As a new father, I have read a great number of books on parenting. In the context of “cosleeping,” the practice of sharing a bed with your child, many of these books mention the phenomenon of “sleep awareness.” This is the knowledge you have of where your body is relative to its suroundings, even in sleep. This is why most adults don’t roll of of the bed at night, even if they toss and turn. Supposedly, this sleep awareness prevents you from rolling over on top of your child, assuming you aren’t under the influence of alchohol or drugs. I’m curious where the parameters your body requires for this come from. When you go to bed, do you make a mental map that you follow in your sleep? What happens if your surrounding change while you are sleeping? If you put a subject who tosses and turns frequently into a bed made of smaller mattresses pushed together, then remove some of them during sleep, is the subject more likely to roll out of bed? Or, in the context of the baby books, suppose my wife brings a child to bed while I am still asleep. Does my body somehow know that he or she is in there with us?
I don’t know about this – but whatever it is, not every species has it. Pandas roll over their kids and kill them. Why we’re spending so much effort on them is beyond me.
One thing to note is that most of the time while you’re asleep you are effectively paralysed. This is done to prevent you from acting out your dreams.
I suspect is has to do with familiarity more than anything else. So if your sleeping arrangements did change substantially, I suspect you would have trouble staying properly positioned. But this is just IMHO.
Here’s my alternate theory: when you have a baby, you become a light sleeper, even if you slept like the dead pre-kid. Babies are pretty noisy, even when sleeping. Put the kid in bed with you, and EVERY SINGLE NOISE she makes will wake you up. Your eyes fly open, the adrenalin hits you a bit, and you look around until you locate the baby and make sure she’s alright.
Do this several times an hour, and you’re always going to know where your body is relative to the kid, which is why you don’t roll over on her.
You’re only paralyzed (from the neck down) during dreaming sleep, not the rest of the time. In camp, did you ever try the trick where you cover a sleeping person’s hand in shaving cream, then tickle them lightly on the face so that they’ll scratch it and get the shaving cream all over themselves? That only works because people can move most of the time while they’re asleep.
I doubt it’s just familiarity - do you find that you usuallly fall out of bed if you’re sleeping in an unusual situation, such as in a hotel or if you’re napping on the couch? I don’t, and I don’t think most people do.