You're trying to fall asleep, and lie in your bed with your eyes closed - and all of a sudden, your whole body twitches and jerks, and you have a sensation of falling.
Then you're wide awake again for a second!
This is something we have all experienced thousands of times before, but only very few people actually know what this phenomenon is or what causes is.
This phenomenon is called Hypnic Jerk or Sleep Start.
If you've ever felt this way, you're not alone - It's suggested that 70% of the population actually experiences these Hypnic Jerks.
"Hypnic" comes from the "Hypnogic state" - the transition phase between sleep and being awake.
This phenomenon can occur in many different variations - it can be triggered by sound or light, and might be accompanied by dreams or actually thinking you're falling deep. Or sometimes, we don't even wake up from the twitch and don't remember it at all.
So why does this actually happen to us?
There are 2 different theories - neurological and evolutionary.
As you fall asleep, two different systems in your brain are fighting over dominance -
1) The Reticular Activating System (RAS) - Arousal centers, which keeps you alert and makes you feel awake.
2) The Ventrolateral Preoptic Nucleus (VLPO) - controls your sleep cycles and is also called the "sleep switch" as it's being activated when you fall asleep.
So as you are trying to fall asleep, the VLPO is slowly dominating over the Reticular Activating System, and takes control.
During this transition phase, the part of our brain that's still alert sometimes stimulates the body's muscles rapidly, causing you to twitch.
This is a sign of the Reticular Activating System wanting to stay in control over your body!
But since we mostly stay in a sleeping position after that happens, the VLPO eventually takes over and we do fall asleep.
So essentially, the reason for Hypnic Jerks is a battle between the sleeping and waking systems of your body.
Our body is transitioning from the conscious state to the unconscious, REM sleep phase.
There's also another theory that's less common, and is based on evolutionary reasons:
Our primate ancestors often rested in quite dangerous places like a tree, and so our brain associates deep relaxation stages and the beginning of sleep with falling out of a tree.
Our body then quickly stimulates all our muscles, causing us to twitch, so that we could react and lessen the damage of the fall.
So this might have been a reflex that stuck with humans over the course of evolution.
Is there anything we can do about these hypnic jerks?
Scientists suggest that this phenomenon is linked to lack of sleep, caffeine, or high stress levels - so those are all things you should avoid if the Hypnic Jerks are upsetting you.
But in the end, no one knows exactly why this strange phenomenon happens to us - and it doesn't actually hurt us, although we sometimes think it will!
Source for the TIL, Images: 1, 2, 3, 4, 5
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