You’re gently drifting off to sleep when suddenly your arms and legs convulse and you jolt yourself awake. Or, perhaps you’re relaxing in bed when, out of the blue, your dozing partner does an almighty twitch, scaring you half to death. This is called a hypnic jerk or sleep start, often accompanied by a sensation of falling or tripping over. An estimated 70 per cent of people will experience this at some point.
So, what causes the hypnic jerk? The truth is, we don’t know exactly why it happens, but sleep researchers have some theories. As we transition from wakefulness to sleep, the nervous system winds down and muscles relax. Sometimes, the brain misreads this relaxation as a sensation of falling or tripping, and sends a quick but powerful signal to the body, resulting in the hypnic jerk.
Hypnic jerks usually affect one side of the body and are painless, though some people may experience a tingling or painful sensation. For most, they are not associated with any health or other sleep problems. However, when they occur frequently over a prolonged period, people may come to anxiously anticipate them, leading to insomnia.
Some research has shown hypnic jerks can be more common among people with certain conditions, such as Parkinson’s disease. A 2016 study suggests they may be a symptom in early stages, but it’s unlikely they would occur without other common symptoms like changes to movement or REM sleep behaviour disorder. Frequent hypnic jerks can also be a side effect of some prescription medications, particularly antidepressants like SSRIs (e.g., escitalopram, sertraline, fluoxetine). Although rare, researchers have reported cases where jerks resolved after stopping the medication.
Other substances containing caffeine or with stimulating effects, such as cocaine, have also been linked to hypnic jerks. Lifestyle factors that can make them more likely include irregular sleep schedules, stress, and excessive caffeine or alcohol consumption. Keeping these factors under control is part of good sleep hygiene, whether or not you are worried by hypnic jerks.
