Why do nation stop falling out of bed?

Do we learn that falling hurts and revise the boundaries subcounsciously? It's so weird =P
When sleeping on my full sized bed I merely use one side of it.. in certainty theirs a dent there.. and on a twin I with the sole purpose use one side too, hahaWhy do nation stop falling out of bed?
We stop falling as soon as we hit the floor.
Sorry, it was too trouble-free and I couldn't help myself.
we revise that it hurts and that it's a rude awakening
yes maybe you are right we probably do know when we are asleep.
I've never fell sour a bed while sleeping before.