Deepening sleep by hypnotic suggestion

Abstract

Study objectives: Slow wave sleep (SWS) plays a critical role in body restoration and promotes brain plasticity; however, it markedly declines across the lifespan. Despite its importance, effective tools to increase SWS are rare. Here we tested whether a hypnotic suggestion to "sleep deeper" extends the amount of SWS.

Design: Within-subject, placebo-controlled crossover design.

Setting: Sleep laboratory at the University of Zurich, Switzerland.

Participants: Seventy healthy females 23.27 ± 3.17 y.

Intervention: Participants listened to an auditory text with hypnotic suggestions or a control tape before napping for 90 min while high-density electroencephalography was recorded.

Measurements and results: After participants listened to the hypnotic suggestion to "sleep deeper" subsequent SWS was increased by 81% and time spent awake was reduced by 67% (with the amount of SWS or wake in the control condition set to 100%). Other sleep stages remained unaffected. Additionally, slow wave activity was significantly enhanced after hypnotic suggestions. During the hypnotic tape, parietal theta power increases predicted the hypnosis-induced extension of SWS. Additional experiments confirmed that the beneficial effect of hypnotic suggestions on SWS was specific to the hypnotic suggestion and did not occur in low suggestible participants.

Conclusions: Our results demonstrate the effectiveness of hypnotic suggestions to specifically increase the amount and duration of slow wave sleep (SWS) in a midday nap using objective measures of sleep in young, healthy, suggestible females. Hypnotic suggestions might be a successful tool with a lower risk of adverse side effects than pharmacological treatments to extend SWS also in clinical and elderly populations.

Keywords: high-density EEG; hypnosis; sleep; slow wave sleep.

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).

Figure 2

Effects of the hypnotic suggestions on sleep. (A) Highly suggestible subjects in experiment 1 almost doubled their amount of slow wave sleep (SWS) after the hypnotic suggestion “to sleep deeper” (white bar), with the SWS amount after the control tape set to 100% (black bar). Using the suggestion “to sleep shallower” the beneficial effect of hypnotic suggestions on SWS in a group of highly suggestible subjects was completely abolished (experiment 2). Similarly in experiment 3, no increase of SWS was observed after listening to verbal information, even though participants were informed previously that listening to verbal information should increase subsequent SWS (demand characteristics). In low suggestible subjects, the suggestion “to sleep deeper” even decreased the amount of SWS (experiment 4), and the direction of the effect was similar when subjects were asked to simulate the effect of hypnotic suggestion on subsequent sleep (experiment 5). (B) The hypnotic suggestion “to sleep deeper” specifically increased the amount of SWS in experiment 1, whereas time awake after sleep onset (W) was marginally reduced, leaving the other sleep stages unaffected (N1/N2: nonrapid eye movement sleep stage 1 and 2, R,: rapid eye movement sleep). Means ± standard error of the mean are indicated. +: P ≤ 0.08; *: P ≤ 0.05; **: P ≤ 0.01. (C) The hypnosis-induced increases in SWS significantly correlated with subjective increases in sleep quality in experiment 1.

Figure 3

Changes in slow wave activity (SWA) after hypnosis and control conditions in experiment 1. (A) Topographical distribution of SWA (0.5–4.5Hz) during nonrapid eye movement (NREM) sleep after listening to the hypnotic suggestion (upper panel) and the control text (lower panel). SWA is indicated as percent of total power (% PTOT; 0.5–50 Hz). Black dots represent electrode positions of the 128-channel electroencephalography (EEG) cap. (B) Topographical distribution of the difference of SWA %PTOT between hypnosis and control condition. White dots represent significant differences between the conditions, indicating a widespread increase in SWA during NREM sleep after the hypnotic suggestion “to sleep deeper”. (C) Representative spectrogram of electrode P4. Black triangles indicate significant increases (P < 0.05) in SWA after hypnosis as compared to the control condition, which are highly specific for SWA (gray area). Mean increases ± standard error of the mean (SEM) are indicated for the hypnosis condition, with the control condition set to 100% (dotted line). (D) Averaged SWA over three topographical regions (frontal, central, parietal, see supplemental material), for the hypnosis and control conditions. SWA is significantly increased after hypnosis as compared to the control condition (P < 0.03), and the effect is stronger over central and parietal as compared to frontal regions (interaction type of text * topography: P = 0.03). Means ± SEM are indicated. * P ≤ 0.05; *** P ≤ 0.001, for post hoc contrasts.

Figure 4

Theta activity during listening to the hypnosis versus the control text before sleep. (A) Topographical distribution of theta activity (4.5–8.0 Hz) during listening to the hypnotic suggestion (upper panel) and the control text (lower panel). Theta power is indicated as percent of total power (% PTOT; 0.5 - 50 Hz). Black dots represent electrode positions of the 128-channel electroencephalography (EEG) cap. (B) Topographical distribution of the difference of theta % PTOT between the hypnosis and the control condition. White dots represent significant differences between the conditions (P < 0.05), indicating a widespread increase in theta power during listening to the hypnotic suggestion “to sleep deeper”. (C) Representative spectrogram of electrode P4. Black triangles indicate significant increases (P < 0.05) in theta power during listening to the hypnosis as compared to the control text, which occur mainly in the lower theta band (gray area). Mean increases ± standard error of the mean are indicated for the hypnosis condition, with the control condition set to 100% (dotted line). (D) Increases in parietal theta activity during listening to the text with hypnotic suggestions as compared to the control text are highly predictive for subsequent changes in slow wave sleep duration between the experimental conditions.