This question was also asked by Curious Joe and Fahim:

What is it like to sleep in zero gravity?
-- Team ISS question

MAGNUS: It is actually kind of tricky. You don't realize it but we are used to gravity when we fall asleep. It holds us down to the bed and holds our heads on the pillow. That was the first thing I noticed the first night I tried to sleep in zero gravity. I felt like I was falling and kept waiting to hit the floor. After time, I was able to strap myself securely into my sleeping bag and also got used to floating (and not feeling a hard surface under me, like a bed). One trick that I have learned is to put my sleeping bag under a couple of bungees and then have them more or less hold me to a surface. That is very comfortable. In any case, you get used to it after a short period of time and can sleep very well.