WHY IS
SLEEP NECESSARY?
Despite ongoing sleep research, scientists currently
have no concrete answer to this question. What has been
repeatedly observed, however, is that sleep IS important.
A study performed more than 10 years ago showed that
after about 10-20 days of sleep deprivation, rats would
die for no apparent reason. There is also a rare disease
in humans called fatal familial insomnia, which causes
death after several months. Lastly, if an organism is
deprived of REM sleep, then that phase of the sleep
cycle lengthens, possibly to make up the REM sleep “debt”.
Explanations that exist today are merely hypotheses
that may or may not have substantial supporting evidence.
For example, an old hypothesis stated that REM sleep
deprivation leads to insanity. This idea has been disproved
by contradictory evidence. Another commonly heard hypothesis
on the function of sleep is that sleep plays a role
in organizing new memories. Like the previous hypothesis,
supporting evidence is weak and actually contradictory.
When REM sleep deprivation (either due to brain damage
or induced by drugs) was studied, subjects experienced
no loss in their ability to remember new information.
In fact, some individuals appeared to have improved
memory. Therefore, this old idea is probably false as
well. New hypotheses, however, are devised from current
scientific knowledge, and may eventually lead to our
complete understanding of the function of sleep.
A possible function of non-REM sleep
A recent hypothesis associates sleep with the repairing
of damaged neurons. When an animal is awake, cell metabolism
results in the production of free-radicals, which are
reactive molecules that can damage or destroy cells.
Normally, free-radical damage is controlled by replacing
cells by mitosis (cell division). However, the majority
of neurons in the human body do not divide after childhood,
so cellular repair is crucial to the proper functioning
of neurons. Brain metabolism slows down and activity
decreases during non-REM sleep, so this phase may be
an ideal time for neurons to repair themselves. A study
done at the University of California at Los Angeles
in 2002 provided support for this hypothesis. The group
of researchers discovered evidence of brain cell damage
in rats as a result of sleep deprivation.
This hypothesis is also based on the observation that
taxonomic classification (grouping based on evolutionary
history) does not appear to be a factor in the sleeping
patterns of animals. Instead, the amount of sleep required
by a species has a stronger correlation to the size
of the animal: it appears that larger animals require
less sleep and smaller animals require more sleep. For
example, an opossum sleeps about 18 hours per day, while
an elephant sleeps only about 3-4 hours per day. This
peculiar variation in sleep time supports this hypothesis,
since small animals would likely experience more free-radical
damage than a larger animal due to a higher metabolic
rate. As a result, more non-REM sleep would be required
to repair this damage.
A possible function of REM-sleep
The above hypothesis could explain the function of the
non-REM part of the sleep cycle, but does not explain
the role of REM-sleep, where brain cells appear to be
fully active. To devise a possible explanation for REM-sleep,
scientists looked at the special characteristics of
this sleep phase. During REM sleep, almost all muscle
tone is lost due to inhibition of the neurons that control
body movement. This occurs due to the cessation of monoamine
neurotransmitter release, which normally allows these
motoneurons to communicate. In 1988, two scientists
hypothesized proper neuron function requires periods
in which neurotransmitter release is stopped. Supporting
this idea are studies done on neurotransmitter receptors,
which have found that the constant release of monoamines
desensitizes receptors. REM sleep could provide the
necessary “rest” period for receptor systems
in the human nervous system, allowing them to regain
full sensitivity.
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