Why do I suddenly shake when I fall asleep

Have you ever had this experience: when you are half asleep and half awake, your body suddenly "shakes" involuntarily, and then you wake up.

This phenomenon is called hypnic jerk and has been documented since 1890. This is an uncontrolled whole-body muscle twitch, often accompanied by a feeling of free falling or vague dreams. Normal sleep twitches will not affect sleep, but if they are too frequent, they may lead to periodic limb movement disorder (PMD), turning the person into a "twitching man" and causing insomnia.

There are many different reasons for hypnic jerks

One theory is that when a person goes from wakefulness to sleep, their heart rate slows, their body temperature drops, their breathing slows, and their muscles relax. The "reticular activation system" is responsible for controlling this conversion process, and sleep jerks may simply be a side effect of errors in the data of the reticular activation system.

Another more interesting explanation is that when people fall asleep, their muscles completely relax and their brain goes into a certain stress response mode, thinking that the body is in free fall. In order to protect itself, the brain directs the muscles of the whole body to immediately grab something during the falling process, resulting in sleep twitching.

The general consensus among researchers seems to favor the second theory.

They believe that when people fall asleep, their muscles begin to relax and enter a state of tranquility, and the brain misinterprets the sensory relaxation signal as "falling down" and then sends signals to the muscles in the arms and legs to try to restore balance. This misconception in the brain may also cause dreams of "descending," along with the sensation of falling.

The American Academy of Sleep Medicine says hypnic jerks have a wide range of potential causes, including anxiety, caffeine, stress, and strenuous activity during the night.