What are the spasms that occur as you fall asleep?

11 January 2009


What about spasms that occur all over the body when you’re asleep?


Chris - I think what you're thinking of is a hypnic jerk. When people fall asleep and you can notice this yourself you suddenly jerk violently. Often enough to wake you up again and certainly anybody who'e sleeping next to you. If you're on a bus with someone and you don't know them it scare the hell out of them because I've done that to people. This is a hypnic jerk and it's to do with the part of the nervous system that paralyses you when you go to sleep. When we go to sleep and we dream you don't want to be acting out your dreams because that could have unfortunate consequences. There's a region of the nervous system called the sub-cerulean nervous system in your brain stem which receives information about when you're going to dream and it also has the ability to switch off the flow of motor systems going down to your spinal cord. It can paralyse your body when you're going to sleep. When you're switching in this system, sometimes it gets a bit carried away. You get these jerks as the system is turning on.

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