This is usually the giveaway as to whether someone has entered some form of sleep, rather than just resting with eyes closed. What sets off the random, brief and occasional twitches?
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When you are asleep, your brain sends signals to your muscles to move as if you were awake and doing the things you are dreaming about.
In order to stop your body from moving around while you are asleep, your body releases a hormone when you fall asleep that prevents your muscles from moving.
After the hormone is released, your brain sends a twitch signal to your body. If the hormone has blocked your muscles, nothing happens, you stay asleep and everything works fine.
If the hormone has not worked, you twitch which wakes you up (usually) and do not then move around in your sleep.
When you wake up, a different hormone is released which counters the first hormone.
Sometime people wake up and the second hormone has not worked yet, so they wake up and are paralyzed.
While you are asleep the amount of the hormone in your system can vary enough to allow your muscles to move when your brain signals them to. The twitching normally occurs when dreaming, because that is when your brain tells your muscles to move.
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