Most people have been there: After finally crawling into bed at the end of a long day, you're comfortably drifting off to sleep when WHAM! Your body twitches hard out of nowhere, probably waking you up (and your significant other) in the process.
As SciShow explains, these horrible spasms are technically called "hypnic jerks" and the reason they occur has a lot to do with how our body transitions from being awake to sleeping and dreaming.
Scientists don't know a whole heck of a lot about why we dream, but we do know that our bodies become basically paralyzed in order to prevent us from acting out our dreams (which is something to be thankful for, next time you dream of jumping off the roof and flying away).
Of course, being paralyzed and unconscious has historically made us pretty darn vulnerable to predators, which isn't great. Some people believe that hypnic jerks could actually be related to an invaluable lifesaving reflex.
Check out the full reason we jump in our sleep here:
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