You fall into bed after a long hard day of adulting, you're just dropping off to sleep until OH GOD PANIC you feel as though you're falling off a cliff and you jerk awake.
This is what is known as a Hypnic Jerk.
There are a couple of theories around the cause of this phenomenon, but no one can seem to agree. The main one - and the one that most often gets repeated knowingly by people in pubs - is that it's a vestige of when human ancestors used to sleep in trees. The hypnic jerk is supposedly a reflex that stops us nodding off and falling off our perch.
They usually occur when a person is falling asleep when they're not completely relaxed, or when they didn't mean to, and this lends credence to the idea that it would serve as a little "are you sure you want to do this?" reminder before you enter the land of nod.
Another theory is that it has something to do with your brain not quite nailing the transition between its sleeping and waking states. When we're asleep, our brain temporarily paralyses our body, this means that you don't get up and act out even your most vivid dreams, but this paralysis can panic the waking brain, this often manifests are a night terror.
It is thought that if the brain gets the whole falling asleep and temporary paralysis thing the wrong way round, it can trick it into thinking that something is terribly wrong, causing it to jerk violently to be sure you haven't actually lost the use of your limbs.
It's also a great way of making your boss completely aware of the fact that you've just dozed off in a meeting. Again.