A ‘Hypnic Jerk’ also known as a ‘Hypnagogic Jerk’, a ‘Sleep Start’ and a ‘Night Start’ – is an involuntary twitch which happens during hypnagogia. This is the stage just as a person is about to fall asleep and it often causes them to suddenly wake up for a moment.
These ‘Hypnic Jerks’ are a physical resemblance of the ‘jump’ experienced when a person is startled and is often accompanied by a falling sensation. Having irregular sleeping patterns is said to increase the likelihood of this happening.
According to the American Academy of Sleep Medicine there is a lot of potential causes as to why this happens, these include: anxiety, caffeine, stress and strenuous activities in the evening.
‘Hypnic Jerks’ are more frequent in childhood with 4-7 per hour at the age ranging from 8 to 12 years old, and it decreases toward 1-2 per hour at 65 to 80 years old.