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Hypnosis Can Increase Deep Sleep

Controlled and blinded experiments using EEG show increase in deep sleep in suggestible women after listening to specific hypnotic suggestions

Posted on January 29 2020

So, we know music can improve sleep, even for clinical insomniacs.

But, Pzizz is a combination of music AND voiceover. So does the voice actually help? Certainly anecdotal evidence would indicate so, but let’s go deeper.

Pzizz’s scripts include many approaches, including progressive relaxation, breathing, hypnosis and lots more.

**Wait, did you just say HYPNOSIS?!**

Yes, hypnosis. Now, don’t get silly about it. This is clinical hypnosis, not the absurd stuff you see in stage shows (as much as I would like to get you clucking like a chicken…).

But clinical or not, there’s still lots of questionable research about hypnosis. It’s hard to find real, unbiased work done on the topic, but we managed to uncover a study in a sleep journal with no connection to hypnosis companies, performed by trained university Ph.D scientists, with proper controls, statistical measures, and methods.

Reading further into it, the study showed that there is evidence that hypnosis, under certain conditions, can be effective.

With the right induction procedure, specific hypnotic suggestions, and suggestible participants, hypnosis can alter sleep patterns for the better. Specifically, hypnosis can increase the time spent in deep sleep (a very important part of the sleep cycle for well-being).

Furthermore, because of the careful controls and multiple versions of the experiment, it seems very clear that these effects are not by chance, nor due to the placebo effect, nor due to participant characteristics, nor due to participants trying to please the researchers.

Ultimately, this study lends weight to the idea that hypnosis can help people achieve increased healthy deep sleep. Which is very much in line with what we find from customer feedback at Pzizz.

Pzizz Researcher Write-Up

Cordi, M. J., Schlarb, A. A., & Rasch, B. (2014). Deepening sleep by hypnotic suggestion. Sleep37(6), 1143–52.


Slow wave sleep is necessary for human health and well-being, as well as memory consolidation. This stage of sleep has been shown to decrease with age, and this decrease has been associated with the development of several neurological illnesses. Prescription drugs aimed at improving sleep (or more accurately: sedating people) have been shown to decrease slow wave sleep. These drugs can also have serious negative side effects, and are often highly addictive. Slow wave sleep (also called deep sleep) is basically stage three of the sleep cycle. It is called slow wave because an EEG will show synchronized low-frequency activity during this stage. EEGs, or electroencephalograms, use electrodes placed on the scalp to measure brain activity by detecting electrical changes.

One possible method for increasing and/or maintaining healthy levels of slow wave sleep is hypnosis. Hypnosis has been defined in many ways, but a simple definition is that it is a state of altered consciousness, achieved after an induction procedure, wherein the hypnotized person is highly ‘focused and absorbed’. Hypnotic suggestions are prompts given to a hypnotized person that are intended to alter their behavior.

While several studies have shown hypnosis to be effective at improving sleep quality, they often have too few participants or lack the proper methodology or measures to be truly valid/reliable. Cordi, Schlarb & Rasch (2014) carefully designed multiple experiments in which their nearly seventy study participants listen to hypnotic suggestions to “sleep deeper” and then used an EEG to measure changes in brain-wave patterns during sleep. The participants were young healthy females. They were given either a hypnotic suggestion tape, or a control tape, which they listened to while lying in bed, and then were permitted to nap afterwards. Several variations of the experiment were conducted.

Participants, Method, and Design

In the first experiment, the hypnotic tape included a standard induction technique followed by a metaphor of a fish swimming deeper and deeper into the sea. This metaphorical story repeatedly contained the suggestion to ‘sleep-deeper’. The control tape was the same length, but contained a recording of someone documenting natural mineral deposits.

Participants were chosen on the basis of being suggestable according to the Harvard Group Scale for Hypnotic Suggestibility. None of the participants had irregular shift work during the previous six weeks, and none of them had a history of neurological or psychiatric disorders. Three participants were excluded because their sleep diaries indicated highly irregular sleep and/or frequent daytime naps. They all indicated normal sleep based on the Pittsburgh Sleep Quality Index. None of the participants took sleep altering medication, nor regularly engage in after-lunch naps, and they were asked to refrain from caffeine or alcohol on the test day. Participants were paid for their time, and an ethics committee at the University of Zurich approved the study.

All participants, except those in experiment 3, were informed of the purpose of the study (i.e. to deepen sleep using hypnosis). Once the participants were in bed, the lights were shut off and the tape was played. Participants listened to the 13-minute tape, and were allowed to fall asleep during or directly after. In all cases they were awakened after 90 minutes. After awakening, participants filled out a subjective sleep quality questionnaire. Before bed participants performed several memory-related tasks.

Five experiments were conducted. Each used a within-subject design, meaning that participants underwent both experimental and control conditions.

Experiment 1

In experiment 1, suggestible participants listened to a hypnotic fish metaphor tape which consisted of a slow, quite, calm voice using relaxing words like ‘deeper’, ‘let go’, ‘relax’, and ‘easily’. The control tape consisted of a normal volume voice at normal speed with no words that were ostensibly arousing or relaxing.

The results showed a statistically significant increase in slow wave sleep in the subsequent nap, for the experimental (hypnotic suggestion) condition. Participants in experiment 1 who listened to the hypnotic suggestion tape nearly doubled the amount of slow wave sleep compared to those who had listened to the control tape. It appears that the hypnotic suggestions had a strong influence on slow wave sleep. This effect was specific for slow wave sleep, whereas other sleep stages were not altered.

