Sleep problems affect a substantial proportion of individuals, and negative thoughts, rumination, and worry are considered key factors underlying persistence and severity. While extensive research has examined how these states disrupt sleep before falling asleep, much less is known about how mental concepts, including thoughts, expectations, intentions, and emotions, continue to influence sleep during sleep itself. Likewise, the potential benefits of sleep-promoting mental concepts during sleep remain underexplored. Here, we propose the Mental Concept Reactivation (MCR)-Hypothesis of Sleep, suggesting that mental concepts activated before sleep continue to influence sleep by spontaneous reactivations during sleep. Drawing on appraisal theories of emotion and evidence for endogenous memory reactivation, the hypothesis rests on three core premises. First, mental concepts relevant to sleep are activated during the day and particularly before bedtime. Second, these pre-sleep appraisals undergo spontaneous reactivations during sleep, with the intensity depending on their self-relevance. Third, these reactivations affect subjective restoration and objective sleep parameters. We present research supporting our hypothesis, particularly focusing on studies from our lab that began to systematically test key aspects of the MCR-Hypothesis and discuss limitations, alternative explanations, and open questions. Overall, the MCR-Hypothesis offers a testable framework for understanding how cognitive and emotional mental concepts influence sleep during this state. It may help to explain interindividual differences in sleep reactivity to stress, worry, and pre-sleep expectations. It offers a theoretical basis for developing non-pharmacological interventions aimed at reducing maladaptive mental concepts and strengthening beneficial ones, enhancing sleep quality in both clinical and non-clinical populations.

As populations age worldwide, pathological neurodegeneration with subsequent cognitive decline has emerged as a pressing public health challenge. As effective treatments remain limited once neurodegeneration has occurred, prevention and delay of decline onset are crucial. While physical exercise is associated with improved cognition, exercise-specific mechanisms remain unclear. We therefore examined the effects of 12 weeks explosive lower-body resistance training (RT) in community-dwelling older adults on proposed markers associated with brain metabolism and sleep. Thirty-six participants (64–81 years) completed pre and post assessments consisting of sensorimotor lactate and N-acetylaspartate (NAA) levels via 7 T magnetic resonance spectroscopy (analyzable lactate data: n = 24; NAA: n = 32), objective sleep architecture with wearable polysomnography (n = 34), and subjective sleep quality of the recorded nights with the revised sleep questionnaire A (n = 25) as well as of the preceding 4 weeks with the Pittsburgh Sleep Quality Index (n = 36). Participants were assigned to RT (3 sessions per week) or a control group (CON; maintaining lifestyle). After significant Group × Time interactions via repeated-measures ANCOVA with sex as covariate, paired t-tests revealed that RT significantly (p < 0.05) reduced resting lactate concentration (d = 0.78), improved objective sleep efficiency (d = 0.75), and prolonged total sleep time (d = 0.61). Our findings suggest that resistance training may be particularly beneficial for older adults experiencing fragmented sleep and impaired neural oxidative metabolism.

Sleep disturbances affect nearly half of all adults aged 60 and above, reducing quality of life and contributing to various health issues. While pharmacological treatments enhancing GABAergic activity can improve sleep, they often have adverse side effects. Given that balance training has been shown to enhance GABAergic inhibition in the sensorimotor cortex, we hypothesized that it could serve as a non-pharmacological intervention to improve sleep quality in older adults. In this study, 36 adults aged 64 to 81 years either completed a three-month balance training programme or served as controls. Before and after the three months, all participants underwent behavioural and neurophysiological measurements. Following a significant group × time interaction with repeated-measures ANOVA, paired t tests revealed that the balance group experienced improved sleep quality assessed with the Pittsburgh Sleep Quality Index questionnaire. However, objective sleep parameters assessed with polysomnography remained unchanged. Neuroimaging revealed increased GABA levels in the sensorimotor cortex (magnetic resonance spectroscopy) and enhanced sensorimotor functional connectivity (functional magnetic resonance imaging). Although intracortical inhibition (transcranial magnetic stimulation) during balancing and sleep did not show significant group × time interactions, individuals with greater increases in inhibition during sleep reported larger improvements in sleep quality. Regression analyses indicated that greater increases in functional connectivity were associated with larger improvements in subjective sleep quality, whereas greater increases in GABA levels were associated with smaller improvements. These findings suggest that balance training improves subjective sleep quality in older adults, possibly by restoring the GABAergic system in the sensorimotor cortex, offering an accessible and non-pharmacological intervention for age-related sleep disturbances. KEY POINTS: Sleep disturbances are common in adults over 60, and medications that enhance GABAergic inhibition often cause unwanted side effects. Previous research suggests that balance training can strengthen GABAergic inhibition in the sensorimotor cortex. In this study, three months of balance training improved subjective sleep quality in older adults, while no changes were observed in objective sleep measures. These improvements may be driven by enhanced GABAergic inhibition in the sensorimotor cortex, as reflected by increased functional connectivity and intracortical inhibition, two functional markers of GABAergic activity. These findings highlight balance training as an accessible and multi-beneficial intervention to promote sleep health in older adults.

