I imagine that it's useful as part of your thermoregulation if your climate is cold. Cold muscles don't work as well, so your reaction time from "sleep to figh

I imagine that it's useful as part of your thermoregulation if your climate is cold. Cold muscles don't work as well, so your reaction time from "sleep to fight" would be improved I guess. As calories have become more plentiful, the selection pressure against the waste has decreased, allowing something which is selected-for with babies to persist into adulthood.