Also, when your jobs are a) finding berries and hunting mammoths and b) having sex, it is not implausible that doing a) during the day and b) during the night g

Also, when your jobs are a) finding berries and hunting mammoths and b) having sex, it is not implausible that doing a) during the day and b) during the night gives you more reproductive success than vice versa. It is fun speculation, but still, OP's self-assured "this is so trivial" tone was not called for.