If you've ever been drifting off to sleep and suddenly felt your entire body freak out, you're not alone. As this video from SciShow explains, about 70 percent of the population experiences "hypnic jerks" (a.k.a. sleep starts). Scientists aren't exactly sure what causes them, but this video explores two ideas—one based on neuroscience and the other on evolution. Most experts say sleep starts aren't dangerous, but if you hate how they feel, try getting more shut-eye, limiting your caffeine intake, exercising in the morning, and managing your stress levels.