A hypnic jerk, hypnagogic jerk, sleep start, sleep twitch, myoclonic jerk, or night start is a brief and sudden involuntary contraction of the muscles of the body which occurs when a person is beginning to fall asleep, often causing the person to jump and awaken suddenly for a moment. Hypnic jerks are one form of involuntary muscle twitches called myoclonus.

Physically, hypnic jerks resemble the "jump" experienced by a person when startled, sometimes accompanied by a falling sensation. Hypnic jerks are associated with a rapid heartbeat, quickened breathing, sweat, and sometimes "a peculiar sensory feeling of 'shock' or 'falling into the void'". It can also be accompanied by a vivid dream experience or hallucination. A higher occurrence is reported in people with irregular sleep schedules. Men have also been known to experience this at a higher rate than women. When they are particularly frequent and severe, hypnic jerks have been reported as a cause of sleep-onset insomnia.

Hypnic jerks are common physiological phenomena. Around 70% of people experience them at least once in their lives with 10% experiencing them daily. They are benign and do not cause any neurological sequelae.