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Comment by HarHarVeryFunny | Hacker News Reader

There is a difference between being physically tired as a result of metabolic effort, and being mentally tired/sleepy. Even if you lie on the couch all day you will still be tired come night time, and can not survive for long if deprived of sleep.

It seems the mental need for sleep comes from the brain needing offline (no sensory input) downtime for "housekeeping" activities - perhaps essentially organizing and filing away the day's short-term memories.

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the brain burns more power when doing mentally exhausting tasks than at idle, so it makes sense to have to recharge mitochondria in there. (the 'more' is not huge, like 5% - so it also makes sense to be tired after a lazy day I guess)

But we're sleepy every night regardless of how much or how little we have done mentally during the day. Doing more work (mental or physical) than usual will make us feel more tired, but the basic need for the 24hr sleep cycle is there regardless.

We fundamentally sleep at night based on circadian rhythm (evolved from earth's 24hr day), not based on activity level. We do also feel tired after a strenuous activity, but recover after a little rest and nutrition - this doesn't appear to be the same thing as the fundamental need for sleep.

The body expends 2000 calories of energy (via mitochondria) simply to be alive, even if you lie in a hospital bed and are unconscious. You do a marathon’s amount of work every day. You need to sleep to deal with that.

We're also alive when we're asleep ... The difference between being asleep or awake lying on the couch seems to have more to do with reduced/different mental activity than energy usage.

Being unconscious, or in a coma, in a hospital bed is more akin to being asleep, which is why you can be in a coma for years without dying.

And frankly, while a long day makes you feel more tired, I don't know that having to focus a lot or working out a bunch really makes me want to go to bed noticeably earlier.

One of the ways this electron leak happens (from the chatGPT) is that fuel (NADH) exceeds energy demand (ATP). So a good way to push off the mental need for sleep is to get your body tired. So the processes aren’t quite perpendicular.

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