This exact theory has been put forward by scientists a few times - that variation in sleep-wake times helps guard a tribe or camp at night. A 2017 study of an

This exact theory has been put forward by scientists a few times - that variation in sleep-wake times helps guard a tribe or camp at night. A 2017 study of an indigenous hunter-gatherer group found that there was virtually always at least one person awake in the group, around the clock. The longest contiguous time the whole group was asleep simultaneously was just 1 minute! Here's one article about it: https://www.discovermagazine.com/planet-earth/chronotypes-evolution-explains-night-owls-and-early-birds https://www.discovermagazine.com/planet-earth/chronotypes-ev...