Why do you jerk when asleep?

This is called a Hypnic Jerk. When you go to sleep at night your brain paralyses your body to stop you acting out all your dreams. It would be dangerous to act out everything that happens in your dreams, especially if you were running or fighting, not just for whoever shares your bed, but also for yourself. It's thought that this 'sleep paralysis' evolved when we slept in trees, as acting out your dreams whilst sleeping high up in the branches would be even more dangerous than doing do while tucked up in bed!As this system kicks in, you can sometimes have these hypnic jerks, where all the muscles contract suddenly and violently. For some reason, these are often associated with dreams of falling.