Normal Hamster Behavior?

This is my buddy Tweek

He was very skittish when we first got him a couple weeks ago. I feel he's really mellowed out since then, though. Sometimes he even lets me hold him. But I've noticed for a while now that he doesn't really come out during the day except to eat or drink. He has a little house in his cage and he just sleeps in it all day. He is active at night. He loves to run on his wheel at night and will do so for a really long time. Is he being active enough? Is he depressed? Or do hamsters just sleep all day?