system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What keeps me from rolling off my bed at night?

If I sleep alone in a Queen size bed, for example, I could theoretically fall asleep on one side of the bed and wake up on the other side. But what keeps me from rolling off entirely?
gpt
Ever seen people falling asleep sitting upright, and then slowly slouching over until they realize it and snap to enough to sit upright again, and then fall immediately back asleep?

Same thing: your body's sense of acceleration doesn't stop when you go to sleep.  Your body knows and wakes up when it suddenly accelerates (like if it's about to fall off a bed).  You probably stop yourself from rolling off a bed more often than you know, because most of the time you go right back to sleep and forget the whole thing.