Why do people jerk awake after falling asleep in class?

Answer 1

Sleep Start

Or myoclonic jerk.

It can sometimes happen just before people enter a deep sleep, no one is completely sure why it happens but the leading theories are that as your muscles relax you brain is fooled into thinking it is falling and wakes you abruptly so you can catch yourself. This may also have something to do with the fact that you may not be in a place where sleep is normal or allowed and your brain knows this, so the logical conclusion based on that, is that you would be falling.

Many fewer people report this sensation in bed, it is usually reported when falling asleep somewhere else.