The brain goes through lots of weird stages and experiences while sleeping that are very different from when you are awake. For one, the brain waves slow down dramatically. When awake the brains frequency is around 30hz and that drops to less than 3.5hz when the person falls asleep. There are around 5 stages of sleep every time you go to bed at night. The first stage includes a very light sleep while the second becomes slightly deeper. The third and fourth stages are the deepest part of it and the fifth stage is where dreaming occurs and their is an increase in heart rate and eye movements. Gravity also plays a role during sleep and is the force that pulls you to the bed and is the reason that sleep would be much different in zero gravity. It would be much more comfortable as you would never have overlapping body parts and may be a way to a better nights sleep!