You did it today. You've probably done it nearly every
day of your life. In fact, you'll spend one third of your life doing it.
So why do we sleep? How is it that we slip into restorative
repose, only to come back at the proper time? Discover the theories
about the purpose and functions of sleeping as a means to restore both
body and mind.
Understanding the Purpose of Sleep
There is as yet no universal agreement about the singular purpose or function of sleep.
Though it is extraordinarily
commonplace, there is much about sleep that remains a mystery. Only in
the last few decades have we even begun to unravel its secrets. There
are at least three common theories of why we sleep, but it is unknown
which (if any) is correct. In fact, it is likely that further research
will reveal other potential purposes to sleeping.
Restorative Theory of Sleep
The
restorative theory of sleep is the most accepted explanation for why we
sleep. It suggests that sleep restores tissue and prepares our bodies
for the next day. This may involve clearing accumulated neurotransmitters
from our brain as well as other tissue repair that occurs throughout
our bodies. The glymphatic system flushes chemicals from the brain
during sleep, including adenosine, the substance largely responsible for increasing levels of sleepiness during wakefulness.
Adaptive Theory of Sleep
This
alternative explanation suggests that sleep increases our ability to
survive. As nighttime can be dangerous, especially in animals at risk
from predators, it makes sense to seek a safe refuge. By avoiding
dangers, the animal lives longer and is more likely to reproduce - thus,
sleep becomes an adaptive advantage. When morning light
returns, it is a powerful stimulus for wakefulness, further preserving
the evolutionary advantage of responding to the natural day-night
cycles.
Energy Conservation Theory
Others
theorize that sleep is a means to conserve energy. In a sense, by
sleeping we are able to spend part of our time functioning at a lower
metabolism, thus our overall caloric needs are reduced. If that time
were spent awake, we may not have enough food to survive. It also allows
time to create glycogen, an energy store that is used as the brain's
fuel reserve. Though metabolism slows during slow-wave sleep, the brain is extremely active during rapid eye movement (REM) sleep, thus not fully accounting for the conservation.
Other Theories
Sleep
seems to have other functions and perhaps these contribute to the
phenomenon of sleep. Sleep helps us to learn, refining and consolidating
our memories during REM sleep. It is important for problem solving. It
may allow for novel solutions to be considered as connections are made
in the brain. Sleep also helps to strengthen our immune defenses,
reducing the risk of infections. Some even argue that REM is critical
for the growth and development of brains in infants.
Regardless
of its purpose or function (and no doubt there are many), sleep is
clearly one of the singular activities of a lifetime, and is perhaps the
one about which we know the least. This leads to opportunities for
speculation and philosophical introspection. So if you if you find
yourself staring up at the ceiling, waiting once again for sleep to
come, ask yourself why we sleep.
Source:
Grigg-Damberger, M. "Normal Sleep: Impact of Age, Circadian Rhythms, and Sleep Debt." Continuum. Neurol 2007; 13(3):31-84.
Horne, J. "Why We Sleep." Oxford University Press, Oxford, 1988.
Grigg-Damberger, M. "Normal Sleep: Impact of Age, Circadian Rhythms, and Sleep Debt." Continuum. Neurol 2007; 13(3):31-84.
Horne, J. "Why We Sleep." Oxford University Press, Oxford, 1988.
