What would make a person jerk a lot while sleeping?

Periodic Limb Move. You may have a condition called periodic limb movement disorder--random movements occuring during sleep. The exact cause is uncertain but possibly "neuronal hyperexcitability" in some area of the brain. Since cause is not known yet, treatment is suppressive. Treating the symptoms to allow better sleep etc...Consult doc if it is affecting your sleep quality or your partner's. Good luck.