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Comment by ben_w | Hacker News Reader

Those are things we do while asleep, but do not explain why we sleep. Why did evolution settle on that path, with all the dangers of being unconscious for 4-20 hours a day depending on species? That variation is already pretty weird just by itself.

Worse, evolution clearly can get around this, dolphins have a trick that lets them (air-breathing mammals living in water) be alert 24/7, so why didn't every other creature get that? What's the thing that dolphins fail to get, where the cost of its absence is only worthwhile when the alternative is as immediately severe as drowning?

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Because dolphins are also substantially less affected by the day/night cycle. It is more energy intensive to hunt in the dark (less heat, less light), unless you are specifically optimized for it.

That's a just-so story, not a reason. Evolution can make something nocturnal, just as it can give alternating-hemisphere sleep. And not just nocturnal, cats are crepuscular. Why does animal sleep vary from 4-20 hours even outside dolphins?

Sure, there's flaws with what evolution can and can't do (it's limited to gradient descent), but why didn't any of these become dominant strategies once they evolved? Why didn't something that was already nocturnal develop the means to stay awake and increase hunting/breeding opportunities?

Why do insects sleep, when they don't have anything like our brains? Do they have "Garbage collection" or "Transfer from short-term to long-term memory"? Again, some insects are nocturnal, why didn't the night-adapted ones also develop 24/7 modes?

Everything about sleep is, at first glance, weird and wrong. There's deep (and surely important) stuff happening there at every level, not just what can be hypothesised about with a few one-line answers.

Yes, actually. Insects have both garbage collection & memory transfer processes during sleep. They rely on the same circadian rhythm for probably the same reasons.

And the answer to "Why not always awake?" is very likely "Irreversible decision due to side effects". Core system decisions like bihemispheric vs unihemispheric sleep can likely only be changed in relatively simple lifeforms because the cost of negative side effects increases in more complex lifeforms due to all the additional systems depending on the core system "API".

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