So you know that feeling when you’re just about to fall asleep and suddenly there’s a rhinoceros charging at you* and you jerk awake? That’s called a hypnic jerk.
A hypnic jerk is often accompanied by a sensation of falling and/or a visual cue, like a daydream or sorts. There are two theories on why it happens, but no one seems to know for sure, which I actually think is pretty cool. But only because they’re harmless. The first theory suggests that it is a byproduct of the change in temperature, breathing and heart rate as we drift into sleep. The second posits that the brain interprets the sudden release by the muscles as a sign that we’re falling and compensates for that by waking us (presumably so we can be conscious in the event that we are, in fact, hurtling through the air).**
*That’s not usually the case, but that’s one of the more vivid ones that I can remember.
**There’s a very interesting article about hypnic jerks here.