Have you experienced being really, really tired and then you suddenly fell asleep and in a snap your body jerks which makes you awake because you felt like falling from a building or a cliff, only to realize you are safe in bed (or wherever you fell asleep) and then you go back sleeping?
I usually experience this whenever I drop down to sleep after having a very, very tiring day from work, travel or even after having Zumba. Are you wondering why this occurs?
Hypnic jerk actually happens when your body relaxes too quickly whilst falling asleep, and then your body jerks itself because your brain thinks you are in danger (like falling or something). Sometimes, this happens when you fell asleep unintentionally.
These are just common and harmless but then it has been associated with quality vs. quantity of sleep. So, my advice is to aim in getting enough sleep and have a good resting environment.