Question:
Why does my son like to sleep under the bed?

My Opinion:
My son sounds much like you describe. We would typically find him sleeping under the bed, in the bookcase, or behind the door, lots of fun when I needed to wake him for surgery early one morning and I could not get the door open. We took video of some of his more creative arrangements. But we kept putting him back in his bed once he was asleep. My mother gave him a very large (king size) and HEAVY blanket when he was 3, which he absolutely LOVES. We put him in his bed with it that night, put the blanket over him and tucked it all around him, and he slept in bed the entire night until morning. He now sleeps in bed with that blanket every night, which is very good because he now shares his room with one of his brothers, and I would not want him bothering his brother at night. He also sleeps 9-10 hours straight each night, which for me is the main thing, as I need my sleep too!!

For me, I preferred the bottom bunk, and hung blankets like curtains so it would be fully enclosed. Then when I was moved to my own room, I put pillows and blankets all around my bed, because it was a captain's bed with drawers under it. On particularly difficult days, I would sleep in the closet, surrounded by pillows. Now, I much prefer to sleep on the couch, because it has one side "enclosed" that I can sleep next to. I also prefer sleeping on my side, fetal position type. For me, it is probably that I was treated so badly by others, that I considered it very comforting to be enclosed, probably Freudian or whatever it is, for the womb feeling. Any place to feel comfortable and accepted.

But I think for many AS kids, altho not necessarily all, the sensory experience of either surrounding pressure, or just simple enclosure, is probably at least partially accurate. Either the child loves the feeling of pressure, or it would be because whether or not the child can articulate what he is experiencing, at some level he knows that he is different and kids do not treat him as they treat other kids, so he likes enclosure and the feeling of aloneness and acceptance. Anyway, that is my opinion about it.