Historically, humans have worked in the daytime because it’s light out and warmer, allowing them to farm/hunt/gather/etc. more easily and safely. The nighttime is more dangerous because some animal predators hunt at night and are not as affected by the lack of light as us humans. Thus, nighttime is sleep-time for most humans because we need to rest and the darkness naturally causes us to be sleepy.
That said, being a night-owl is arguably more fun