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Comment by Arch485 | Hacker News Reader

Increasing skin temperature is known to induce sleep (can't find a source currently, sorry). Something about your skin being warmer allowing your body to cool more effectively, I think.

So a hot shower before bed is actually great for sleep, because you get the increased skin temp, relaxed muscles from the warm water, and general relaxation because showers are (for many people) relaxing.

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That's funny, I find it much easier to fall asleep in a cold environment. Then again, I also like to use a heavy blanket, so maybe it's the weight more than the cold that's helping me.

I think that's also consistent with the idea behind a hot shower. The shower doesn't help by increasing your body temperature, in fact it does the opposite. The hot shower induces the body to try to cool down, so near-skin blood vessels swell, and that dumps heat into the cold air, which reduces your core temperature, and a reduced core temperature helps you fall asleep.

I think where I read about this was Why We Sleep from Matthew Walker. But he suggests just washing your face with warm water, as opposed to a shower.

Hot shower heats up your body, which causes it to direct blood away from the core to cool it down so it doesn't overheat. Dropping core temp triggers the brain to ramp up melatonin production. Or so I heard.

Conversely, when the temperature drops, your body directs blood away from your hands and legs because core has higher priority for survival

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