> iirc, biphasic sleep is the “usual way” people sleep without the advent of electricity, yes? “the waking hour”, and all that.

I honestly believe that this is not "natural", but rather an adaptation to the absurdly long European winter nights. Note that we didn't evolve there, so it seems unlikely that bi-phasic sleep is the most primal way.

Some of us have partially evolved for that environment. My ancestors eyes turned blue to see in the dim light and their hair and skin changed from Ethiopian Black to pale and blonde af.

Seems reasonable to expect some degree of genetic drift in sleep regulation as well, but I know of no data on this.

A bigger pupil would work better for dim light vision. Light through the iris would sacrifice daylight vision. The light skin on the other hand does have an obvious benefit in that climate for Vitamin D.

Everything is an adaptation to something. The one 8 hours sleep a night is also an adaptation for the life style we adapted for in the last 100+ plus years.