[zoomablemind]

The body and mind need to slow down to free up time slices for cleaning up the waste. Humans "in wild" are mostly daytime creatures, that's when they're supposed to actively look for food, deal with danger, maybe compete for mates, handle the offsprings. So not much safety to slow down in daytime.

On the other hand, nighttime affords this naturally, though modern society arguably blurrs the utility and safety aspect of nighttime.

Counterexample is some marine mammals, like dolphins and seals, which can shutdown half of their brain for sleep, while the other one is active for safety reasons.