Why Do People ‘Twitch’ When Falling Asleep?
A hypnagogic jerk is an involuntary muscle spasm that occurs as a person is drifting off to sleep.
The phenomenon is so named in reference to the hypnogogic state — the transitional period between wakefulness and sleep. Hypnagogic jerks are also commonly known as hypnic jerks or sleep starts.
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Source: Ultra Facts