So, you're falling asleep. The stress of work and the pile of unwashed clothes and unanswered emails and rent and everything else is finally receding. Then, what the hell was that? You're jerked back awake by some kind of muscle spasm.
If that's happened to you, you're a part of an estimated 70% of people that experience hypnic jerks. While it sounds like a bunch of bullies who got super into Jack Kerouac in the 70s, it's actually that feeling of falling, followed by a muscle twitch when you're trying to fall asleep.
It's not 100% clear why that happens, but the SciShow takes on the topic and offers several prominent theories as to why that early stage of sleep is so often ruined.
The two dominant theories are based on neuroscience and evolution. One explanation contends that the twitch is the byproduct of how your brain hands off control of functions to another part of your brain as you drift into that sweet netherworld of sleep.
The evolutionary theory contends that everyone is basically still a monkey at heart and really wants falling asleep to not involve falling out of a tree.