From The Art and Popular Culture Encyclopedia
A hypnic jerk, hypnagogic jerk, sleep start, or night start, is an involuntary myoclonic twitch which occurs during hypnagogia, just as the subject is beginning to fall asleep, often causing him or her to awaken suddenly. Physically, hypnic jerks resemble the "jump" experienced by a person when startled, often accompanied by a falling sensation. It is commonly caused by irregular sleep schedules.
Hypnic jerks are usually felt once or twice per night. More regular, and usually less intense, hypnic jerks often occur during normal sleep. In extreme cases, however, this is classified as a disorder called periodic limb movement. People with the disorder usually sleep through these events.
When a subject is deprived of sleep and is trying to fight sleep, hypnic jerks can sometimes occur more frequently. This normally happens to subjects who have deprived themselves of sleep for longer than 24 hours, or to those who have recently awakened after insufficient sleep.
In popular fiction
Hypnic jerks—or more properly, the simulation of the effect they have on a dreaming person, potentially waking him/her up—played a central role in Inception, a sci-fi heist film involving people entering other people's dreams to retrieve or implant memories. In the plot, hypnic jerks are referred to as "kicks" and a number of elaborate kicks, from falling sensations to immersion in icy water, are used to reawaken sedated dreamers at various, precisely-timed intervals.
- Periodic limb movement disorder
- Rapid eye movement
- Exploding head syndrome, a related sleep phenomenon which may result from fatigue