Agreed - your brain and sensory system don’t shut off when you’re asleep. One example would be the phenomenon of sleeping soundly through routine noises (which may be loud) but waking up to a strange noise (even if somewhat faint).
IOW, I think you learn how to stay in a bed, and it becomes a skill your brain handles with no difficulty. Perhaps falling out of one a time or two when young is part of how you learn.
You have enough concept of time when sleeping to actually wake up at some previously “chosen” time 
Heh. A college buddy of mine who sometimes drank to excess once told me, with the air of someone imparting timeless wisdom, “Remember, you can’t fall off the floor!”
I don’t think I ever fell out of a bunkbed when I was a kid, and I’ve never fallen out of a regular bed as an adult. I sleep pretty soundly and tend not to move much.
I can do that, too. Despite a weekend of staying up to all hours and sleeping in late, come Monday, I usually wake up just before the alarm goes off.
Okay, but I assume that your alarm is set at the same time during the weekdays, and that you usually get up around the same time during the weekdays.
My confusion is coming from the “choosing,” or at least to what extent. Say that you usually have an alarm set for 6 AM. Say that you will often wake up at 5:55 or something, before the alarm ever goes off. I can understand that.
But the other poster was sounding like he could, before bed, choose to wake up at some arbitrary time, say 7:00 AM or even 4 AM, and convince himself to wake up close to those times.
I have an irregular schedule and it is very rare that my alarm clock wakes me up. In my case I tend to wake up an hour to a half hour early, check the time and then drift in and out of light sleep until I have to get up. The more often I need to wake up at a particular time, the better I get at waking up close to that time.
I can do that too. I can’t explain how, but I can. Probably lots of others can too.
I think you are reading too much into this. That wasn’t the impression I got, but I guess I could be wrong.
In the winter, I have flannel sheets and a flannel nightie. Couldn’t fall out if I wanted to. Can barely get out when the alarm goes off (and it’s soooo nice and warm in there.)
Summer, silk nighty, cotton sheets. I may very well fall out of bed, in fact, I have. I usually catch myself before I go over.
My kids’ bunkbed had a rail. Even so, they managed to fall out about once apiece. I believe there was some horseplay going on and the kid in question wasn’t really asleep at the time.
A woman? Hah! We had bets before every big ceremony (HQ has a lot of thsoe) on how many would fall out. The medics gave us a count afterward. For 500 people, I’d guess 6 or 7.
I can wake up at a given time withing a half hour. It’s particularly easy if it’s something important, like an early tournament, meeting, or flight. 3 hours after bed seems to be my most accurate time.
She must have locked her knees up. I always thought the women had it worse in inspections and ceremonies because they were usually in some type of heels.
Actually, though, in our 1-2 hour long inspections in NROTC, I used to pray that one of the women near me would start to sway, so you had an excuse to break ranks and keep them from hitting the deck. (Because I had been standing at attention or parade rest that whole time, and any change of position was blessed relief.)