Fluctuations in hypnotic susceptibility and imaging ability over a 16-hour period

Within-subject variability for hypnotic susceptibility as measured by the Harvard Group Scale of Hypnotic Susceptibility, Form A and for imaging ability as measured by the Vividness of Visual Imagery Questionnaire was determined over a 16-hour period. Half of the subjects were day persons, those most alert during daytime hours (as determined by the Alertness Questionnaire); the remaining subjects were night persons. For day persons, hypnotic susceptibility was greatest at 10:00 a.m. and 2:00 p.m.; for night persons, susceptibility was greatest at 1:00 p.m. and between 6:00 p.m. and 9:00 p.m. Imaging ability also varied as a function of time of administration. However, these peak periods occurred before and after hypnotic susceptibility peaks. Such a pattern was interpreted as indicating the possible existence of an ultradian cycle for imaging ability.