What Is a Hypnagogic Jerk and What Causes Sleep Starts?

Frequent Sudden Movements in Sleep May Require Further Evaluation

A woman sleeping in bed may experience hypnagogic jerks or sleep starts
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Just after falling asleep, you may wake with a sudden jerking movement. What causes these so-called sleep starts? Learn about hypnagogic jerks, or hypnic jerks, including the most common symptoms, and whether further evaluation and treatment may be necessary.

What Is a Hypnagogic Jerk or Hypnic Jerk?

A hypnagogic jerk is a sudden and strong involuntary twitch, or muscle contraction, that occurs while an individual is beginning to fall asleep. The same phenomenon is called a hypnic jerk if it occurs upon awakening. Both are often known as a sleep start. It may affect only part of the body, like an arm or leg. It may more commonly seem to cause the entire body to jolt suddenly. In some cases, a vocalization or sharp cry may occur.

These movements may occur without waking the affected person. If an awakening does occur, these sudden movements are often associated with a brief mental image. For example, you might believe that you were falling. A leg movement may incite a fragmentary dream image that you were perhaps kicking a soccer ball. It is believed that the movements occur first, perhaps due to an electrical discharge along the body's nerves, and that the mental image or explanation follows. In a sense, the brain creates a story to account for the movement.

Why Do Sleep Starts Occur?

Sleep starts can be a normal part of sleep. It is estimated that 60 to 70 percent of people recall experiencing them. If they occur infrequently, as they often do, they are not usually distressing. However, frequent events may lead to anxiety about falling asleep and insomnia, especially if the recalled explanation for the movement is upsetting (such as falling from a great height).

Hypnagogic jerks typically occur during stage 1 sleep. This is the lightest stage of sleep that occurs immediately after falling asleep. It may be misinterpreted as wakefulness, leading to confusion about when sleep starts occur. It may occur periodically later in the night, but these events are less likely to be recalled.

Sleep starts occur more often with increased use of caffeine and other stimulants. It may be provoked by physical exercise or emotional stress. It is possible for sleep fragmentation to be caused by another sleep disorder, such as obstructive sleep apnea. Frequent episodes may prompt further evaluation.

Other Causes of Movements in Sleep

Beyond sleep starts, there are other conditions that may contribute to movements during the transition to or state of sleep. These other causes may be considered:

  • Restless legs syndrome—Characterized by an uncomfortable feeling often affecting the legs when lying down in the evening that is associated with an urge to move that is relieved by movement. It occurs during wakefulness.
  • Periodic limb movements of sleep—Occurring during sleep, often these movements consist of flexion and extension of the foot (and sometimes knee) in a rhythmic fashion. These occur periodically in trains of events, but may come and go in the night. Unlike hypnagogic or hypnic jerks, these are not single, isolated events.
  • Seizures—Depending on the type of seizure, there may be a large movement of the body. Generalized tonic-clonic seizures may be associated with tongue or mouth biting, loss of bladder control with incontinence, and even injuries. After the episode, there may be a period of confusion. These last 1 minute on average and are usually easy to distinguish from sleep starts. 
  • Shivering—If the sleep environment is cold, or a fever exists, it is possible that shivering may cause movements in sleep. This high frequency movement may affect the extremities with the whole body. These typically last longer than sleep starts, which are often sudden events that immediately resolve, and resolve with warming the body or treating the fever.
  • Fasciculations—If a specific muscle, or group of muscles twitch, this may be called a fasciculation. It appears like quivering and may even look like a "bag of worms." These movements may occur across a joint and cause the contraction to move the extremity. These generally are more persistent than a sleep start and may be noted during wakefulness.

Further Evaluation and Treatment of Sleep Starts

In general, it is not necessary to seek further testing or treatment for sleep starts. Reassurance that this is a normal phenomenon is often all that is needed. If frequent movements occur—especially if they are associated with other complaints such as physical injury, mouth or tongue biting, bedwetting, or confusion upon awakening—it may be helpful to visit with a doctor to rule out other conditions.

Sometimes these awakenings can be provoked by another sleep disorder, most commonly disrupted breathing like sleep apnea. Sleep starts may be exacerbated by medication or substance use. In addition, movements in sleep can sometimes suggest seizures. It may be necessary to have a diagnostic sleep study called a polysomnogram. Alternatively, an EEG may be ordered if seizures are suspected to be causing the uncontrolled movements.

A Word From Verywell

If you have frequent sleep starts that are disturbing your sleep, or the sleep of your bed partner, you may want to speak with a board-certified sleep physician to get the evaluation and treatment that you need.

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Article Sources
  • Berry RB et al. "The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications, Version 2.2." Darien, Illinois: American Academy of Sleep Medicine, 2015.
  • Kryger, MH et al. "Principles and Practice of Sleep Medicine." Elsevier, 6th edition, 2017.