You're experiencing a hypnic jerk, also called a hypnagogic massive jerk.
These mostly happen during hypnagogia, the phase between being awake and being asleep, but can also occur during REM sleep, as shown in the video. Most people will have four or five hypnic jerks while sleeping, but they are rarely strong enough to wake them. If you've shaken yourself awake during a dream, you experienced a hypnic jerk.
There are two theories on why this happens:
As you fall asleep, your breathing and heart rate slows, releasing tension on heart and lung muscles. While in REM sleep all of your muscles are completely relaxed. In either situation, your brain interprets this sudden relaxation as a fall and tenses your muscles to pull you upright. It's common for people in REM sleep to dream that they are falling just before the jerk wakes them up.
The brain may mistake falling asleep as dying, causing it to twitch muscles to get your body active again. This results in a more general movement of the body than the sitting up of the falling reaction.
Severe hypnic jerks are more common in people suffering from sleep deprivation.
Posted 1916 day ago