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I'm curious if there is any neurological mechanism that explains why falling asleep is easier in the dark.

I recognize that this isn't true universally -- a phobia of darkness might make it easier to sleep with the lights on -- but it seems to be true for most people.

I'm particularly curious if there is a general explanation that might explain why it is easier to fall asleep in the absence of much sensory stimulation, i.e. a quiet environment versus a noisy one, or if darkness is a special case due to our circadian rhythms.

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Please see Light, melatonin and the sleep-wake cycle. It's a start. –  anongoodnurse Aug 15 '14 at 2:55
Yep, melatonin is definitely going to be a piece of the puzzle. I'm wondering if it's the whole story, though. –  Josh Aug 15 '14 at 12:42
No, it's not the whole picture, hence the comment, it's a start. –  anongoodnurse Aug 15 '14 at 23:11

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