[necovek]

It's so weird that all this research and discussion never once mentions daylight length difference between summer and winter in parts of the world far out of the equatorial belt.

The difference is between almost 16h of night time (depending on your definition, of course, since some light will be there even if Sun is not out) in late December and 8h of night time in late June at 45 degrees of latitude.

I wouldn't be surprised if sleeping twice during the night was more frequent during winter time in Europe.

Still, ever since fire was introduced into houses, there was an artificial light source as well.