Hypnic jerk on polysomnography

This figure shows a PSG of a patient with hypnic jerks. Hypnic jerks are short body contractions that affect the body asymmetrically during sleep onset. EMG shows that the involved muscles have, singly or in succession, brief high amplitude potentials that are generally 75 to 250 milliseconds in duration. EEG shows that stage N1 sleep patterns may occur with hypnic jerks.