system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why we sleep at night instead of during the daytime?
gpt
While many mammals turned nocturnal during the age of dinosaurs, modern primates have turned diurnal for probably a lot of reasons.    
-  Most importantly we don't have very dominant diurnal predators anymore.   
-  Early primates learned to live out the night in the trees which offered some protection from nocturnal predators.  (incidentally, this is the theory of why we sometimes spasm or jerk ourselves out of sleep, it's a holdover instinct from our tree dwelling days to keep us from falling out of trees while unprepared for sleep)  
- We don't have to compete with nocturnal predators.     
- Our food is easier to see during the daytime (as are our enemies and potential mates)   
-  Primate eyes are specialized to work better in daylight as opposed to some mammals who have made compromises, i.e. the cat's tapetum lucidum which allows their eyes to collect far more light than our eyes (their eyes sort of reflect light back through the retina twice, which is why their eyes appear to glow at night) but hinders image resolution a bit.  Mammals lost most of their color vision during the age of dinosaurs but we gained a lot back pretty quickly.  Still tho we are far behind the visual acuity of birds, who made it through the KT extinction with a massive head start on daytime vision, although to be fair we don't need the same acuity they need.  Primates are one of the few mammal groups that can see red because it helps us identify ripe and nutritious food.  At night anyone's color vision is shit  
- We have relatively efficient thermoregulation methods and can weather heat relatively well.     

So for these and probably many other reasons (other will probably list more) may as well sleep when it's dark.