Jolt on the way to dreamland: Hypnic jerk

Hi Alice,

How come when you just start to fall asleep, you get a jolt in the leg or arms? What is this called and what causes it?

Dear Reader,

What you’re describing sounds like what’s called a hypnic jerk (HJ). Or, if you prefer, other names include sleep starts, hypnagogic jerks, or night starts. Hypnic jerks are sudden, brief contractions of the whole body or one or more limbs as a person falls asleep. For many, this is a common occurrence, happening sporadically, and affecting all ages. While little is known about this phenomenon, it’s considered benign. However, if you’re experiencing repetitive movements or the hypnic jerks are causing you to feel anxious about falling asleep, it may be worth talking with a medical professional to rule out any other conditions or sleep disorders that may be contributing factors. Keep reading for more about these night movements.

A hypnic jerk is a type of myoclonus, which is an involuntary spasm of a muscle or group of muscles. While there are a number of other types of myoclonus, these in particular are characterized by movements that occur as a person transitions between sleep and wakefulness. The good news is that hypnic jerks are said to be a normal phenomenon that occurs in healthy people. In fact, approximately 60 to 70 percent of the general population experience them. Though many people experience these sleepy jolts, they often won’t recognize them since they aren’t strong enough to wake them up. Sometimes, though, they’re accompanied by a sense of falling or a sensory hallucination. They may also increase in severity and intensity as a result of sleep deprivation, excessive caffeine intake, and emotional stress.

While the occasional HJ is not typically a cause for concern, there are some sleep-related movements that may be worth exploring further. One type of sleep disorders, periodic limb movement disorder (PLMD), is characterized by a series of jerks at precise time intervals that may go on for hours. This repetitive cramping or kicking of the legs is the only movement disorder that occurs during sleep. Periodic limb movement is also sometimes associated with other disorders, such as restless leg syndrome. If you think you may have PLMD or related issues, it may be helpful to talk with a health care provider. They can work with you to determine a diagnosis and help you manage the condition.

Having these sudden intense jolts, either one at a time or one after another may also lead to anxiety around falling asleep. If the movement itself, or the fear of them, are keeping you from getting enough sleep, you may consider talking with a sleep specialist to help you get to the bottom of the issue. However, if these are just occasional and your question is mostly out of curiosity, it’s possible that what you’ve experienced is a harmless hypnic jerk as you slip into dreamland.

Sweet dreams,

Last updated Mar 10, 2017
Originally published Nov 22, 2002