You may have experienced sudden, jerky body movements as you drift into sleep. The sensation is common, and if paired with a dream, can feel like you've suddenly moved or fallen.
When it seems like it's part of a dream, say falling through the air, this is called dream incorporation, and reveals our mind's amazing capacity to improvise, wrote Tom Stafford in his BBC Future Neurohacks column on the phenomenon in 2012.
The experience is known as a 'hypnic jerk' and it sheds light onto the conflict in our brains as we shut down for sleep.
What causes a hypnic jerk?
In sleep our bodies are paralysed, and we become oblivious to events in the outside world. But our muscle control isn't flicked off like a switch.
An area of the brain called the reticular activating system controls our basic functions, like breathing, and tells us whether we feel alert. In contrast, the ventrolateral preoptic nucleus, located near the optic nerve, dictates tiredness. As we descend into sleep, the reticular activating system releases control of our body and the venterolateral preoptic nucleus takes over. The process is like a slow fade of a dimmer switch, but it isn't always smooth.
Random bursts of our remaining wakeful energy occasionally surface in the form of jerky movements, for reasons that aren't fully clear. Unlike rapid eye movements, these have nothing to do with our dreaming brains, but are the last vestiges of our day.
Is it dangerous?
A strange and very unpleasant phenomenon called 'exploding head syndrome' follows a similar pattern of behaviour - our wakeful and sleeping minds attempting to wrest control from each other - and results in the sensation of seeing flashing lights and hearing loud bangs.
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