The Effect of Time of Day on Hypnotizability: A Brief Communication

Hypnotizability has been shown to be very stable for long periods of time in a person's adult life when no attempts at modification are made (Morgan, Johnson, & Hilgard, 1974). Even though it can be improved by several different means, attempts to modify individual hypnotizability may only allow people to reach a "plateau" level of hypnotizability that is predetermined for each person. The present experiment addressed the question of whether hypnotizability like body temperature, reflex response intensity, and performance on various mental and motor tasks, fluctuates in a circadian rhythm. The Harvard Group Scale of Hypnotic Susceptibility, Form A (HGSHS:A) of Shor and E. Orne (1962) was used to test compliance with hypnotic suggestions among several large groups of Ss at different hours of the day. A plot of the mean HGSHS:A scores obtained at each time of day showed a bimodal distribution, with the highest mean score significantly different from 2 of the 3 minima. Although the second peak did not differ significantly from the minima, its Occurrence was consistent with the bi-modal distributions which sometimes occur in other circadian rhythms, such as body temperature and complex mental and motor skill performance. One possible implication for the results of this study is that they suggest that hypnotherapy could be more effective for individuals at different times of the day, as they approach their "plateau" level of suggestibility.