“Why do we sleep?”, your kid might ask in an annoying, but curious voice over and over again as you actually start falling asleep. But do you actually know the answer to that? “Just resting our eyes and shutting down our brains, sweetie!”
Wrong again, Karen. If you think it shuts down, like a computer too tired to deal with your endless open tabs, then you are about to learn something new (provided you haven’t already closed this browser tab). There are three main ideas behind why we sleep. The first one refers to restoration and metabolic pathways, and it’s ingeniously called the “restorative theory”. It feeds into the second concept regarding energy conservation, proposing that we mainly sleep to slow down metabolic processes and replenish cellular components that are used up during the day, such as neurotransmitters. Kind of like what happens when they stealthily restock your favorite food items in stores when you’re not looking. Energy conservation would refer to saving up energy and not consuming calories when you don’t have much to do, but sleep only saves you, on average, around 100 calories, so this one isn’t too interesting to discuss. It links to the restorative theory inasmuch as the slowing of metabolic processes is concerned. The most interesting aspect of sleep, besides restocking on your favorite neurotransmitters, is brain function. We all know our brains feel best when we’ve had a good night’s sleep, and that’s not really because THEY rest and do nothing. So, what happens?
Turns out, our brains need some “me time” to sort out through all of our “mental clutter”. When I say “mental clutter”, I mostly mean the information we hoard throughout the day. Our brains are the nice guys that hang around after a party and tidy up. They pick up the pieces of that shattered beer bottle and throw it away. Sometimes, if they’re feeling adventurous, they put that bottle back together (sometimes not resembling bottle in question, but the pieces are all there). Sure, our brains are also the ones bringing all the bottles and having the party in the first place, but oh well. At least they clean up.
In normal people terms, your brain consolidates your memories. If you’re not familiar with the term “consolidate”, you should look it up (just kidding, we’re here to explain these things for you, you lazy human). When a brain consolidates memories, it mainly strengthens or weakens certain connections between different neurons in different brain regions, based on importance. A sort of “filter” or “sort by” function on your favorite sites. Is it really that relevant for you to remember the eye color of that random person who walked by you on the train platform? Not really, unless you’re a psychopath detective maybe? So, your brain, which doesn’t really know in advance what information you will need, records mostly everything. Then you sleep, and your brain is like “okay, I have a hoarding problem, time to tidy up!”. It is believed that the brain does so by “pruning” away certain brain connections (or synapses), weakening them ultimately. This theory is called “synaptic homeostasis hypothesis” – “synaptic”, because, well, synapses and “homeostasis”, referring to equilibrium or balance. So for your brain to be clutter free, just like your home environment maybe, it needs to declutter, clean up, and throw away those funny, sparkly, plastic cowboy hats that you got from a novelty shop because you had too much to drink and thought it would be a great purchase. In the end, you probably forgot there was a chair underneath all those clothes you piled up on it. The brain works similarly, connecting and strengthening those things that we need and that are important to us, like a chair to sit down on, and it works to disconnect, weaken, fade, or get rid of those things which are just there, useless information that piles up to hide the things we really need. Does that make sense?
Another interesting thing that seems to happen when you sleep is yet another form of clean-up. Your brain doesn’t only declutter, it also removes waste and toxins that accumulated during the day. It does so, it seems, by increasing the volume of interstitial fluid (cerebrospinal fluid, that liquid that keeps your brain afloat – literally!). Well, actually, it has a system which draws the fluid in and out of the brain, like a washing machine, and with that fluid goes the waste as well (by waste, we obviously mean neuron poop, but we’re trying to be fancy here).
Okay, so, we see that the brain wants to be in a state of equilibrium and that sleep helps it achieve that. But what helps your brain make you sleep so that it can reach this homeostatic state? Melatonin. No, it’s not a skin pigment (that’s melanin). Melatonin is a hormone secreted by your brain (the pineal gland found in the brain, actually) to help you regulate sleep. You’ve probably heard of the circadian rhythm – an intrinsic process that helps you regulate your sleep-wake cycles. Well, melatonin plays a big role in that cycle, because, when it accumulates in the thalamus and hypothalamus, it kind of tells your brain that it’s time for some shuteye. We find that the highest level of melatonin is achieved during sleep, which further strengthens the idea that melatonin contributes to making you sleepy.
You probably have a lot of questions now (so do we, there’s much that’s left undiscovered), so go discover! Meanwhile, we’ll cook up some more on this topic, soon.
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References
https://science.sciencemag.org/content/366/6465/628
https://pubmed.ncbi.nlm.nih.gov/28590688/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3507109/#sec1-8title
https://www.youtube.com/watch?v=LWULB9Aoopc&ab_channel=TED
https://neurosciencenews.com/neuroscience-sleep-7876/
https://pubmed.ncbi.nlm.nih.gov/24411729/