Hypnic jerks are a of the sleep process, so many doctors believe it\’s just the body \”twitching\” as it slowly shuts down for rest. Another leading theory, Breus says, is that it happens when the body goes through the first sleep stage too quickly, because it\’s so exhausted. During the first stage of sleep, which usually only lasts a few minutes, your breathing and heart rate slows down and your sleep is still very light. If the body speeds through this stage and \”shuts down\” too fast, it might trigger the brain to think your vitals are actually failing and in response, it jerks the body awake. \”It might be a kind of protective mechanism, but we really aren\’t sure because it\’s difficult to study,\” Breus says.
According to the 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 and 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.
According to a study on sleep disturbances in the Journal of Neural Transmission, a hypnic jerk occurs during the 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.