Hypnic jerk is a phenomenon that many individuals have experienced but may not be aware of its name or cause. It is a sudden, involuntary muscle contraction or twitch that occurs just as a person is falling asleep. This can be accompanied by a feeling of falling or a sensation of being startled, leading to a brief moment of wakefulness. Hypnic jerk is a common occurrence, with about 60%-70% of people experiencing it at some point in their lives. Despite its widespread occurrence, there is still much mystery surrounding the cause and purpose of this phenomenon. In this essay, we will explore the characteristics of hypnic jerk, its potential causes, and the current theories surrounding its occurrence.
A hypnic jerk, hypnagogic jerk, sleep start, sleep twitch or night start is an involuntary twitch which occurs just as a person is beginning to fall asleep, often causing them to awaken suddenly for a moment. Physically, hypnic jerks resemble the “jump” experienced by a person when startled, sometimes accompanied by a falling sensation. Hypnic jerks are associated with a rapid heartbeat, quickened breathing, sweat, and sometimes “a peculiar sensory feeling of ‘shock’ or ‘falling into the void’”. A higher occurrence is reported in people with irregular sleep schedules.
Causes
According to the American Academy of Sleep Medicine there is a wide range of potential causes, including anxiety, caffeine, stress and strenuous activities in the evening. However, most hypnic jerks occur essentially at random in healthy people.
Another hypothesis is evolutionary, stretching back to our primate ancestors. A study at the University of Colorado has suggested that a hypnic jerk could be “an archaic reflex to the brain’s misinterpretation of muscle relaxation with the onset of sleep as a signal that a sleeping primate is falling out of a tree. The reflex may also have had selective value by having the sleeper readjust or review his or her sleeping position in a nest or on a branch in order to assure that a fall did not occur.”
During an epilepsy and intensive care study, the lack of a preceding spike discharge measured on an epilepsy monitoring unit, along with the presence only at sleep onset, helped differentiate hypnic jerks from epileptic myoclonus.
According to a study on sleep disturbances in the Journal of Neural Transmission, a hypnic jerk occurs during the non-rapid eye movement sleep cycle and is an “abrupt muscle action flexing movement, generalized or partial and asymmetric, which may cause arousal, with an illusion of falling”. Hypnic jerks are more frequent in childhood with 4 to 7 per hour in the age range from 8 to 12 years old, and they decrease toward 1 or 2 per hour by 65 to 80 years old.