In 1986 Coleman developed the Owl and Lark Questionnaire to differentiate morning people from evening people, with owl individuals being more alert during the evening phase and lark individuals being more alert during the morning phase. Rossi has hypothesized that the bimodal peaks of hypnotizability found by Aldrich and Bernstein in 1987 were caused by alterations in owl and lark circadian rhythms. In the current study I used the Harvard Group Scale of Hypnotic Susceptibility, Form A to test compliance with hypnotic suggestions among 42 graduate students at three times of the day: in the morning, in the evening, and, as a control, in the middle of the night. Owls were more hypnotizable than larks in the morning, and larks were also significantly more hypnotizable in the evening than owls. There was no difference between the two groups in the middle of the night. A possible implication of this study is that one fundamental mechanism of therapeutic hypnosis is the entrainment of psychobiological rhythms.