Ideas are the most powerful things in existence. Here are mine.
Videos and music become popular because people like to minimize their risk of wasting time listening to or watching something unenjoyable. Actually, I suspect something similar lies at the heart of popularity in any context.
Here’s my hypothesis: Similar to illness behavior, it prevents you from hunting when conditions are unfavorable to you. Dreams allow simulations of threatening events instead. Fasting long enough overrides the clock (it does; you can change your circadian rhythm by hours at a time by selectively fasting); eventually the need for energy outweighs the risk of hunting at night.