In undergraduate, you may have had time to finish your studying or projects each night, watch a little TV or read a book (for fun!), and get up each morning to go for a run. Maybe you went dancing every Thursday night, spent weekends at a friend's place, and only ever didn't receive a full 8 hours of sleep by choice.
In veterinary school, you enter this Twilight Zone where I swear there are only like 6 hours in the day -where did the other 18 go!?