I've done more night riding randonneuring than touring, and more still commuting. Agree with everything John says, as a baseline.
A few more notes (of course!):
Night riding at its best is almost magical. The visual world compresses, but the auditory world expands. On a lonesome road you'll notice the occasional house, church, or open convenience store, and welcome it, even more than during the day. Also of note is that the temperature drops; even when it's in the 90s during the day, it can drop to the 50s at night. How welcome that cool is may depend on how many layers you have available.
Make sure you have spare batteries for any battery-powered lights. I try to avoid any night-time riding without some kind of reflective garb, and my Ortlieb panniers have reflective patches. You'll want to have a light (helmet or spare flashlight) in case you need to fix a flat.
Route selection is critical. You'll want to avoid rough roads, because your reaction time disappears when you approach a pothole or other obstacle in the dark. Busy roads with minimal or no shoulders are pretty scary when you're passed with inches to spare. Also try to avoid anything but the smallest hamlets when the bars are closing; Friday and Saturday and holidays are good days to ride early (get up at 3:00 am if you want to ride at night), but settle down early on those days to avoid trouble. Definitely try to avoid shift changes at large plants, mines, hospitals, etc.
One big downside to riding until midnight is that you'll have to set up camp, assuming you're camping, in the dark. You'll probably also miss the early morning rides when the temperatures are cool and the wind is calm.
Supply can be a problem. Want to refill water bottles, buy a Coke, grab something to snack on? Better to plan ahead than try to get to the convenience store that's going to close at 10 p.m., only to get there and find it closed an hour earlier than you planned.