In line with what Care_Baer said; I figure everybody has to work a shift and sleep a shift (or the time equivalent of one), so it doesn't really matter which one you sleep and which one you work. I just do the opposite of what most do.
At very first (I've been doing third shift in a hospital for about a year now) it feels mostly weird, it's hard and you feel exhausted all the time...after a few months it got a lot harder for me as the constant schedule switching was wearing me down, I didn't have a routine, didn't know what kind of things helped me handle it personally.
Now I have a feel for it and it's getting to feel a lot more natural. For example, I know about how many hours of sleep I need to feel human, I know when to drink (or not drink) caffeine, when to nap, etc. Taking a nap, even an hour or 1/2 hour in the evening, really helps.
I sleep until 3:30 P.M. if I work that night. If I don't have work that night, I only sleep until noon or so, because I know I'll just be able to make up for it that night. And no, I don't keep the same hours when I don't work. When I have a night off, I sleep! Trust me, you'll want to.
Some positive things: It's really nice driving home Monday morning, with everybody else just headed into work and knowing you're headed home to bed, especially if the weather is crappy. Also, you get the entire day off before work; so if you work a Saturday you still get to enjoy the entire day even though you technically work that day. Another positive thing is that there are no bosses there at night; it's so much more relaxed, the people are usually more laid-back (the whole attitude, actually), and there aren't tons of people swarming everywhere. It's quieter and generally a less stressful work environment. You'll have less demanding/upset patients to deal with because they'll be sleeping most of the time, and also you won't have to deal with family/visitors. The workload on nights is generally substantially lighter.
I won't lie; at first it will probably be difficult. The very first night or couple of nights you may get nauseous in the middle of the night from being so tired, you'll be falling asleep, but it gets better. I would advise you to NOT wait to start drinking caffeine until you feel dead tired. Give it several months...like I said, I've been there a year or so and I have the feeling I'm still adjusting. Give it time to learn what works for you, for your body to adjust. There are plenty of good things too...I don't see myself doing this forever but honestly, though it sounds crazy to say this, the thought of going back to a busy, strict day shift is almost unappealing compared to nights. Who knows, you may love it! Let us know how things turn out and good luck! Sorry for writing a novel...
I don't eat during the night. Just never got into the habit of it, so I think that helps keep the weight off. I haven't really gained any. I eat breakfast when I get home from work in the morning (cereal or something) and then go immediately to bed. I then eat when I wake up at noon or 3:30; a lunch-type item. I eat 'dinner' later at night, 7 PM or so, so my three meals of the day remain at roughly the same times as a day shift schedule.