Does Sleep Have A Core Function?

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Main Category: Sleep / Sleep Disorders / Insomnia
Also Included In: Biology / Biochemistry
Article Date: 26 Aug 2008 - 0:00 PDT

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The potential necessary functions of sleep were recently discussed in an article released on August 25, 2008 in the open-access journal PLoS Biology.

While it is largely publicly accepted that sleep is essential, there are still many mysteries surrounding its function and how essential it is. Sleep scientists continue to research these questions and find the core function of sleep, which is discussed in the piece by Chiara Cirelli and Giulio Tononi.  This is a complex process can require information on many levels, including cells, organs, and the whole organism. "Everybody knows that sleep is important, yet the function of sleep seems like the mythological phoenix,"says Cirelli.

One theory, referred to as the "null hypothesis," states that sleep simply is a manifestation of a quiet, immobile state known as rest. This theory implies that sleep is not individually important in mammals and birds. The null hypothesis is systematically rejected by the authors through three central postulates: that there are animals that do not sleep, that there are animals that do not need recovery sleep when deprived, and that there are no serious consequences to sleep deprivation. 

The first, that there are animals that do not sleep, is rejected by refuting the few commonly cited examples: the bullfrog and the coral reef. They claim that the investigations have not been widely performed and further research is needed, at least, to make the claim that they do not actually sleep. Cirelli explains: "We don't understand the purpose of sleep, but it must be important because all animals do it."

Even dophins, which move continuously, sleep in one hemisphere of the brain at the time, with one eye closed and half of its brain in 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," says Cirelli.

Animals that do not need recovery sleep form the second statement. In recovery sleep, sleep deprivation is followed by this rebound period in which the organism either sleeps longer or spends more time in deeper sleep, as characterized by brain waves. There are many studies in birds and in zebrafish that have shown that the body regulates sleep homeostatically.

Finally, it has been shown in many species that adverse effects are experienced by organisms that are sleep deprived. In fact, sleep deprivation, affects cognitive function in animals of many families. "Microsleep epsidodes" are often experienced by deprived animals, including humans, which prevent them from reacting properly in dangerous situations. Notably, deprivation, or at least its effects, have been shown to kill rats, flies, and cockroaches, and even humans with genetic insomnia can die from the deprivation.

The authors argue that sleep is universal, regulated in the body, and essential to prevent harm. It follows that some important core function, which likely is related to the functioning of the brain. They hypothesize that sleep allows reassembly of tired neurons by allowing their synapses to dampen back to baseline levels of their communication chemicals. During the day, this maintenance occupies 80% of the brain's energy. "Sleep may be the price you pay so your brain can be plastic the next  day," they sa.

This could allow the brain to "forget" the random, unimportant impressions that day, leaving more room for new information the next day. This would help explain why, in the midst of resting sleep, there are regular periods of high activity.

The authors conclude that this important question is worth further perusal. "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."

Is sleep essential?
Citation: Cirelli C, Tononi G
PLoS Biol 6(8): e216.
doi:10.1371/journal.pbio.0060216
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Written by Anna Sophia McKenney
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