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Why Do We Sleep Research Paper Pdf

An average human sleeps for a third of his or her life (1). Other animals can display great variation in their time spent sleeping, ranging from the koala (up to 22 hours) to the giraffe (as little as 1.5 hours) (1).

Numerous studies affirm the importance of sleep to health and well-being, in a manner comparable to that of food (2).  A human will, on average, die more quickly from sleep deprivation than from food deprivation (1). After all, an average person can last about two weeks without food (if also supplied with water); he or she would last only 10 days without sleep (1).

To evaluate the importance of sleep, one may see what happens when it is deprived. In the short term, performance and alertness drop—a reduction of 1.5 hours of sleep can reduce alertness by as much as 32% (3). Memory formation and cognition are impaired as well (3).

In the long term, numerous negative health effects occur; high blood pressure, heart attacks, heart failure, strokes, Attention Deficit Disorder (ADD), depression and other mood disorders, mental impairment, and generally poorer quality of life have all been correlated to long term sleep deprivation (3). In addition, those who sleep less than six hours per night have increased mortality risk—a risk even greater than that of smoking, high blood pressure, and heart disease (3).

The effects of total sleep deprivation (TSD) have been examined in animals. In one study at the University of Chicago, a group of rats were subjected to TSD. All rats died or were sacrificed when death appeared imminent. The rats displayed no anatomical causes of death, though they did possess very debilitated appearances, lesions on tails and paws, as well as weight loss despite increased food intake (4).

Sleepiness is a natural protective mechanism comparable to hunger, though hunger fulfills a physical necessity for nutrients and proper metabolism (2). Both are regulated by powerful internal drives that discomfort the organism even before sleep or food deprivation has a marked physiological effect.

Though scientists understand sleep intimately from years of study, the question of why sleep exists still has not been definitively answered (2).  They have, however, developed many theories over the years. The complete answer likely lies in some combination of these theories.

First, Inactivity Theory (also called the adaptive/evolutionary theory) explains that inactivity at night or day is an adaptation for survival at a time of daily vulnerability through low visibility or oppressive temperature (2). For example, animals may sleep through periods of time during which they are most often preyed upon.

Through natural selection, those animals that stayed quiet through these times survived in greater proportions and eventually came to the mechanism of sleep (2). However, some have challenged this theory, asserting that it would be safer to remain conscious to react to emergencies (2).

Second, the Energy Conservation Theory proposes reduction of energy demand and expenditure is aided by a period of inactivity such as sleep. In natural selection, competition for and efficient use of energy resources is vital—if a species uses its given energy resources even slightly more effectively, it would have an advantage over similar species (2).

Especially if this period occurs during a part of the day or night with little opportunity to obtain food, energy metabolism can be reduced. Research shows that energy metabolism is reduced by as much as 10% in humans (and even more in certain other species) (2).

Third, restorative theories propose that sleep restores, repairs, and rejuvenates the body. After all, many major restorative functions—such as muscle growth, tissue repair, protein synthesis, and growth hormone release occur mostly or entirely during sleep (2).

Another segment of the theories explains that neurons need sleep to clear away adenosine accumulated as a byproduct of neuron activity (2) . The build-up of adenosine results in the perception of tiredness and leads to a “drive to sleep” (2). Remaining awake creates increasing amounts of buildup that will eventually become crippling (2).

Even in adults, sleep also assists in memory formation. A study that administered aptitude tests to groups of students demonstrated that those with sleep had better recall of test patterns, even if the sleep came from daytime naps (5).

Finally, the brain plasticity theory suggests that sleep is correlated to changes in structure and organization of the brain. In particular, sleep has a critical role in brain development of infants and young children (2). Sleep also improves learning ability and task performance in adults (2).

Indeed, rats trained to navigate a maze showed the same activity patterns during sleep as when they physically were performing the task. Sleep helped the brain reconstruct the experience and improved the rats’ performance when they ran the mazes again (Stafford).


1. The Better Sleep Council, Sleep Statistics and Research. Available at http://bettersleep.org/better-sleep/the-science-of-sleep/sleep-statistics-research (5 April 2013).

2. Division of Sleep Medicine at Harvard Medical School, Why Do We Sleep, Anyways? (18 December 2007). Available at http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/why-do-we-sleep (1 April 2013).

3. M. Breus, Sleep Habits: More Important Than You Think. Available at http://www.webmd.com/sleep-disorders/features/important-sleep-habits (5 April 2013).

4.  C.A. Everson et al., Sleep Deprivation in the rat: III. Total Sleep Deprivation (1989). Available at http://www.ncbi.nlm.nih.gov/pubmed/2928622 (6 April 2013).