Experiment 2

In experiment 2, a new group of highly suggestable young adult females listened to hypnotic and control tapes, but this time the hypnotic suggestion was to ‘sleep shallower’ (i.e. a metaphor of a boat resting on the surface of the ocean). The induction procedure, the slow/relaxing voice, and the inclusion of relaxing words were all the same as experiment 1. Because all participants were told before the experiments that hypnotic suggestion could help them sleep more deeply, it is possible that the results of experiment 1 are mealy due to the placebo effect. However, the results of experiment 2, showed a non-significant decrease in the slow wave sleep of those in the experimental condition (i.e. boat on the surface of the sea version of the hypnotic suggestion). This indicates that the results of experiment 1 were not due to the placebo effect, but rather are due to the specific kind of hypnotic suggestion used.

Experiment 3

In experiment 3, researchers wanted to test whether it was the demand characteristics of the study that caused changes in slow wave sleep. To test this, researchers told people that just listening to verbal information about sleep would increase slow wave sleep. The researchers used the mineral deposit tape (the control in all the other experiments) as the experimental condition, and they used an incomprehensible version of the text as the control. They made the text incomprehensible by filtering out the high frequencies, which kept the intonation and length, but made the words unrecognizable (almost as if being listened to through a wall). Participants were told that the comprehensible version would improve slow wave sleep, whereas the incomprehensible version would not. The experimental condition had no significant effect, implying that the demand characteristics of the test (or the tendency for study participants to conform to the perceived demands of the experimenters) were not the cause of the changes in experiment 1 or 2.

Experiment 4

Experiment 4 used the same procedure as experiment 1, except the participants were people ranked as having low suggestibility on the Harvard Group Scale for Hypnotic Suggestibility. In contrast to the highly suggestable group, the relatively unsuggestable women in experiment 4 showed no effect of listening to the hypnotic suggestions. This indicates that suggestibility is an important factor in experiment 1 and 2’s results.

Experiment 5

In experiment 5, highly suggestible participants were asked to simulate the effects of hypnotic suggestion. Despite the hypnotic suggestions, and the instructions to ‘simulate’ a hypnotic effect, no significant change in slow wave sleep was observed. Interestingly, the combined results of experiment four and five showed that the experimental condition (hypnotic suggestion) decreased slow wave sleep. This implies that people with low suggestibility may actively counteract the effects of the hypnosis.

General Discussion

In all the experiments using an audio tape, the same male voice was heard. Sleep latency, or the time taken to fall asleep, on average was longer then the tapes, and did not differ significantly between groups. This implies that sleep latency differences between the experiments is not responsible for the observed changes in slow wave sleep.

In experiment 1, control and experimental groups did not show an average difference in subjective (self-report) sleep quality, yet on an individual level, participants rated sleep after the hypnosis as being of a better quality then their sleep after the control tape. The memory tasks showed no effect. In all experiments which used the deep sleep suggestions on highly suggestable participants (1, 2, and 4), an increase in Theta activity (4.5–8 hz brain waves) was observed while listening to the hypnotic (experimental) tapes. Theta activity is thought to be related to hypnotic trances, as well as feelings of sleepiness or drowsiness. No subjective measures of drowsiness were taken. The relation between increase of Theta activity and increase in slow wave sleep almost reached significant levels. This implies that the immediate impact of the hypnosis is related to its later effects on slow wave sleep duration.

Sleep measures were scored by two experts, blind to the conditions of the participants. Standard statistical measures for within subject design were used, and the P-Value cutoff used was the standard value in psychology research (.05).

These results indicate that hypnotic suggestion can improve slow wave sleep, depending on the type of suggestion, and assuming that the participant is highly suggestable.


Limitations of the study include a lack of a non-intervention control, meaning that all the controls involved some sort activity (although this can also be seen as a strength). The memory tasks beforehand could also possibly have had some effect, although this would not explain the differences between control and experimental conditions. Further limitations include the limited range of participants, who were all young healthy females, making the study less generalizable. Also, the fact that this study focused on naps, limits its generalizability to bedtime sleep, although there is no immediately apparent reason that these results might be replicated in nightime sleep.

The differences between experiment 1 (sleep deeper) and experiment 2 (sleep shallower) imply that it was not the instructions beforehand that caused the differences, but rather the specifics of the hypnotic suggestion. The inverse effect of the suggestion on unsuggestable participants (experiment 4 and 5) is in keeping with previous research showing active counteracting of hypnotic suggestion on the part of unsuggestable individuals. However, previous research has shown that emphasizing the procedure as a test of imagination ameliorates this effect to some degree. This inverse effect adds even more evidence to the notion that it was not simply the relaxing nature of the hypnotic tape, but rather a combination of specific suggestions and the participants suggestibility that contributed to extended slow wave sleep.

Methodology and Authenticity

This study meets all the basic criteria for a clinical valid study, including blinding, controls, and proper statistic measures. It was published by a reputable journal (based on the impact factor, it’s inclusion in medical databases, and the fact that it is peer reviewed and sponsored by medical associations). No red flags were observed. In addition, the authors included a disclosure statement (below).

Disclosure Statement

“This was not an industry supported study. Financial support was from a Grant of the Swiss National Foundation (SNF) (PP00P1_133685) and the Clinical Research Priority Program (CRPP) “Sleep and Health” of the University of Zurich. The work was performed at the University of Zurich, Institute of Psychology, Department of Biopsychology, Zurich, Switzerland. The authors have indicated no financial conflicts of interest.”

Short Study Summary

“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 years

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.”


What we already knew

What this paper added

  • Hypnosis can increase time spent in deep sleep (a.k.a slow wave sleep).
  • This study is one of the first that uses a large enough sample size, proper controls, and EEG measurements to demonstrate the effectiveness of hypnosis.
  • Hypnotic effects do not appear to be based on the placebo effect, or particpants trying to meet researcher’s expectations.
  • The effectiveness of hypnotic suggestion is dependent on the specific nature of the suggestion, as well as the suggestability of the participants.

Further Readings

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