I sleep on the top side of the bed. I mean, sleeping underneath the bed itself is just silly. Unless you have a bunk bed, in which case I always prefer to sleep underneath someone.
But to answer the question properly

I don't share a bed with anyone (yet), but despite that I sleep on whichever side is convenient for me i.e. access to clock radio, personal belongings, phone, but this has usually tended to gravitate towards the left side than the right. I've found this even when sleeping in a hotel room by myself with a double bed. I sleep on the right alone when access is on the right hand side only, as is the case at my own home. Curiously, in my bed at my parents' house, I enter the bed from the right but sleep on the left.
Now, I'm no bedroom psychologist [go on, insert your own joke here] but I guess my sleeping positional habits reflect a need to stay in touch with the world and to be on the ball. Interestingly, in my last flat I also slept on the side of the bed furthest away from the exit. This might suggest I like to remain in my own comfort space when I need to be, and don't compromise when it comes to trying to get rest. That I also prefer to sleep on the bottom of a bunk bed may reflect Falling Out Of Bed Anxiety™, a need to stay attached to a cradle of support and security at all times. But what do I know, eh?
