[Cthulhu_]

Anecdotal / from memory, they did a study (at least) once where people didn't have artificial light (not sure if they had candles or nothing at all), and found that people naturally went to sleep when it went dark - not much else to do - but also woke up after a few hours.

But I'm not entirely sure; how much can you do awake if it's dark?

But then, it's rare for it to be truly dark, with moonlight and the like.

[ianburrell]

I wonder if this changed with the seasons. In the winter, the night is longer than normal sleep amount and might make sense putting extra time in the middle of night. In the summer, the night is shorter and would sleep through.