I Feel tired at around 3-5?

How come on some day's I feel really tired? It started in the summer after a lesson or doing a lot of work I felt tired. (I was a Paid-Intern). Yesterday it happened because I have school from 8-11 then I go straight to internship. I worked hard yesterday and when I got home I felt tired and went to bed at 5:30 and woke up the next morning at 7? Which is way too long for me, I usually go to bed at 11. Is this because I'm over whelmed and think to much, or could it be mono? I also feel a little tired now :(