You're just about to fall asleep, you're in the perfect position and you feel yourself dozing off. Next thing you know, your spouse who's sleeping beside you has a huge jerk, which scares the living crap out of you. Your sleep is now ruined and you are slightly terrified that your spouse was just taken over by a demonic spirit.
Luckily for both of you, it's a common thing, and it happens to most people.
This pre-sleep twitch is called a hypnagogic jerk, and it's considered an involuntary muscle spasm that occurs as a person is falling asleep.
If you've ever been woken up by the sensation of falling, it is the same thing as the twitch before you fall asleep, and it isn't completely random.
These muscle spasms are caused by sound, light or anything external.
Although there hasn't been a lot of research done around these hypnagogic jerks, there are a lot of running theories around why they happen.
Some experts believe these jerks date back to our evolutionary background. They believe that once our body feels all of our muscles relaxing at once, the brain essentially misinterprets the relaxation as a sign that the sleeping primate is falling out of a tree, and causes the muscles to quickly react.
Here are a few more backed up theories: