Hypnic jerks (a.k.a. Hypnagogic jerks, sleep starts, sleep twitches, myoclonic jerks, night starts etc) are brief and sudden involuntary muscle contractions (twitches) which happen as a person is beginning to fall asleep. They can be severe enough to wake the sleeper. Approximately 70% of people have experienced them.
They are benign, and occur randomly in healthy people.
Various factors are known to increase the likelihood of HJ,- such as caffeine, nicotine, exercise, stress etc etc. The cause is unknown, and none of the several theories that have attempted to explain it has been fully accepted.
Note: Hypnic jerks are not thought to be linked with