A quick, sharp, sudden movement.

Head jerks during sleep in a patient with [[obstructive sleep apnea]]
((Lim JA, Jun JS, Yang TW, Park B, Kim KT, Lee HE, Lee SK, Jung KY. Head jerks
during sleep in a patient with obstructive sleep apnea. Sleep Med. 2019 Feb
28;57:141-143. doi: 10.1016/j.sleep.2019.02.013. [Epub ahead of print] PubMed
PMID: 30981958.
)).