Sleeping and waking hours are very complicated in my opinion.
For instance, women tend to ovulate late afternoon, so I would assume our circadium rhythm affects our natural cycles.
If people tend to be awake during light hours and sleep during dark hours, that made a lot of sense before electricity, so we naturally were awake when we could see and do our work. People who naturally seem to be nightowls possibly served to protect the group. However, groups that traveled a lot, nomads, it served them well to all wake together, travel, and rest at the same times. There is a theory that yawning is contagious so a group would rest at the same time.
Most adults move through all stages of sleep in about 1.5 hours. If you are short on sleep and take a nap, check to see how long you nap if allowed to sleep uninterrupted. I usually nap 1.5 or 3 hours, so that reinforces this idea to me, but I don’t really know a lot of facts about sleep. Anyway, infants also tend to sleep at about 1.5 intervals while breast feeding. This seems like an efficient way for mommy to get sufficient sleep. Although, I would guess even in caveman days parents were exhausted with new infants.