Why do you twitch as you go to sleep?

This is called a hypnic jerk, and the reason we think it happens is that when you go to sleep, your body is completely paralysed. The reason for that, we think, is that you don't want to be acting out your dreams because otherwise it could get nasty! You must have had that dream when someone's chasing you and you have that horrible sensation of running through treacle. That's because you're literally paralysed in your sleep. However the process of that paralysis kicking in occasionally causes these funny jerks. People have done experiments on cats where they temporarily turn off the bit of the brain that causes you to be paralysed during sleep. The result is that you get cats prancing around and acting out these dreams.