep time has been reduced, our tendency to fall asleep the next day increases, and we tend to sleep more deeply the next night. When we obtain substantial "extra" sleep, we are less likely to fall asleep or become drowsy on the next day, and we may not sleep as deeply the next night. The obvious purpose of this homeostatic process is to ensure that each of us will obtain a certain amount of sleep as a daily average.
Sleep need
Each of us needs a certain amount of sleep which, if obtained on a daily basis, will maintain our homeostatic equilibrium. In other words, if we are fulfilling our daily sleep requirement, the consequences described in the preceding paragraph would not occur. Our tendency to fall asleep in the daytime would neither increase nor decrease. Although there are individual differences, most people's daily sleep requirement is around eight hours.
Long term sleep deprivation studies have been carried out mainly in human beings and rodents, primarily laboratory rats. In the case of rats, the studies of Professor Allan Rechtschaffen at the University of Chicago have established that total sleep deprivation is fatal in 16 to 20 days without exception. After demonstrating 100 percent mortality from prolonged sleep loss, the Rechtschaffen group has spent over a decade attempting to work backward from the terminal event to understand what causes the animals to die and by implication the vital necessity for sleep. At this point, the direct cause of death in the rodents appears to be huge numbers of live bacteria in the blood stream and body tissues. To see bacterial invasion in moribund animals that are very debilitated at the end of the sleep deprivation is not very compelling. However, Dr. Carol Everson has reported finding live bacteria in abdominal lymph nodes as early as only five days of sleep loss at which time the animals are quite sleepy but are not at all debilitated. Her preliminary hypothesis is that daily sleep of some amount is necessary to maintain an intact immune system which will prevent bacterial invasion.
The question of whether sleep is vitally necessary for humans would be relatively easy to answer if there were an easy, nonstressful method to maintain very prolonged wakefulness. However, the increasing strength of the sleep drive and the increasing difficulty of preventing sleep makes prolonged sleep deprivation studies in humans very difficult. After a few days without sleep, methods that are considered dangerous and unethical would be required to continue.
So, what does sleep do for us? It might not be vitally necessary but sleep is certainly helpful to human existence. Human beings evolved as a day active species and are highly dependent upon vision to interact with their environment and are pitifully helpless in the dark. Accordingly, one huge advantage of the homeostatic drive to sleep is to force the human species to seek a safe place in which to sleep during the nocturnal hours. Not only are human beings drawn towards day activity by their highly developed visual senses, but also the timing mechanisms of the circadian system have evolved such that the biological clock promotes wakefulness and activity during the daylight hours. Another huge adaptive advantage of sleeping every night is the substantial reduction in energy expenditure. If human beings were fully active 24 hours a day, they would need many more calories and their food requirement would be perhaps 50% higher, creating a vulnerability to food shortages.
There have been several reports of studies of individuals who appeared to need very small amounts of sleep, as little as one hour in some cases. It is not clear, however, if these individuals were fully alert in the daytime. Presumably they were normally alert, but tests to demonstrate this were not utilized. Many years ago, I studied one man for seven or eight consecutive nights; a Stanford professor whose nightly sleep was only 4 hours, and he claimed he never napped during the daytime and was fully alert. Approximately two years ago I received a letter from his daughter saying she was a short sleeper and that her daughter was also a short sleeper.
In conclusion, it seems possible that the vital functions of sleep, the ones that keep us alive, might be carried out in a relatively short period of time. In contrast, the protective homeostatic mechanisms underlying the drive to sleep evolved to require much longer sleep periods to achieve the additional adaptive advantages outlined above.
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