I learned in My psychology class that humans aren't Natrually supposed to just sleep at night, but rather when we're tired. But we've made it so that we're awake during the day (generally speaking) because that's when everything is more visible. I always feel awake at night, unless I just haven't had sleep, and I've found that for me, It's easier for me to fall asleep during the day. Not sure why that is though.