Why are freshman college students (especially) so tired?

I’m a freshman in college, and in a lot of my classes where I’ve made some friends, they’ve mentioned once in awhile that they take naps after class – only like an hour or so, but still. When I went back to my highschool one-year reunion, also, a lot of the girls agreed that they liked taking naps. The sophomores and juniors in college, however, didn’t seem to have that problem.

It isn’t like we have a huge workload – they’re all pretty manageable as far as I had heard from them. There weren’t any all-nighters or anything. So, what do you think is going on?