It should be one of the most relaxing times of the day. You climb into bed, get comfortable and cosy, start to feel your brain slowing down… and then suddenly you experience a shocking falling sensation.
It’s like you misjudged the number of stairs you were walking down, leaving your leg in mid air for just a bit longer than you expected. Not pleasant.
This bedtime tumbling sensation is the phenomenon known as the “hypnic jerk” and may sometimes be accompanied by a visual hallucination. You may have heard it called a “sleep start”, the “hypnagogic jerk” or the “myoclonic jerk”, but for the sake of sanity we’ll just stick with the former.
So what is it?
The hypnic jerk occurs when the muscles, usually in the legs (although they can be observed throughout the body), involuntarily contract quickly, almost like a twitch or spasm. Although the reasons behind this are not that well understood, the evolutionary perspective suggests that it serves at least two important but interrelated functions, the former of which is still relevant today.
First, this sudden awakening allows us to check our environment one last time, an opportunity to ensure that it really is safe to go to sleep by creating a startle-like response. You might have accidentally dropped off somewhere dangerous, after all.
Watch: This is why sleep is so damn important. (Post continues after video.)
Another suggested evolutionary function is that it allowed us – or at least our early ancestors – to check the stability of our body position before we went to sleep, especially if we started to fall asleep in a tree. The jerk would allow us to test our “footing” before unconsciousness set in.
The other main theory suggests that the hypnic jerk is merely a symptom of our active physiological system finally giving in, albeit sometimes reluctantly, to our sleep drive, moving from active and volitional motor control to a state of relaxation and eventual bodily paralysis.
In essence, the hypnic jerk may be a sign of the eventual switch over between the brain’s recticular activating system (which uses arousal neurotransmitters to aid wakefulness) and the ventrolateral preoptic nucleus (which utilises inhibitory neurotransmitters to reduce wakefulness and promote sleep).