Why do people twitch in their sleep? At what sleep stage does this happen?

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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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Anecdotal evidence here, but I notice that this happens to me only when I am stressed or sleeping untimely. This is when I have a clenched fist or a raised leg or some other tensed muscle. I have attributed the twitch sensation to my muscles suddenly relaxing due to onset of sleep and my brain interpreting it as falling off a tree.

I never get this twitch when I make sure to relax myself before I sleep

Researchers are still not 100% sure of what causes hypnic (short for hypnagogic) jerks, which are a type of myoclonus (involuntary (no control over) rapid muscle contractions). It’s hypothesized that these involuntary jerks are due to brain misfires during stage 1 of sleep or during the transition from stage 1 to stage 2 sleep.

The same area in your brain responsible for the startle reflex/response, is also responsible for hypnic jerks and other myoclonus, which leads to the theory that as your muscles relax when you fall asleep, your silly brain misfires and interprets the muscle relaxation as “oh no, body free-falling”, so your brain sends a signal to one of your body parts (usually an appendage), causing that body part to twitch, sometimes violently.

Stimulants, such as caffeine, stress, sleep deprivation, and intense exercise prior to sleep can increase the chances of experiencing hypnic jerks.

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.