You're nearly under. Then your whole body kicks, and you're wide awake with your heart going, absolutely certain you just fell off something.
It has a name — a hypnic jerk, or a sleep start — and most people get them. They're harmless. But the interesting part isn't the twitch. It's the falling.
What's actually happening
As you drop off, your brain starts switching your muscles off. It doesn't happen all at once, and it doesn't happen neatly. Somewhere in that handover, a single burst of activity gets past the gate that's closing, runs down the line and reaches your muscles as one hard contraction.
That's it. That's the whole event: one signal that left slightly too late. Caffeine, stress, exercise late in the evening and being short on sleep all seem to make them more frequent, which fits — those are all things that keep your arousal system busier than it should be at that moment.
But you didn't fall
The twitch comes first. The falling is something your brain writes afterwards.
Here's the part almost nobody knows. Your cortex is already half offline when this happens. It gets hit with a violent motor signal it has no explanation for — and it does what it always does with things it can't account for. It makes one up.
Falling is the obvious story. Your muscles went slack and then something jolted, so: you must have been falling. The sensation feels like it came first, because that's how your brain filed it. It didn't. You were lying still in a bed the entire time.
And the thing about sleeping in trees?
You've probably read that this is left over from ancestors who slept in branches, where letting go meant falling. It's a lovely explanation and it might even be right.
But it's worth knowing that it's a guess. It came from a single proposal, no study has ever supported it, and it's very hard to see how one could. It spread because it's satisfying, not because it was demonstrated. Treat it as a nice story rather than a finding.
So what do you do about it
Mostly nothing, and that's genuinely the answer. Hypnic jerks aren't a sign that anything is wrong. If you're getting them a lot, the usual suspects are worth checking: caffeine later than you think, a hard workout close to bedtime, and going to bed already exhausted after a short night — being overtired makes them more likely, not less.
If the twitching is frequent, painful, or continues once you're properly asleep, that's a different thing and worth a doctor's opinion.