5. T. Stafford, Why do we need to sleep? (28 February 2012). Available at http://www.bbc.com/future/story/20120228-why-do-we-need-to-sleep (1 April 2013).



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Still, scientists know far more than they once did.

The discovery of rapid eye movement, or REM, sleep in 1953 awakened scientists to the realization that sleep was not ''a simple turning off of the brain,'' but an active, organized physiological process, said Dr. Jerome Siegel, a professor of psychiatry and biobehavioral sciences at the University of California at Los Angeles.

Five decades later, few researchers would dispute that sleep serves some critical -- if unknown -- biological purpose.

All mammals, birds and reptiles engage in some form of sleep, Dr. Rechtschaffen noted in a 1998 paper, even if they do it perched on a tree branch or, like the dolphin, while swimming, with one half of the brain at a time. Sleep has also endured through the eons, despite the fact that it interferes with other survival-enhancing activities.

''While we sleep, we do not procreate, protect or nurture the young, gather food, earn money, write papers, et cetera,'' Dr. Rechtschaffen wrote.

Equally telling is the finding that when humans and other animals lose sleep, they proceed to make it up, paying off the ''debt'' by sleeping longer or more intensely.

Sleep deprivation over long periods appears to have serious consequences, though what they are is still debated, because it is difficult to separate the effects of lost sleep from those of stress or other factors.

''One can't remove sleep and change nothing else,'' Dr. Siegel said.

Researchers once thought that a prolonged lack of sleep produced mental illness. They now know that this is not the case, though waking subjects up every few minutes, early studies showed, made them cranky. Nor is there proof that humans have expired from a lack of sleep. But rats deprived of sleep die in two to three weeks, or in five to six weeks if they are deprived only of REM, a sleep stage in which brain activity is similar to that in waking. Whether the rats die from massive heat loss, infection or other cause is unclear.

What is it about sleep that makes it essential to life? Experts say that, despite widespread belief, it is not simply the fact that humans and other animals need rest.

''You can rest all you like and you still need sleep,'' Dr. Rechtschaffen said.

Another theory holds that sleep may serve to protect animals, by taking them out of circulation during the dangerous hours when predators roam. Yet this theory, Dr. Rechtschaffen and others point out, cannot explain why the sleep winks lost one night are made up the next or why the impact of long-term sleep deprivation is so severe.

''It's clear,'' said Dr. J. Allan Hobson, a Harvard sleep researcher, that sleep is ''not just to get you off the street and save you a few calories.'' Dr. Hobson; Dr. Robert Stickgold, also of Harvard; and other experts have argued that REM sleep helps consolidate memory and advance learning, and a number of studies have examined this premise, including two reports published in the journal Nature last month.

But other researchers, including Dr. Siegel, have challenged this theory. People who take antidepressants called monoamine oxidase inhibitors, which suppress REM sleep, do not show memory deficits, Dr. Siegel noted in a 2001 review.

Similarly, patients with brain injuries that do away with REM appear to suffer no problems in memory, Dr. Siegel said. One Israeli man, injured by shrapnel, went to law school and served as the puzzle editor for a local newspaper. Nor are the animals that spend the most time in REM -- the platypus, for example, which averages eight hours of REM each day compared with the two hours typical of humans -- known for their learning ability or powers of recall.

Dr. Hobson responded: ''It's not to say that memory depends on REM sleep. It is to say that certain aspects of mnemonic function are enhanced during REM.''

Dr. Siegel himself has waded into the mysteries of REM sleep. As in waking, most neurons in the brain fire actively during REM. The exception is nerve cells involved with the transmitter chemicals serotonin, norepinephrine and histamine, which remain inactive. It is possible, Dr. Siegel and others have suggested, that these neurons become overused, and that REM allows them to rest and regain their sensitivity.

Smaller animals, studies have found, sleep longer than large ones: a horse snoozes for 3 hours a day, a ferret close to 15. The fact that an animal's metabolic rate slows with size has led to yet another hypothesis about sleep's purpose: that it may act to repair cell damage caused by free radicals, chemicals released during the metabolic process.

The most promising theory so far, some experts believe, proposes that REM sleep plays a role in brain development. Newborns spend more time in REM than adults. Animals that spend long periods in REM are also more immature at birth.

In the meantime, the search continues. The answer, experts say, may turn out to be something obvious or something not yet dreamed of.

''There is something tremendous out there,'' Dr. Rechtschaffen said, ''and we just haven't found it.''

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