Eli5: Why do we need to sleep so much, what happens in the body when asleep that cannot happen while normal resting or relaxing? Or is it just for the brain?
In: Biology
The best answer I’ve seen is your body actually doesn’t do much different while you sleep but does a lot of things faster such as cell growth. It has to do with how humans use energy – most creatures create energy as it’s needed, for the tasks needed at any given time. More complex systems have more energy demand, but have the benefit of adaptability – a simple creature that only eats algae will only have organs needed to intake and convert algae to energy, and likely will convert it as it’s needed, but will as a result have limited energy for things such as moving quickly away from predators. There does seem to be a link to energy demand and sleep – creatures that perform high intensity activities during their time awake tend to sleep more while slow moving creatures tend to sleep rarely and some never sleep.
The reason we do things such as close our eyes and relax is it allows the body to mostly guarantee that energy that COULD be spent on running, analyzing the environment, so on that COULD be needed immediately won’t happen, so it can safely move energy to body systems for maintenance. Think about a time you had a bad injury – even if it was just a really bad hit to the shin, you are going to feel less energetic. You usually won’t fall asleep though – the body can repair itself while you’re awake but it does it slower.
This is also what is happening when fighters get knocked out – they are more or less going to sleep as the body recognizes the energy demand to repair damage and prevent fatal injury exceeds the availability.
So in short, nothing particularly unique happens while you’re asleep, it’s more like a spaceship rerouting power to systems as they are needed – and by having a period where your body can guarantee with almost certainty you won’t need your full vision, sense of smell, hearing, or muscles, it can slow those systems down and focus on healing. This is also why you may notice if you get a cut in the morning, by night it might look roughly the same, but when you then go to sleep and wake it will appear notably smaller.
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