Get it? "sleeper?" Get it?
OK, so they can't all be great ones. But circadian rhythms run the whole show. It has recently been discovered that every cell in the body has a clock just vibrating away and each of those clocks is synchronized by a master clock in the brain. The master clock, in turn, is set by light.
I notice that if I get up at 2:00 AM my functioning is a lot worse than if I get up at 4:00 AM. I really don't think that it is just a matter of dopamine production but also a matter of receptor sensitivity and also cortisol rhythms.