SCIENTIFIC PROOF OF SLEEP:SLEEPY THEORIES

At a Glance
Our bodies regulate sleep in much the same way
that they regulate eating, drinking, and breathing.
This suggests that sleep serves a similar critical
role in our health and well-being.
Although it is difficult to answer the question, "Why
do we sleep?" scientists have developed several
theories that together may help explain why we
spend a third of our lives sleeping.
Understanding these theories can help deepen our
appreciation of the function of sleep in our lives.
As with eating well, good sleep is a
staple of optimal health.
Arctic Fox at rest.
Lions conserving energy after a meal.
PET scan showing brain activity in a
20-year-old.
Bookshelf
Siegel JM. 2005. Clues to the functions
of mammalian sleep. Nature.
437:1264-1271.
Porkka-Heiskanen T. 1999. Adenosine
in sleep and wakefulness. Annals of
Medicine. 31:125-129.
Frank MG. 2006. The mystery of sleep
function: current perspectives and
future directions. Reviews in the
Neurosciences. 17:375-392.
Why Do We Sleep, Anyway?
Hunger and Eating; Sleepiness and Sleep
While we may not often think about why we sleep,
most of us acknowledge at some level that sleep
makes us feel better. We feel more alert, more
energetic, happier, and better able to function
following a good night of sleep. However, the fact
that sleep makes us feel better and that going
without sleep makes us feel worse only begins to
explain why sleep might be necessary.
One way to think about the function of sleep is to
compare it to another of our life-sustaining
activities: eating. Hunger is a protective mechanism
that has evolved to ensure that we consume the
nutrients our bodies require to grow, repair tissues,
and function properly. And although it is relatively
easy to grasp the role that eating serves— given
that it involves physically consuming the
substances our bodies need—eating and sleeping
are not as different as they might seem.
Both eating and sleeping are regulated by powerful
internal drives. Going without food produces the
uncomfortable sensation of hunger, while going
without sleep makes us feel overwhelmingly sleepy.
And just as eating relieves hunger and ensures that
we obtain the nutrients we need, sleeping relieves
sleepiness and ensures that we obtain the sleep we
need. Still, the question remains: Why do we need
sleep at all? Is there a single primary function of
sleep, or does sleep serve many functions?
An Unanswerable Question?
Scientists have explored the question of why we
sleep from many different angles. They have
examined, for example, what happens when
humans or other animals are deprived of sleep. In
other studies, they have looked at sleep patterns in
a variety of organisms to see if similarities or
differences among species might reveal something
about sleep's functions. Yet, despite decades of
research and many discoveries about other
aspects of sleep, the question of why we sleep has
been difficult to answer.
The lack of a clear answer to this challenging
question does not mean that this research has been
a waste of time. In fact, we now know much more
about the function of sleep, and scientists have
developed several promising theories to explain
why we sleep. In light of the evidence they have
gathered, it seems likely that no single theory will
ever be proven correct. Instead, we may find that
sleep is explained by two or more of these
explanations. The hope is that by better
understanding why we sleep, we will learn to
respect sleep's functions more and enjoy the health
benefits it affords.
This essay outlines several current theories of why
we sleep. To learn more about them, be sure to
check out the "Bookshelf" feature at the end of this
essay. There you'll find links to articles by
researchers who are studying this fascinating
question.
Theories of Why We Sleep
Inactivity Theory
One of the earliest theories of sleep, sometimes
called the adaptive or evolutionary theory, suggests
that inactivity at night is an adaptation that served a
survival function by keeping organisms out of
harm’s way at times when they would be
particularly vulnerable. The theory suggests that
animals that were able to stay still and quiet during
these periods of vulnerability had an advantage
over other animals that remained active. These
animals did not have accidents during activities in
the dark, for example, and were not killed by
predators. Through natural selection, this behavioral
strategy presumably evolved to become what we
now recognize as sleep.
A simple counter-argument to this theory is that it
is always safer to remain conscious in order to be
able to react to an emergency (even if lying still in
the dark at night). Thus, there does not seem to be
any advantage of being unconscious and asleep if
safety is paramount.
Energy Conservation Theory
Although it may be less apparent to people living in
societies in which food sources are plentiful, one of
the strongest factors in natural selection is
competition for and effective utilization of energy
resources. The energy conservation theory
suggests that the primary function of sleep is to
reduce an individual’s energy demand and
expenditure during part of the day or night,
especially at times when it is least efficient to
search for food.
Research has shown that energy metabolism is
significantly reduced during sleep (by as much as
10 percent in humans and even more in other
species). For example, both body temperature and
caloric demand decrease during sleep, as
compared to wakefulness. Such evidence supports
the proposition that one of the primary functions of
sleep is to help organisms conserve their energy
resources. Many scientists consider this theory to
be related to, and part of, the inactivity theory.
Restorative Theories
Another explanation for why we sleep is based on
the long-held belief that sleep in some way serves
to "restore" what is lost in the body while we are
awake. Sleep provides an opportunity for the body
to repair and rejuvenate itself. In recent years,
these ideas have gained support from empirical
evidence collected in human and animal studies.
The most striking of these is that animals deprived
entirely of sleep lose all immune function and die in
just a matter of weeks. This is further supported by
findings that many of the major restorative functions
in the body like muscle growth, tissue repair,
protein synthesis, and growth hormone release
occur mostly, or in some cases only, during sleep.
Other rejuvenating aspects of sleep are specific to
the brain and cognitive function. For example, while
we are awake, neurons in the brain produce
adenosine, a by-product of the cells' activities. The
build-up of adenosine in the brain is thought to be
one factor that leads to our perception of being
tired. (Incidentally, this feeling is counteracted by
the use of caffeine, which blocks the actions of
adenosine in the brain and keeps us alert.)
Scientists think that this build-up of adenosine
during wakefulness may promote the "drive to
sleep." As long as we are awake, adenosine
accumulates and remains high. During sleep, the
body has a chance to clear adenosine from the
system, and, as a result, we feel more alert when
we wake.
Brain Plasticity Theory
One of the most recent and compelling explanations
for why we sleep is based on findings that sleep is
correlated to changes in the structure and
organization of the brain. This phenomenon, known
as brain plasticity, is not entirely understood, but its
connection to sleep has several critical
implications. It is becoming clear, for example, that
sleep plays a critical role in brain development in
infants and young children. Infants spend about 13
to 14 hours per day sleeping, and about half of that
time is spent in REM sleep, the stage in which most
dreams occur. A link between sleep and brain
plasticity is becoming clear in adults as well. This is
seen in the effect that sleep and sleep deprivation
have on people's ability to learn and perform a
variety of tasks.
This theory and the role of sleep in learning are
covered in greater detail in Sleep, Learning, and
Memory .
Although these theories remain unproven, science
has made tremendous strides in discovering what
happens during sleep and what mechanisms in the
body control the cycles of sleep and wakefulness
that help define our lives. While this research does
not directly answer the question, "Why do we
sleep?" it does set the stage for putting that
question in a new context and generating new
knowledge about this essential part of life.