This article includes discussion of sleep starts, hypnagogic jerks, and hypnic jerks. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
Also termed “hypnic” or “hypnagogic” jerks, sleep starts are characterized by a myoclonic jerk, especially of the axial or limb muscles, often associated with a sensation of falling in space (siderealism) and represent a physiological and universal accompaniment of the process of falling asleep. As such, they should be differentiated from other pathologic myoclonic activity of sleep, eg, the startle reaction of hyperekplexia, the epileptic myoclonias, the periodic limb movements of sleep, and the propriospinal myoclonus occurring at the transition from wakefulness to sleep. Occasionally, however, sleep starts of excessive frequency and intensity are a cause of anxiety and sleep onset insomnia.
• Sleep starts are myoclonic jerks involving the whole body or some or 1 limb, associated with a subjective impression of falling, or with some other impression.
• Sleep starts occur mainly at the transition from wake to sleep, and represent a physiological accompaniment of the process of falling asleep.
• When excessive, however, sleep starts may impede falling asleep and may be a cause of sleep onset insomnia.
Historical note and terminology
Sleep starts, also known as hypnic or hypnagogic jerks, represent a completely physiological accompaniment of falling asleep and, as such, are experienced by many normal people. When excessive, sleep starts may cause sleep onset insomnia. The syndrome was recognized and described more than 100 years ago by S Weir Mitchell (Mitchell 1890). Formerly classified within the sleep-wake transition disorders, sleep starts are currently included in the Isolated Symptoms, Apparently Normal Variants and Unresolved Issues section VII of the newly revised ICSD-II (American Sleep Disorders Association 2005), which comprises sleep-related symptoms that either lie at the borderline between normal and abnormal sleep or that exist on the continuum of normal to abnormal sleep.
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