Does Sleep Have A Core Function?

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.В  This is a complex process can
require
information on many levels, includingВ 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.В 



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 nextВ  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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Here for Full Length Article



Written by Anna Sophia McKenney