Participants

Ten female students with mean (M) age = 21.6 years (standard deviation, SD = 2.5) were selected to test three different conditions on three consecutive nights, counterbalanced in order. Inclusion criteria included good dream recall, no previous lucid dream experience, and a positive attitude toward hypnosis. Together with a questionnaire on inclusion criteria, applicants were sent information about the study, explaining its aim, important theoretical constructs, and a short walkthrough of the study. In addition, they were instructed to train in the performance of specific eye movements at home. At the time of data collection (2009–2010), ethical review and approval was not required for the study on human participants in accordance with local legislation and institutional requirements (Institute of Sports and Sports Sciences, Heidelberg University, Germany). Participants provided written informed consent before the beginning of the study and the experiment was conducted in accordance with the Declaration of Helsinki. Participation was compensated by 90 €.

Polysomnography

Polysomnography (PSG) was applied using silver cup electrodes attached according to the American Academy of Sleep Medicine’s (AASM) guidelines for PSG using the 10–20 system. Sixteen electrodes were used in total, with six electroencephalography (EEG) electrodes, two each for frontal (F3/F4), central (C3/C4), and occipital (O1/O2) regions. Two electrooculogram (EOG) channels, two electromyogram (EMG) channels on the chin, and one electrocardiogram (ECG) channel were used. Reference electrodes (M1/M2) for EEG and EOG were placed on the mastoids. The EMG reference was placed on the chin, and the ECG electrode was in the middle of the lower right rib. An electrode above the nasion served as the primary reference. The visualization of PSG used sampling rates of 500 Hz and the lower and upper filter cut-off frequencies were set in accordance with AASM guidelines. The EEG channels had an indicated sensitivity of 7 μV, the EOG had a sensitivity of 30 μV, and the EMG and ECG had a sensitivity of 20 μV. For lucid dream verification, the participant executes a specific sequence of predetermined eye movements upon lucidity, which the EOG electrodes then pick up. In this study, a left–right–left–right (LRLR) sequence was used.

During the study, participants slept in a soundproof room located in the sleep laboratory while experimenters sat in a separate control room. The participants were able to communicate with the experimenter through an intercom system. Sleep recordings were done using an Xltek amplifier (model: TrexTM Ambulatory EEG and PSG System), and data were transferred to the recording computer using the Xltek NeuroWorks program (Natus, Middleton, WI, USA). The auditory stimulus was recorded and edited with the Audacity® program (Audacity Team; https://audacityteam.org/) and transmitted via loudspeakers. An additional amplifier channel was used for PSG to mark stimulation times in the recording.

Assessment of susceptibility

In the present study, the term hypnotic enhancement is used as opposed to posthypnotic suggestion, as an acoustic suggestion was used as a cue, with additional hypnosis to potentially strengthen its efficacy. The suggestibility of participants was assessed using the German version of the Stanford Hypnotic Susceptibility Scale [4]. First, the session induces a suggestible state in the participant, followed by 12 test suggestions (e.g., lower the outstretched right arm). The duration of the assessment was about 1 h. If the participant was susceptible to one of the 12 test suggestions, a point was awarded, resulting in a final score of 0–12. The participants in this study had an average susceptibility score of M = 6.2 (SD = 2.6). One participant scored 2, three participants scored 4, and the rest scored 6 or higher.

Assessment of hearing and wake-up threshold

Sensory stimulation during sleep requires a particular intensity and is controlled by a delicate balance. If the stimulation is too low in intensity, the cue might not reach the dreamer during sleep. On the other hand, if the stimulation is too high in intensity, the participant might experience arousal or even awakening. In the case of acoustic stimulation, the intensity levels apply to the volume of the presented stimulus [5].

Because of this, it is essential to find the perfect volume thresholds. Thus, a hearing and a wake-up volume threshold were established before sleep. The hearing threshold test was performed in a lying, supine position in bed. For this test, the volume of the acoustic stimulation was altered, and the participant had to indicate whether they were able to hear the stimulus. The hearing threshold was set at the lowest volume value at which the sentence was perceived during wakefulness but not necessarily understood.

