I didn't regularly work overnights when I was at Target, but I did work them a few times. Especially right before the Christmas season, they would put out a call for volunteers because they needed about double the staff.
Wal-Mart will be a bit different because you'll still have guests in the store. We worked when the store was closed and empty so it was nice, we got TONS done and nobody was around to bug us and ask us where the hairdryers were...
I will say that while I liked plenty of the people who worked overnights and was friends with several of them (some of them were people who worked other shifts during their time at Target so I did work with them other times), many of them were weird individuals for lack of a better word. One of my current co-workers is actually married to one of my Target co-workers and she agrees with me but it's hard to explain. There's just something about them... their social skills aren't very good. They were not pleasant to be around. Several of them were just flat out rude. I worked with a few other people who were like that, but it mattered less in overnights since they didn't have guests to be rude to, so they didn't get fired. =P It just tends to just be a different culture.
I would never want to do it long term honestly, but if that was the only option I had, I would do it in a heartbeat. It wasn't unpleasant, not the worst job I've ever had, just different. And like I said, I was friends with many of the people who worked overnights. It can be hard for your social life because you kind of live on an opposite schedule of everybody else, but depending on what you're comfortable with as far as your sleep schedule goes, it's not like you CAN'T be social and go out with friends and stuff.