No. 26: Mar-Apr 1983
Observations of individuals undergoing hypnosis suggest that intervals of particular susceptibility come along about every 90 minutes. Expert hypnotherapists are cognizant of this psychophysical cycle and often wait for it to appear. By closely watching the subject's swallowing, eye-blinking, respiration, etc., the hynotizer can take advantage of these periods of heightened susceptibility.
(Rossi, Ernest L.; "Hypnosis and Ultradian Cycles: A New State(s) Theory of Hypnosis?" American Journal of Clinical Hypnosis, 25:21, 1982.)
Comment. The reason for this 90-minute rhythm and the control system behind it are obscure.