Exploring the function of sleep

August 26, 2008

Is sleep essential? Ask that question to a sleep-deprived new parent or a student who has just pulled an "all-nighter," and the answer will be a grouchy, "Of course!"

But to a sleep scientist, the question of what constitutes sleep is so complex that scientists are still trying to define the essential function of something we do every night. A study published this week in PLoS Biology by Chiara Cirelli and Giulio Tononi addresses this pressing question.

The search for the core function of sleep can seem as elusive as the search for the mythological phoenix, says Cirelli, an associate professor of psychiatry at the University of Wisconsin School of Medicine and Public Health in Madison.

Some scientists argue that sleep is merely a way to impose a quiet, immobile state (rest), and isn't important by itself in mammals and birds. This is the so-called "null hypothesis," and Cirelli and Tononi reject it.

"We don't understand the purpose of sleep, but it must be important because all animals do it," Cirelli says.

There's no clear evidence of an animal species that doesn't sleep, she says. Even the dolphin-which is sometimes held up as an animal that doesn't sleep because it moves continuously-will show "unihemispheric sleep" with one eye closed and one half its brain showing the slow waves characteristic of deep sleep.

"The very fact that dolphins have developed the remarkable specialization . . ., rather than merely getting rid of sleep altogether, should count as evidence that sleep must serve some essential function and cannot be eliminated," Cirelli says.

She also argues that sleep is strictly regulated by the brain, because sleep deprivation is followed by a rebound, in which the sleep-deprived animal either sleeps longer, or spends more time in the deeper sleep characterized by large slow brain waves.

Prolonged sleep deprivation has been shown to kill rats, flies and cockroaches. Humans who have a genetic insomnia can also die. In less extreme cases, sleep deprivation affects cognitive function in animals ranging from flies to rodents. Rats kept awake will engage in "micro-sleep" episodes, and sleep-deprived humans tend to fall asleep even in the most dangerous circumstances.

Because it is universal, tightly regulated, and cannot be lost without serious harm, Cirelli argues that sleep must have an important core function. But what?

"Sleep may be the price you pay so your brain can be plastic the next day," Cirelli and Tononi say.

Their hypothesis is that sleep allows the brain to regroup after a hard day of learning by giving the synapses, which increase in strength during the day, a chance to damp down to baseline levels. This is important because the brain uses up to 80 percent of its energy to sustain synaptic activity.

Sleep may also be important for consolidating new memories, and to allow the brain to "forget" the random, unimportant impressions of the day, so there is room for more learning the next day. This could be why the brain waves are so active during certain periods of sleep.

"While there may still be no consensus on why animals need to sleep, it would seem that searching for a core function of sleep, particularly at the cellular level, is still a worthwhile exercise," she concludes.

Public Library of Science

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	The Harvard Medical School Guide to a Good Night's Sleep (Harvard Medical School Guides) by Lawrence Epstein (Author), Steven Mardon (Author) Put your sleep problems to rest with this proven six-step plan How many times have you heard it's important to get a good night's sleep? It sounds simple, but it isn't always easy. Now one of the nation's leading sleep experts gives you a step-by-step program for overcoming sleep problems from insomnia and snoring to restless legs syndrome and sleep apnea. Dr. Lawrence Epstein of Harvard Medical School reveals his proven six-step plan to maximize your nights and energize your days. He explains the health benefits of sleep and identifies signs of sleep problems as he gives in-depth advice on how to: Turn your bedroom into the optimal sleep environment Finally overcome insomnia Silence buzz-saw snoring Relax restless legs Deal with daytime exhaustion ...
	Sleep Thieves by Stanley Coren (Author) The A to ZZZ of sleep is offered by the bestselling author of The Intelligence of Dogs. In an engrossing blend of entertaining anecdotes and scientific data, bestselling author Stanley Coren explores the world of sleep and sleeplessness.
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	Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem by Committee on Sleep Medicine and Research (Author), Harvey R. Colten (Editor), Bruce M. Altevogt (Editor) Clinical practice related to sleep problems and sleep disorders has been expanding rapidly in the last few years, but scientific research is not keeping pace. Sleep apnea, insomnia, and restless legs syndrome are three examples of very common disorders for which we have little biological information. This new book cuts across a variety of medical disciplines such as neurology, pulmonology, pediatrics, internal medicine, psychiatry, psychology, otolaryngology, and nursing, as well as other medical practices with an interest in the management of sleep pathology. This area of research is not limited to very young and old patients - sleep disorders reach across all ages and ethnicities. "Sleep Disorders and Sleep Deprivation" presents a structured analysis that explores the following:...
	Sleep, It Does a Family Good: How Busy Families Can Overcome Sleep Deprivation by Archibald D. Hart (Author) People today are sleep-deprived, and nowhere are the negative effects felt more acutely than in families. Most parents realize that their families aren’t getting enough sleep, though they likely don’t understand how serious the problem is and need help finding a solution. In this book, renowned psychologist and best-selling author Arch Hart explores why sleep is so important, what happens when family members are chronically sleep deprived, and how families can start developing healthy sleep habits.
	Frequently Asked Questions about Sleep and Sleep Deprivation (FAQ: Teen Life) by Judy Monroe Peterson (Author) Readers learn about the importance of getting enough sleep and get information on what it do if sleep deprivation becomes a problem. Includes photos, glossary, further resources, web sites, and index. 64 pages.
	Sleep Deprivation Chamber - Acting Edition by Adam P. Kennedy and Adrienne Kennedy (Author) The newest play--and first collaboration--by renowned playwright Adrienne Kennedy.
	Surrendered Sleep: A Biblical Perspective by camino real publishers Surrendered Sleep explores insomnia and sleep disorders from a biblical perspective discusses attitudes that may emerge as we lay down for rest.
	Good Nights: How to Stop Sleep Deprivation, Overcome Insomnia, and Get the Sleep You Need by Gary Zammit M. D. (Author), Jean Zevnik (Editor) St. Lukes-Roosevelt Hospital, New York City, NY. Addresses the need for quality sleep and what may be causing sleep deprivation. For consumers. Covers: sleep cycles, tips, behavioral treatments, sleeping pills, sleep rhythms, children and adolescents, sleep disorders, and consulting a sleep specialist. Hardcover edition was published in 1997. Softcover.
	WHAT IS CAUSING EXCESSIVE DAYTIME SLEEPINESS?: Evaluation to distinguish sleep deprivation from sleep disorders (Postgraduate Medicine) by JTE Multimedia Many people have a temporary spell, often in early afternoon, when they feel drowsy. This passing desire for a quick nap is completely different from excessive daytime sleepiness, which is a much more significant problem. Considering the potentially dire personal and economic consequences of falling asleep unintentionally or at inappropriate times, excessive daytime sleepiness must be taken very seriously. A thorough evaluation, as described by Dr Mahowald, virtually always leads to a specific underlying cause, allowing effective treatment recommendations. Mahowald MW. What is causing excessive daytime skeepiness?: evaluation to distinguish sleep deprivation from sleep disorders. Original Publication Date: March 2000