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.