Nearly every animal sleeps, from flies to lizards, to people. Yet losing
consciousness every day has some serious drawbacks.
This means that sleep must perform some essential biological duty otherwise no
animal would engage in such a dangerous waste of time. One purpose of sleep is to
promote learning and memory. This is obvious to anyone that's ever taken a
test after just one night without sleep. But researchers are still sorting out
how exactly sleep benefits the brain. One possibility is the synaptic homeostasis
hypothesis. Inside your head are billions of spindly cells called neurons. Between
these neurons are quadrillions of tiny gaps called synapses. these synaptic
connections can be strengthened or weakened, increasing or decreasing the
flow of information between two connected neurons. Because these changes
in signal strength are long-lasting, researchers believe the synapse is the
physical basis of memory. Your first kiss, your brother's hair color, the lyrics to
your favorite song-- each of these physically exists inside your brain as a
spiderweb of strong and weak synaptic connections. According to the synaptic
homeostasis hypothesis, synapses are strengthened during the day and weakened
during sleep. So why would a brain do this? Imagine two people attempting to
read a difficult textbook chapter full of acronyms and footnotes. The first
person reads the chapter straight through while the other takes a break
after every section to reflect on the main points being discussed.
Chances are, the second person will come away with a better understanding of the
material. Similarly, sleep is the brain's
opportunity to pause and reflect on a lot of complicated data in order to
build a comprehensive picture of the most important details of the day. By
weakening all the synapses at once during sleep, the brain is able to
distinguish the important memories, the highly strengthened synapses, from
the noise, the weakly triggered synapses. The most important information is
preserved while the fluff is cleared away. This nightly separation of the
synaptic wheat from the synaptic chaff is the reason you can remember what you
had for breakfast today, a relatively unimportant memory, but not what you had
for breakfast last Tuesday. And it ensures the brain is only devoting
cellular supplies and storage space to important information. Weakening synapses
during sleep also prevents them from hitting a signal strength roof. Like a
fader style light switch, synapses can be low, high, or in-between. But once they
reach a maximum threshold no new information can be stored that requires
an increase in synaptic strength. It would be like trying to turn up a dimmer
switch that's already set to maximum brightness. Scaling down all the synapses
at once during sleep prevents them from reaching that maximum threshold, thereby
enabling new information to be stored the following day. Even one night without
sleep can have a big impact on brain function, in part because it disrupts
this nightly scaling down of synapses. It should be no surprise then that a
chronic lack of sleep might have serious health consequences. Alzheimer's disease,
schizophrenia, and autism spectrum disorder are all associated with poor
sleep quality. It's possible that long-term lack of sleep may actually be
contributing to symptoms associated with these disorders. Developing better ways
to improve sleep may eventually lead to new treatment options.
In this way, studying sleep may allow us to find new ways to treat disease. We
will also begin to unravel a truth that the animal brain has known for millions
of years. That in order to think, you must be able to rest. And in order to remember
you must also be able to forget.