Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A hypnic jerk is defined as an involuntary spasm or twitch of the muscles, occurring during the lightest stage of sleep. Most common in children, hypnic jerks are experienced by up to 70% of people, being a completely normal occurrence, according to medics. It is believed that fatigue, sleep deprivation, stress and anxiety, could be the main triggers of the hypnagogic jerks, however, the exact causes of the phenomenon still remain elusive. Learn more about the hypnic jerks and their causes in this book.