Answer: Been Startled Awake

You climb in bed after a long day, you’re just drifting to sleep, and then it happens: your body jerks, you tense your body as if you’re falling, and you wake up with a feeling that you almost, no matter how improbable it is, were about to tumble out of bed and onto the floor.

In that moment that nearly everyone experiences at some point in their life (if not many times), you’ve experienced a “hypnic jerk”. It’s an involuntary muscle twitch that occurs right around the time someone transitions from consciousness to sleep. While the exact cause of hypnic jerks is unknown, they occur with greater frequency in childhood and more frequently in adults with irregular sleep schedules, anxiety issues, stress, caffeine in their system, or who perform strenuous activities later in the evening.

One theory on the origin of the phenomenon comes to us via evolutionary biology. The argument is that hypnic jerks are a holdover from an evolutionary mechanism designed to protect us, that our brains sometimes mistake the relaxation of our body in our beds for the muscle relaxation that would have sent our primate ancestors tumbling out of a tree.