Hypnic jerks — also called sleep starts — are sudden, involuntary muscle contractions you may experience as you are falling asleep1. Hypnic jerks are a type of myoclonus, which is a category of rapid, involuntary muscle movements. Hiccups are another type of myoclonus. Hypnic is short for hypnagogic, a word that describes the transition between wakefulness to sleep, which is when these jerks occur.
Hypnic jerks occur seemingly at random as you’re falling asleep, and typically only affect one side of the body, such as your left arm and left leg. You may experience a single jerk, or multiple in succession, before your body relaxes again.
In addition to the jerking movement, it’s common to experience other sensations or mental imagery along with a hypnic jerk, such as a dream or hallucination. People often report feeling like they’re falling, seeing flashing or blinding lights, or hearing banging, crackling, or snapping sounds. For the most part, hypnic jerks are painless, although some people do report a tingling or painful sensation.
Hypnic jerks can feel different at different times. Sometimes they’re strong enough to jolt a person awake and disrupt the process of falling asleep. Other times, they’re so mild that the affected person doesn’t notice them at all — although their sleep partner might.
Hypnic jerks occur at any age, but they’re more common among adults. In part, this may be due to the fact that some of their potential causes, such as caffeine consumption and elevated stress levels, are also more common in adulthood.
Researchers do not know for sure what causes hypnic jerks, but they have a few theories. Hypnic jerks and other types of myoclonus start in the same part of your brain that controls your startle response. When you fall asleep, researchers suspect that a misfire sometimes occurs between nerves in the reticular brainstem, creating a reaction that leads to a hypnic jerk.
For example, it may be that when your muscles relax completely, even though that’s a normal part of falling asleep, your brain mistakenly assumes you are really falling and reacts by twitching your muscles. It’s also possible that hypnic jerks are a physical reaction to the dream-like imagery that accompanies them.
Certain risk factors may increase your likelihood of experiencing a hypnic jerk, including excessive caffeine and stimulant consumption, vigorous exercise before sleep, emotional stress, and sleep deprivation.
Stimulants like caffeine and nicotine wake up your brain. These substances can also stay in your system for several hours, disrupting sleep. In one study, people who stopped drinking coffee a full six hours before bed still had trouble falling asleep. Having too much caffeine or nicotine, or consuming these substances too close to bedtime, may lead to hypnic jerks.
In general, exercise is almost always a good idea when it comes to sleep. Regular exercise has been consistently demonstrated to improve sleep quality. However, it’s important to recognize that exercise is an energizing activity that makes you feel more alert, rather than tired. For that reason, exercising too vigorously late in the evening may lead to hypnic jerks.
Trouble sleeping and lack of regular sleep overall, whether due to chronic insomnia or a poor night’s sleep, can both lead to sleep deprivation. Among other unwanted side effects, like poor mood and focus, sleep deprivation may increase your risk of hypnic jerks.
Both everyday stress and diagnosed anxiety disorders can contribute to insomnia, which leads to the kind of sleep deprivation that increases your risk of hypnic jerks. When you’re stressed or anxious, your cortisol levels remain elevated during sleep, which makes your sleep less restful. Anxious thoughts can also keep you up at night, making it hard for you to relax into sleep and disrupting the transition between wakefulness and sleep, potentially triggering a hypnic jerk.
Some people who experience hypnic jerks frequently may even develop anxiety around sleep itself, which only increases their likelihood of experiencing sleep deprivation and more hypnic jerks.
Hypnic jerks can be unsettling, but they’re not dangerous. In fact, they’re considered a normal part of falling asleep. Up to 70% of people experience hypnic jerks.
Hypnic jerks can be annoying and disrupt the sleep of you or your partner, but that’s typically the worst they can do. While it is possible that a particularly violent jerk could lead to a minor injury, it’s not common.
Hypnic jerks are different from other movements that can occur while you are awake or sleeping. Hypnic jerks occur during the transition from wakefulness into sleep, happen quickly, and are generally considered harmless. If you only experience hypnic jerks, you probably don’t need to see a doctor. Symptoms similar to hypnic jerks could require medical attention, however.
If, during the daytime, you experience multiple, persistent contractions in your muscles that spread to other parts of your body, you could be experiencing a different type of myoclonus, not a hypnic jerk. These types of myoclonus can be symptomatic of epilepsy, nervous system disorders, a head or spinal cord injury, or organ failure.
If you experience other types of jerking movements during sleep beyond hypnic jerks as you fall asleep, they could be symptoms of periodic limb movement disorder. If you’re concerned your muscle jerks are a symptom of another issue, speak to your doctor.
Hypnic jerks are a normal, albeit unpredictable, part of the experience of falling asleep. It’s unlikely that you’ll be able to completely eradicate them from your life. However, you can reduce their frequency and intensity, and improve your sleep at the same time, with a few relatively simple techniques.
Improving your sleep hygiene can make it easier for you to sleep better more consistently, which may reduce the occurrence of hypnic jerks. Follow these tips:
Reducing stress could lead to a reduction in hypnic jerks. Explore relaxation techniques that can relieve your stress. Meditation, deep breathing, and yoga, can all help. Fill your bedtime routine with calming activities, like a warm bath or reading a book. If your stress and thoughts interfere with your quality of life, speak to a doctor or therapist.
Exercise daily to enjoy more restful sleep. Regular exercise can also help relieve stress. If you prefer a vigorous workout, schedule your exercise for earlier in the day to prevent it from disrupting your sleep. If you can only exercise at night, opt for low or moderate-intensity exercises like walking or yoga. Aim to finish your workout at least 90 minutes before bed to allow your heart rate to slow back down and prevent the occurrence of hypnic jerks.
Caffeine can provide some beneficial energy-boosting effects during the daytime, but consuming too much, especially later in the day, can interfere with your ability to sleep soundly. If you’re experiencing hypnic jerks and having trouble sleeping as a result, caffeine might be the culprit. Avoid consuming more than 400 milligrams per day, and schedule your last cup of coffee for at least six hours before bedtime.
Nicotine is a stimulant that can impair your brain’s ability to wind down at night. It can also disrupt your sleep quality once you are asleep. Although alcohol is a sedative, it can also disrupt your sleep architecture, leading to the sleep deprivation that increases your risk for hypnic jerks.
Be aware that even after implementing these tips, you may still experience hypnic jerks on occasion. They’re a normal part of falling asleep. If you experience other movements that are disrupting your sleep, however, consult your doctor.