Whenever i find my mojo.
I think for a lot of people its so set by their sleep schedule as well and where they study.
During highschool no doubt it was the afternoon (i was always so dead tired) to early (?) evening, 4pm - 9pm. It just felt right, everything was winding down and being quiet for the day.
Now at uni, it's all over the fucking place. What people not in uni might not understand is the work usually comes in waves, there's no constant assessment (for the most part) and there's often no real need to stay immediately on-top of things or be with your class. Right now for instance i'm like 4 weeks behind.
Sometimes its almost no study, sometimes its an insane amount. Lately though for me its been late night to early morning, often i've been up til like 9-10am no sleep knocking off an assignment but that's what uni does to you.