Tool Module: The Possible Functions
We spend one-third of our lives sleeping, so there
is no doubt that we need to sleep. And if we are deprived of sleep for one night,
we tend to “catch up” on our lost sleep the next, which further indicates
the fundamental importance of sleep for the human organism. Likewise, from an
evolutionary standpoint, the function of sleep has been universally preserved
in birds as well as mammals. Yet astonishingly, scientists still don’t know
exactly why we sleep.
Despite all these indications of the great functional
importance of sleep, there is no consensus on what precisely makes it so important.
Scientists have proposed many theories, and not all of them are mutually exclusive.
But all of them have shown their limitations or been contradicted by other scientists’
experimental findings. The most commonly accepted theories of the function of
sleep can be divided into two groups: restoration theories and
Restoration theories make common
sense: at the end of the day, you’re tired. You go to sleep, and when you
make up the next morning, you feel restored. In other words, according to this
theory, just like hunger and thirst, mental and physical fatigue induce a homeostatic
response–in this case, sleep–that restores an equilibrium in the central
In one example of experiments that support this theory,
the more exercise animals got during the daytime, the longer their periods of
non-REM sleep at night. But scientists still have not determined at what level
of the central nervous system this restoration may occur, and whether it involves
synthesizing molecules or breaking them down.
As far as the possibility
of synthesis is concerned, there does not seem to be any evidence that any particular
body tissues are rebuilt during sleep. But the observation that the neurotransmitter
adenosine builds up in the brain during the daytime and declines during sleep
has inspired a different kind of restorative theory: the “protective theory”
of sleep, according to which we sleep mainly to protect our bodies from the negative
effects of prolonged wakefulness.
Yet another restoration theory that
some authors have proposed is that we sleep to save energy. The reasoning is as
follows: during non-REM sleep, animals consume less glucose and oxygen both in
their brains and in their bodies as a whole. Hence, if animals that were always
awake had ever existed, they would have needed to eat more food than animals that
could sleep (or hibernate), and over the course of evolution, natural selection
would therefore have favoured those animals that could sleep.
theory leaves the following contradiction unresolved. During hibernation, animals
expend almost no energy at all. Yet animals that hibernate periodically emerge
from hibernation in order to sleep. In the process, they raise their body temperatures
by a few degrees Celsius, to about 38°C. Hence it is hard to see how the role
of their sleep could be to save energy.
Another counterargument to all
these theories is that sleep presumably serves some function beyond mere resting.
(Try resting in bed all night without going to sleep, and you’ll have no
trouble agreeing that you don’t feel as refreshed as if you had slept!)
The other major group of theories of the role of sleep instead focuses
on its adaptive value. According to these theories, for many animals, the absence
of light and the lower temperatures at night made life more dangerous, so that
it was more adaptive for them to remain hidden and motionless at night. Sleep
was simply the most adaptive way to implement this protective strategy, and also
let them save energy. Hence they evolved so that they slept at night. But that
still doesn’t explain why nocturnal animals, which function so well at night,
also need to sleep.
Another widely held adaptive theory of sleep is
that REM sleep in particular may help the nervous system to mature. This theory
is well supported by the strong correlation between how immature the individuals
of a given species are at birth and how much time they spend in REM sleep. The
intense neural activity of REM sleep may thus play a decisive role in the maturation
of the central nervous system during a period when the brain receives fewer external
stimuli. But this theory still doesn’t explain why REM sleep persists in
The connection between sleep and learning and memorizing also
has been studied extensively but still remains unclear. On the one hand, sleep
definitely plays an important role in the optimal learning of new tasks. For example,
several studies have suggested that when the REM sleep of experimental subjects
is intentionally interrupted, they take longer to accomplish certain kinds of
learning. REM sleep seems especially important for acquiring new visual and motor
skills. When athletes are learning a new sequence of movements, such as a tennis
serve, during the daytime, they spend appreciably more time in REM sleep the following
night. In experiments where such athletes are deliberately awakened, some from
REM sleep and others from non-REM sleep, the ones who are awakened from REM sleep
have considerably more trouble in storing their new skill in memory.
On the other hand, it is also clear that people can successfully learn and memorize
things without getting any REM sleep. Here is one substantial piece of evidence
against a predominant role for REM sleep in the formation of memories: certain
medications suppress REM sleep, but people who have taken them for years have
not experienced any impairment of their ability to memorize.
debate is about which type of sleep, non-REM or REM, plays a role in learning.
In humans, learning of some tasks seems to be facilitated more by REM sleep, while
learning of others, such as those involving spatial orientation, seems to be facilitated
more by non-REM sleep. Hence many scientists believe that both types of sleep
may contribute to memory, in complementary ways, and that it is their alternating
phases over a night’s sleep that enable the brain to correctly sort the
information that it has accumulated during the day. For example, the long periods
of non-REM sleep at the start of the night might amplify the memory trace, whereas
the briefer but intense episodes of REM sleep might trigger the expression of
genes that are necessary for storing the information processed during non-REM
The abundance of theories on the role of sleep suggests that it
may in fact have a number of functions that are not mutually exclusive. Sleep
may also serve different functions at different stages of life. In children, who
dream a lot, sleep may plausibly play a role in building neuronal circuits. In
adults, sleep may assist in selecting the relevant information learned during
the day and placing it in long-term memory. Lastly, sleep may play a role in some
as yet unidentified cellular function that is the foundation not only for maturational
processes in children, but also for regulation of body temperature and for higher
cognitive functions such as memory in adults.