The day of a launch to low Earth orbit, astronauts have a significant chunk of downtime. They depart for the launch pad at about T-3 hours, and after final check and a pre-launch pee, they climb into the cockpit—where there's just not that much to do.
Marsha Ivins would know. She's made space shuttle trip five times. She explains to Wired:
Everyone imagines that when you’re sitting on the launchpad atop 7 million pounds of explosive rocket fuel, you’re nervous and worried; but the truth is, there isn’t much to do for those two hours after you climb into the shuttle. Many astronauts just take a nap. You’re strapped in like a sack of potatoes while the system goes through thousands of prelaunch checks. Occasionally you have to wake up and say “Roger” or “Loud and clear.”
Astronaut Leroy Chiao notes that astronauts are wearing adult diapers during this time, just in case, and explains why pre-launch is so peaceful, on Gizmodo:
This is kind of a time to relax a bit. The environment is totally familiar, thanks to the hours upon hours spent in the simulators. For once, nobody is talking to you. Nobody is asking you for something. It's not unusual to doze off.
Once astronauts arrive in space, the sleeping situation continues to be pretty strange. Astronauts zip into sleeping bags with straps that secure them in zero gravity. The bags have arm holes, too. Ivins explains what happens if you don't tuck in your limbs after tightening the straps:
[T]hey drift out in front of you. Sometimes you wake up in the morning to see an arm floating in front of your face and think, “Whoa! What is that?” until you realize it’s yours.
Circadian rhythms don't have much meaning in space: the sun rises 16 times a day when you're orbiting the globe. But this doesn't mess with astronauts' sleep schedules, in particular. Ivins says that they're so busy with experiments that they have to make a special effort to make the time to take it in.