The experience of people working the night shift, who use blackout curtains to sleep during the day, would disagree.

Wow, I didn't know my own experience disagreed with me...

Or that during my childhood when my dad was swing shift, he was apparently a freak of nature too...

But that’s for a relatively highly regimented sleep cycle. If you slept and worked completely at your leisure, you might end up with one shorter sleep period at night, and one even shorter nap during the day. And without any day-night cycle at all, some people naturally adopt cycles of varying lengths.

Again, human variation is a big thing.

But an individual will change their sleep schedule as they age, which is another supporting point for what I'm saying.

Evolutionary biologists hypothesis that it was so out of an entire tribe of early hominds, at least some members were likely to be awake. It wasn't an inate guard duty rotation. But kids and middle age went to bed early, teens went to bed super late, and by then the elderly were waking up.

If something happened, someone screamed and everyone woke up. And the fires stayed lit all night.