See Waste Collection System for more about the orbiter's toilet. The humidity here is at about 20 percent onboard the shuttle, so we do notice that things dry out rather quickly, including us, and we try to drink more water than we normally do on the ground. During a single-shift flight, we all sleep at the same time. We have a group of people in Mission Control who watch over us, and they have ways of waking us up. They can set off an alarm up here in the shuttle that will certainly get us out of the rack and get us looking at what they want us to look at.
Some crew members like to set their alarms a few minutes early so that they are alert by wake-up.
It's a nice time to be awake since the ground is not going to call them unless there's an emergency. I believe that weightlessness does affect the types of dreams I have experienced. I have not had, that I can recall, the experience on the ground of dreaming of being weightless, although ever since I was a young boy I dreamed that I could fly. I know that's a fairly common experience, but perhaps that was anticipation of what I'm doing now, and perhaps now when I dream of flying when I'm on Earth, maybe that's reaching back into these memories.