Science Sundays: Hypnic Jerks

A few nights back, just as I was falling asleep my legs decided to give a sudden, spastic kick. Much like Elaine doing the little kicks dance.

It's like a full body dry heave set to music!

This sudden kicks on the edge of sleeping are fairly common for me, and for people in general, especially children. However, I sometimes startle not only myself out of sleep but Kevin as well. Having woken both of us up one night, I told him that I thought I read somewhere that you have a twitch like that because sometimes falling asleep tricks your brain into believe you're dying, so it gives a big kick to make sure you're still alive.

Kevin laughed at me.

So the next morning I thought I'd read up on what exactly is happening when I give a little kick in my sleep.

It turns out the name for this phenomena is the Hypnic Jerk or Myoclunus. Potentially anxiety, caffeine before bed, stress or extreme tiredness can cause these pre-sleep kicks, but they can also happen in people experiencing none of these things. There's no harm done by a kick or two, but if your jerks and spasms are keeping you up through the night then you should probably consult your physician.

But the real question is why do they happen? What is going on in my brain right on the edge of sleep to make me suddenly spasm?

Unlike the rapid eye movements we experience in REM sleep, which may be linked to the intensity of dreaming, the hypnic jerk is believed to be independent of your dreams. However, I do notice that mine sometimes are accompanied by a sudden sensation of falling. This seems to fit with one popular hypothesis of the origin of the hypnic jerk.

It is a common believe that the hypnic jerk is a left over response developed by our tree dwelling ancestors. As our furrier relatives settled in for the night in a cozy tree nook, their muscles would relax as they fall asleep. Relax too far however and you're falling out of the tree. Perhaps the hypnic jerk monitors the loss of tension in your muscles, if you become too slack and in danger of falling, then a sudden wakeful spasm might just keep you in the tree.

So while I am not in any danger of falling out of a queen sized bed, part of my ancestral instincts still worry it might happen. It's good to know that my subconscious brain is looking out for me.

Are you among the 70% of people who experience hypnic jerks?