A hypnic jerk (also called a hypnagogic massive jerk) is a common, though poorly researched phenomenon that happens when people are about to drift off to sleep. Many experience a kind of electric jolt, or a sensation of falling—usually when they are exhausted or are in an uncomfortable sleeping position. Eight out of 10 people experience hypnic jerks on an occasional basis; only 1 in 10 claim to rarely, or never, experience them.
While scientists haven’t completely found the explanation for this phenomenon, some theories have been presented. The first involves changes in body temperature and muscle tension. Apparently, as people relax, the brain gets confused and thinks the muscles are failing or the body needs to be stimulated. It then jerks the limbs in an effort to “wake up” and keep the body functioning.
The hypnic jerk is a reflex used to keep the body functioning. The brain might register falling asleep as a situation in which the body needs to be stimulated.
Other scientists trace hypnic jerks to R.E.M or rapid eye movement, a part of th sleep cycle where people dream. Usually R.E.M. is accompanied by lower muscle tone and involuntary twitching. Some also theorize that people who go through hypnic jerks have entered the dream cycle but are unable to relax muscles – and hence “act out” their dreams.
Some people experience hypnic jerks when they wake up. They go through “brain glitches” where they are blinded by light or hear a loud snapping, cracking noise. These cases are far rarer, however.
Every night, one in 10 people experience hypnic jerks.