Once a participant entered the first or second REM stage, the wake-up threshold was established. The auditory stimulation for inducing a lucid dream, i.e., the phrase “You are dreaming,” was presented through two speakers attached about 2 m above the head of the bed. The wake-up threshold was determined by starting stimulation with a volume of 20 steps above the hearing threshold. Then, every 30s, the volume was increased by two steps until the participant woke up. On the first night, the mean of two test wake-up thresholds was used as the final wake-up threshold of the first night. The wake-up threshold of the second night was the mean of the first night’s final wake-up threshold and the second night’s test wake-up threshold.

Conditions and procedureConditions

In this experiment, three different conditions were tested for their potential to induce lucid dreams on three consecutive nights that were counterbalanced in order.

Acoustic suggestion.

The acoustic suggestion included presentation of the phrase “You are dreaming” from the third REM phase onward. This condition tests acoustic suggestion during sleep and simultaneously acts as a control for the hypnotic enhancement condition in this study. This first condition tests the potential of the acoustic suggestion on its own.

Acoustic stimulation.

As a control condition for the acoustic suggestion, acoustic stimulation was included in the procedure, controlling for acoustic suggestion by presenting solely the participant’s name, avoiding any literal suggestion of becoming lucid.

Hypnotic enhancement of acoustic suggestion.

Finally, a condition that enhances the acoustic suggestion using a hypnosis protocol was used. In this condition, the hypnotic enhancement was first performed after the hearing threshold had been established. It started with fractionation to bring the participant into a relaxed state [15]. Afterward, the lucid dream experience was suggested, followed by practicing the reaction to becoming lucid while presenting the acoustic suggestion “You are dreaming.” The simulated lucid dream was signaled by LRLR eye movement. After the hypnosis session was finished, the participant was instructed to sleep. This procedure was repeated from the third REM phase onward.

Experimental procedure

Lucid dream induction was performed after the wake-up threshold had been successfully established, usually starting from the third REM phase. Stimulation began 5 min after the occurrence of the first rapid eye movement. The volume was increased continuously, starting at 10 volume steps below the wake-up threshold. The stimulation intervals were increased to 60 s compared to the 30-second intervals during the wake-up threshold condition to avoid overstimulation and give participants enough time to incorporate the stimulus into the dream. Stimulation was stopped if the participant switched from REM to another sleep stage.

In some cases, REM sleep reappeared within a few minutes, allowing stimulation to continue. If the participant became lucid during stimulation, they were instructed to perform three LRLR eye movements, each separated by a 5-second break. After a maximum of 10 stimulations and an additional final waiting period of 60 s, the participants were awakened for a dream report. After each REM awakening, any thoughts before awakening were collected, together with the potential perception of a stimulus and whether this stimulus had led to waking up.

Dream content analysis

Before dream content analysis, the original dream transcripts were processed [23]. Thereafter, based on the dream reports, the research team (DE and FM) established whether the participant had realized that they were dreaming in the dream.

Criterion for successful lucid dream induction

The lucid dream induction counts as successful if three different types of proof hold [8]: (1) the participant’s subjective self-rating of lucidity; (2) the dream report rated by external judgement as having either possible or clear signs of lucidity; (3) the participant reports LRLR eye signaling, which can be unambiguously identified on the PSG recording by external rating by the research team. All three proofs must hold true for the “strict” criterion of successful lucid dream induction. For the “loose” criterion, (1) and (2) were considered sufficient.

Data analysis

Using the Neuroworks program, each PSG recording was scored following the AASM manual for sleep scoring [13]. The hypnogram data were exported and further analyzed. Sleep parameters such as sleep onset latency, sleep duration, REM sleep duration, and others were compared between the three nights and conditions to control for any confounders affecting sleep quality other than the three different conditions. An ANOVA test was performed for each sleep parameter. The percentage of lucid dreams was calculated following the liberal and conservative approaches.