Figure 1.

Overview of the experimental procedure. (A) Healthy young females listened either to a tape with hypnotic suggestions or a control tape while lying in bed, and were allowed to fall asleep afterward. The hypnosis tape included a standardized induction procedure, followed by a specifically developed metaphor of a fish swimming deep into the sea, repeatedly containing the suggestion to “sleep deeper” (see supplemental material, for a more detailed description). The control text had the same length and consisted of a documentation of natural mineral deposits. (B) All subjects participated in a hypnosis and a control condition, separated by 1 w. In the main experiment (experiment 1), participants suggestible to hypnosis (HS) listened to the hypnotic suggestion “to sleep deeper” (i.e., the fish). In experiment 2, only the hypnotic suggestion was altered now suggesting “to sleep shallower” (i.e., a boat, resting on the surface). In experiment 3 (demand characteristics), suggestible participants were informed that listening to verbal information before sleep increases subsequent slow wave sleep as the brain tries to consolidate the learned information. An incomprehensible version of the text was used as control condition. In experiment 4, low suggestible participants (LS) listened to the suggestion “to sleep deeper”. In experiment 5, LS participants were asked to simulate the effects of the hypnotic suggestion. In the control condition, all participants listened to the same neutral text (except in experiment 3).