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 (or other mammal) 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 hypnic jerks - such as caffeine, nicotine, exercise, stress etc etc. The cause is unknown, and none of the several theories that have attempted to explain them has been fully accepted.

See Wikipedia

Note: Hypnic jerks are not thought to be linked with Restless leg syndromeplugin-autotooltip__plain plugin-autotooltip_bigRestless leg syndrome

Restless leg syndrome, also known as Willis-Ekbom disease, is a common condition of the nervous system that causes an overwhelming and irresistible urge to move the legs. Estimates are that around 10% of people suffer from RLS at some stage in their lives. It often occurs during sleeping hours and can cause severe insomnia. In the majority of cases there's no obvious cause (i.e. it's termed