Why do we dream, and why do we suddenly fall?

The full narration of the video.

decnoelleee asked: why do we dream when we sleep? And why do we suddenly feel like we've stepped off a ledge? You've been there: you lie down, you're drifting off, then it's like missing a step, your whole body jolts, and you're awake. Most people think it's a dream about falling. But it comes too early for that.

You don't fall asleep all at once. You sink, floor by floor, like an elevator going down. The first few minutes are the shallowest floor, and your brain is still half awake. The really vivid dreams don't start until about ninety minutes later. So who missed that step, so early?

As you fall asleep, your muscles relax one by one, like someone switching off the lights in your body, one at a time. But your brain has a night guard in charge of balance, and it's still on duty. Suddenly it notices: the body just went limp! It can't tell sleeping from falling, so it grabs first. Every muscle tightens at once, and that's the jolt.

It's a neat story, but unproven, just one of several guesses. What we do know: it's common. About seven in ten people have felt it, and it's usually harmless. Being tired, stressed, or full of coffee makes it more likely. So where are the real dreams? The elevator sinks to the bottom, then slowly rises to a special floor: the brain lights up, and the eyes dart around under the lids. That's where most vivid dreams happen.

Here the body's switch is pulled hardest; your arms and legs barely move. So however fast you run in a dream, your legs stay still. In one night the elevator makes several round trips, so you dream several times. So why do we dream? Honestly, scientists haven't settled it.

The idea with a bit more evidence: while you sleep, your brain tidies up the day, saving what's worth keeping and dulling what hurt. Dreams may be pictures that leak out while it tidies. Others think dreams have no job at all, just a side effect of a busy brain. So that missed step isn't a falling dream. You're at the door of sleep; the lights went out too fast, so your brain thought you fell. That flash of a missed step is a picture it adds on the spot. decnoelleee, what did you used to think that jolt was? Tell us in the comments.