Hypnagogic myoclonic jerk or hypnic jerk occurs when your body suddenly jerks just as you are about to drift off to sleep. From what I could understand from the articles I read, this happens when your brain misinterprets the feeling of falling asleep with the feeling of actually falling. In response, your body jerks to brace itself for the fall.
Another possibility is periodic limb movement, also kown as nocturnal myoclonus. This is similar to restless leg syndrome and causes the legs- and sometimes the arms- to continuously cramp, jerk or move while a person is sleeping. This movement is sometimes strong enough to wake the person, resulting in a sleepless night. There are two types of this disorder. One, primary PLM occurs for unknown reasons. The other, secondary periodic limb movement can be caused by the following:

•Diabetes mellitus


•Iron deficiency


•Spinal cord tumor


•Spinal cord injury


•Sleep apnea syndrome - Breathing difficulties that disrupt sleep, causing daytime sleepiness and a number of other problems


•Narcolepsy - A sleep disorder involving excessive sleepiness and overpowering urge to sleep during waking hours


•Uremia - Build-up of waste products in the blood because of poor kidney function


•Anemia - Low level of hemoglobin, the substance that carries oxygen in the blood


•Medication - Neuroleptics and other antidopaminergic agents such as Haldol, dopaminergic agents such as Sinemet (despite the fact that Sinemet is often a treatment for PLMD), or tricyclic antidepressants such as amitriptyline (Elavil)


•Withdrawal from sedative medications such as barbiturates or benzodiazepines (such as Valium)