Day persons, night persons, and variability in hypnotic susceptibility.

Day persons (those most alert during daytime hours) and night persons participated in 2 experiments to examine within-subject variability of scores over time on the Harvard Group Scale of Hypnotic Susceptibility, Form A, and the Stanford Hypnotic Susceptibility Scale, Form C. Regardless of scale used, day persons exhibited peak susceptibility at 10 a.m. and 2 p.m.; for night persons, peak susceptibility was found at 1 p.m. and between 6 p.m. and 9 p.m. Furthermore, 2 peaks of temperature increase (1 p.m. and 6 p.m.) appeared to be associated with peaks in hypnotic susceptibility. However, such was shown to be associated with periods of food intake rather than with increases in hypnotic susceptibility. Results are examined with respect to methodological concerns and the potential existence of ultradian rhythms for hypnotic responsiveness.