What is a "hypnic jerk"?

A hypnic jerk is a twitch while asleep. Not all movement while falling asleep is a hypnic jerk. Restless leg and periodic limb movement disorder have different causes.

According to the American Academy of Sleep Medicine (AASM), there is a wide range of potential causes, including anxiety, stimulants like caffeine and nicotine, stress, and strenuous activities in the evening. It also may be facilitated by fatigue or sleep deprivation.

https://en.wikipedia.org/wiki/Hypnic_jerk

A hypnic jerk, also known as a sleep start or hypnagogic jerk, is a sudden and brief involuntary muscle contraction that typically occurs as a person is falling asleep. This action is often accompanied by a falling sensation and can sometimes startle the individual awake. It's certainly jarring to the person sleeping or falling asleep next to them...

https://www.medicalnewstoday.com/articles/324666#what-is-a-hypnic-jerk