Many people have experienced the startling sensation of suddenly jerking awake just as they are about to fall asleep. Often accompanied by the feeling of tripping, slipping, or falling from a height, this common phenomenon is known as a hypnic jerk, or sleep start. Although it can feel alarming, experts generally consider it a normal part of the body’s transition from wakefulness to sleep.

A hypnic jerk is an involuntary muscle contraction that occurs as your muscles begin to relax and your heart rate and breathing slow down. Before the brain fully enters sleep, the nervous system may briefly trigger a sudden movement involving a single limb or even the entire body. Many people also experience a vivid sensation of falling or stumbling at the exact same moment, likely because the brain combines the muscle contraction with dream-like imagery during the earliest stages of sleep.

Several factors may increase the likelihood of experiencing hypnic jerks. Sleep deprivation, physical exhaustion, stress, anxiety, caffeine, nicotine, certain medications, and even vigorous exercise close to bedtime can all make these sudden movements more noticeable. Fortunately, occasional hypnic jerks are generally harmless and do not require medical treatment.