Why do we sleep at night, why does that make sense?

We're so vulnerable when we sleep. Why do we have to add our other weakness... darkness? I read through some previous answers to this same question and someone had the gall to say, "a long time ago we evaded nighttime predators by holing up in a cave and sleeping." If something were to come knocking on our cave in the daytime, while we slept, at least we'd be able to see the danger.