Attributed to: Sri Ramana Maharishi
We spend about one third of our lives sleeping. And when we are not sleeping, many of us want to be! We organize our time, our work, our relationships to meet the demands of sleep. Research and personal experience strongly suggest that sleep, or the lack thereof, has an significant impact on general health. (1) According to surveys conducted by the National Sleep Foundation, more than 40 percent of adults experience daytime sleepiness severe enough to interfere with their daily activities at least a few days each month, while 20 percent report problem sleepiness a few days a week or more. (2) Most of us are familiar with the powerful need for sleep. But does the body need sleep, in the way that it needs food and water? If so, why? In this paper, I ask what is the physiological function of sleep, examine some theories, and draw some conclusions.
To answer the question of why we sleep, it is first necessary to ask, "what is sleep?" This is a bit like saying we need to answer the question "are we alone in the universe?" before we ask "is there life on other planets?" That is, if we knew the answer to one, we would probably know the answer to the other. Major questions about the nature of sleep, including what happens during sleep and what are the mechanisms of sleep, remain largely unanswered. But some progress has been made.
Until the 1950s, sleep was considered a state of inactivity. The brain was thought to be "turned off", and the body rested. We now know that the sleeping brain is equally and sometimes more active than the waking brain. Sleep, as well as wakefulness, is generated by the discharge of specific neurons in certain parts of the brain. Neurotransmitters produced in the brainstem, such as serotonin and norepinephrine, appear to play a role in keeping parts of the brain active while we are awake. As we fall asleep, neurons at the base of the brain begin firing and appear to "switch off" the signals that keep us awake. Research also suggests that adenosine builds up in our blood while we are awake and causes drowsiness, then gradually breaks down while we sleep. (3)
Furthermore, at least two distinct phases of sleep have been identified. During rapid eye movement (REM) sleep, synchronous rapid eye movements are seen, muscle atonia occurs, and the activity of the autonomic nervous system is irregular and accelerated. Non-REM (NREM) sleep displays lower, but not nonexistent, central nervous system activity, as seen in a slower and more regular EEG pattern. Clearly, sleep is an active and dynamic state.
Animal studies show that sleep is necessary for survival. For example, while rats normally live for two to three years, those deprived of REM sleep survive only about five weeks on average. Rats deprived of all sleep stages live only about three weeks. Sleep-deprived rats also develop low body temperatures and sores on their tails and paws. The sores may develop because the rats' immune systems become impaired. (4) So the evidence suggests that there is a function of sleep other than to prevent sleepiness, just as there is a function of eating other than to prevent hunger. But what is this function? Various hypotheses that relate sleep to metabolic, immune, endocrine and brain functions have been proposed to answer this question. I will limit my discussion to the function of sleep as a whole (versus particular stages of sleep, such as REM), and to some theories of sleep function that seem more plausible than others.
Restoration Hypothesis
A classic view of sleep function is that sleep allows the central nervous system to repair and/or restore itself. According to this theory, sleep allows neurons that are used in wakefulness to shut down and repair themselves. Without sleep, neurons may become depleted in energy or "polluted" with the byproducts of normal cellular activity, and malfunctioning may result. Given the detrimental physical, mental and behavioral effects of lack of sleep, this hypothesis seems intuitively reasonable.
Data to support this hypothesis is scant, but not non-existent. During deep sleep, the secretion of growth hormone occurs in children and young adults. Some cells in the body have shown increased production and reduced breakdown of proteins during deep sleep. Since proteins are needed for cell growth and repair, this may indicate that deep sleep has restorative power. Also, activity in parts of the brain that are known to control emotions, decision-making processes, and social interations is greatly reduced during deep sleep. Thus deep sleep may help maintain optimal emotional and social functioning during wakefulness. A study in rats also showed that certain nerve signaling pathways generated by the rats while awake were repeated during deep sleep. (5) Scientists have theorized that this repitition may play a role in memory and learning.
