Around here (KY), most new grads start out on nights. Once you get use to staying up all night and sleeping during the day, it isn't bad. I'm currently on days right now, but will likely be going back to nights soon.
As for staying in shape on nights, I always make sure I bring healthy food to snack on (fresh fruit, veggies, broth-based soups), coffee, and I take walks when I'm feeling super -tired, especially in the summer, when the air is warm.
When I get home from work at 8:00, I'm usually wide awake, so I take a run and workout, which helps tire me and makes me get a good day's sleep in before the next shift.
I don't find that staying in shape and healthy on nights is any harder than days. It might be a little easier. On day shifts, I get home at 8:00, and have to make dinner for my husband, spend time with him, clean the house, walk the dog, etc. When I get home from a night shift, the house is empty and still clean from the evening before :-)