Sometimes I feel disconnected - a lone island in a vast expanse of timeless emptiness. Then I consider the complex strands of DNA that run my physical and emotional body - all inherited from the genetic pool of my grandparents, their grandparents, and all my ancestors - separate families from diverse global cultures. We are all interconnected in amazing ways. Until I investigated hypnic jerks, I neglected to include the connection we have as a species - the influence of thousands, no, millions of years of evolution. Yes, I understood it as a concept, but consider this...

Our brain cells retain the vestiges of our evolution. Have you ever had the experience of drifting off to sleep, then suddenly there's a mild, but noticeable muscle jerk in your body and you're awake again? Sometimes the muscle movement is accompanied by a feeling of falling through space. When this happens to me, the muscle twitch and the sensation of falling, startle me from drowsiness. Suddenly awake, I recognize that I'm in my own bed. The name for this phenomenon is hypnic jerk, and it's harmless.

Hypnic jerks are considered more of a curiosity than a medical problem. It's a random reaction that occurs to about 70% of people. You may not remember the muscle twitch, especially if it's not strong enough to wake you up, but that's what it's designed to do. Hypnic jerks are part of our heritage as primates - a remnant of the evolution from our animal ancestors who lived in the trees of tropical forests 12 to 22 million years ago. Sometimes primates fell asleep in the trees. As their muscles relaxed they could have a dangerous fall. The survival response became a muscle twitch combined with the feeling of falling to wake them up. 

The primitive part of our brains still remembers this survival threat. Just as happened millions of years ago, our muscles naturally relax as we drift to sleep. To wake us up, the brain recreates both the sensation of falling, and the muscle twitch. The hypnic jerk is designed to save us from harm now, as it did then.

Here's a more familiar version of the hypnic jerk. Sometimes when we're nodding off in a boring meeting or a humdrum movie, our heads jerk, waking us up with a start. In this situation, the head-bobbing happens as we’re falling asleep while sitting up. It's likely the same evolutionary dynamic in action. Now it protects us from the danger of embarrassment rather than from the injury of falling from a tree!  

I’ve come to be delighted by the hypnic jerk when it occurs. It’s one of those odd aspects of our physiology that makes the concept of evolution real - an amazing connection to our past.