Research has shown an increased amount of sleep in humans and other mammals after 24 hours of sleep deprivation. However, studies on humans sleeping in constant environments without external time cues show that sleep episodes tend to be shorter following long periods of wakefulness. Studies on the effects of exercise on subsequent sleep also do not appear to support the body restoration hypothesis. According to this theory, one would predict increased sleep following elevated catabolism produced by physical exertion, but studies show no effect on post-exercise sleep. (6) Finally, I wonder, why would the brain get "tired" and need to restore itself any more than, say, the liver?
Energy Conservation Hypothesis
A second theory is that sleep serves an energy conservation function. According to this hypothesis, the body needs sleep in order to offset the high energy costs of endothermy. Sleep reduces the metabolic rate and body temperature in endothermic animals. When animals fall asleep their metabolic rates decrease by approximately ten percent. Heat is dissipated from the body through peripheral vasodilation, leading to a one to two degree reduction in body temperature. Data in favor of this hypothesis rest on the similarities between hibernation and sleep. (7) Because hibernation has been shown to have an energy conservation function and because hibernation is an extension of some physiological processes of sleep, then it is argued that sleep has an energy conservation function. Furthermore, at lower ambient temperatures, energy savings can be much greater; heat loss from the body increases as the difference in temperature between the body and the environment increases. By sleeping, an animal can keep this temperature difference smaller by lowering the body's metabolic rate (and temperature), which could give an animal an advantage in environments with low ambient temperatures or scarce food sources.
Immune System/Sleep Relationship Hypothesis
A related theory to those above is that sleep may help the body conserve energy and other resources needed by the immune system to attack pathogens. Infections and the ensuing fever often induce sleep because cytokines, chemicals produced by the immune system to fight infection, have sleep-inducing properties. Furthermore, the current view of cytokines is that they are predominantly inflammatory mediators. There is increasing evidence that cytokines are expressed and perform functions in the normal brain. (8) Specifically, blocking interleukin (IL)-1 and tumor necrosis factor (TNF-alpha) in the normal brain alters the regulation of sleep. Researchers are just beginning to unravel the complex interrelationship between the immune system and the sleep function, but the possibilities are intriguing.
Conclusion
So what can we conclude? First, the theories discussed here and others rest more on speculation than empirical data. Sleep is one of the most complex biological processes, and it likely has multiple primary functions. Clearly, we need more basic research to investigate the functions of sleep and the molecular and cellular processes underlying it.
Second, in looking at theories of sleep function, some inherent difficulties in testing these hypotheses become evident. First, many theories rest on the assumption that sleep deprivation is equivalent to extirpating an organ to determine the impairments produced. Sleep is a complex process involving virtually all organs and systems of the body. Sleep itself cannot be removed without substituting something else in its place - wakefulness - that has its own properties. That sleep deprivation results in significant impairments, even death, does not explain the primary function of sleep. That is tantamount to saying that the function of the liver is to prevent the negative effects of liver-deprivation. It is also difficult to know what kinds of evidence would support or refute a hypothesis about sleep function.
Ultimately, to understand sleep, we need to understand the functions of sleep. And to understand the brain, we need to understand sleep. If the brain and behavior are the same thing, then we have a large gap to fill in understanding a behavior that consumes about one-third of our lives.
2)Home page of the National Sleep Foundation (NSF), NSF is an independent nonprofit organization that seeks to increase public understanding of sleep and sleep disorders in order to improve public health and safety.
3)Home page for the National Institute of Neurological Disorders and Stroke, NINDS is an agency of the National Institutes of Health and U.S. Public Health Service. Has good basics on sleep.
4)Brain basics- Understanding sleep, from NINDS
5)Brain basics-Understanding sleep, from NINDS
6)Basics of sleep behavior, A great overview of sleep, types of sleep, mechanisms, functions, regulation and disorders.
7)Journal of Neurochemistry, Vitkovic, L., et.al. "Inflammatory Cytokines." Journal of Neurochemistry, Vol. 74, No.2, 2000 457-471.
Other Sleep Resources
Sleep Research Online, allows limited searches on sleep topics.