Musical chords represent the most basic musical elements conveying emotional information. During wakefulness, different musical chord categories elicit distinct neuronal correlates and emotions, with major chords typically inducing more positive and minor chords with negative ones. However, it remains unclear whether the brain continues to process musical chords differently when presented during sleep. To address this question, we conducted a proof-of-concept study and presented musical major, minor, and dissonant chords to 47 healthy participants during nocturnal non-rapid eye movement (NREM) sleep. Prior to sleep, participants rated the chords in valence and arousal. Sleep was recorded using polysomnography. Our analysis of event-related responses during sleep revealed significant differences between the three musical chord categories. Major chords induced the strongest negative amplitude approximately 800 ms after chord onset, indicated as peak-to-peak (PTP) amplitude from the earlier positive peak. Minor chords showed intermediate PTP amplitudes, while dissonant chords elicited the lowest PTP amplitudes. In the time-frequency domain, these differences were also apparent, including differences in slow-wave, theta, alpha and sleep spindle bands across chord categories. Notably, experience in playing a musical instrument induced stronger differentiation of musical harmony during sleep compared with participants who never played a musical instrument. In conclusion, our findings suggest that the different processing of single musical chords persists during sleep, influenced by harmonic features and musical expertise in the context of Western musical conventions. Future research should explore whether longer and more complex harmonic features (e.g., chord sequences or musical pieces) are differentially processed by the sleeping brain. Statement of significance Whether the sleeping brain can process musical harmony remains an open question. In this study, we investigated the perception of musical harmony during sleep. Our results demonstrate that the sleeping brain is capable of differentiating between musical chords, and that this sensitivity is modulated by the level of musical expertise. Given the strong link between musical harmony and emotional processing, these findings have important implications for the use of music to influence emotions during sleep and dreaming and may even contribute to novel approaches for improving sleep quality in clinical settings.

Sleep is vital for physical and mental health and plays an important role in general well-being. Given the high prevalence of sleep disturbances in contemporary society, developing effective sleep-enhancing interventions, including non-pharmacological approaches such as hypnosis, is important. In this chapter, we will first discuss the nature of sleep and the factors that can disturb it. We will present scientific evidence on how hypnotherapy can improve sleep parameters and disturbances. This will be followed by the presentation of experimental studies highlighting the potential of hypnotic suggestions to modulate objective parameters of sleep depth. In conclusion, we will hypothesise on a potential mechanism by which hypnotic suggestions might be capable of modulating sleep.

Anticipation of stressful events can impair sleep quality. In a recent study, we reported that anticipating a stressful task before a nap led to negative changes in sleep parameters, particularly at the end of the nap. In our previous study, we compared stress anticipation with the anticipation of relaxation; thus, the observed effects may have been amplified by sleep quality improvements in the relaxation condition. In the current study, we aimed to replicate these findings using an alternative neutral control condition. The data from a newly collected sample (n = 31) were compared with the data from our previous study (n = 33) using identical analyses. The results reveal an opposite pattern from our previous study: participants in the neutral control condition showed poorer sleep (longer sleep onset latency, reduced slow-wave sleep, and lower SWA/beta ratio) compared to those anticipating stress. In a direct comparison of both studies, sleep parameters in the stress conditions were highly similar across the two studies, suggesting that the divergent outcomes are driven by differences in the control conditions. The temporal dynamic changes observed in our previous study could not be replicated. These findings highlight the importance of carefully considering control conditions in experimental sleep research and suggest that even "neutral" instructions can evoke anticipatory effects. Moreover, the observed benefits of anticipating post-sleep relaxation highlight opportunities for relaxation-based interventions to improve sleep quality.

Psychosocial stress induces stress responses at physiological and cognitive levels which disrupt subsequent sleep. A recent study from our group has shown that stress-induced changes are dynamic: stress before sleep led to prolonged sleep latency and changes in the early sleep period, while anticipated post-sleep stress resulted in changes in the late sleep period. A possible explanation is that anticipated stress is spontaneously reactivated during sleep in a dynamic fashion. As reactivations might be linked to dream content, we investigated the influence of anticipated stress vs. pre-sleep stress on dream content in early and late sleep periods. One group of participants performed either a stress or a relaxation task before sleep (pre-sleep group); while another group was informed before sleep that the stress or relaxation task will occur in the morning (post-sleep group). During sleep, they were regularly woken up in the early and late sleep periods and asked about their cognitive activity, while polysomnographic data was recorded. Fifty-five subjects participated and a total of 668 dreams was collected. While there was no difference in the incorporation of threat- and stress-related elements in the dreams, we did find a temporal difference in their emotionality. The threat intensity was influenced in a dynamic manner, with higher impact on early dreams when stress occurred before sleep and higher threat intensity in late dreams when stress was anticipated after sleep. Our results speak for a reactivation of mental activity in temporal proximity to the stressor for some parameters, mainly with impacts on the dream emotionality and length.

Sleep is essentially contributing to human health and well-being through multiple biological functions, including restoration and biosynthesis, brain clearance, energy metabolism, immunological and endocrine processing, synaptic plasticity, memory consolidation, and regulation of cognitive and emotional processes. Sleep disturbances are highly prevalent and are both a symptom and a contributing risk factor for psychiatric, neurological, and somatic disorders. Given the limitations of pharmacological interventions, noninvasive neuromodulation techniques ranging from noninvasive transcranial [transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), transcranial random noise stimulation (tRNS), temporal interference stimulation (tTIS), and transcranial ultrasound stimulation (TUS)] to peripheral sensory (auditory, olfactory, visual, tactile, vestibular) and electrical nerve (galvanic vestibular, transcutaneous vagus nerve, and median nerve) stimulation have gained increasing attention as potential tools to modulate sleep physiology. These techniques offer promising avenues for both therapeutic applications and fundamental research into sleep-dependent neuroplasticity, interregional communication, and oscillatory activity. However, sleep is not a uniform state but a highly complex and dynamic phenomenon, with intricate macrostructural [e.g., non-rapid eye movement (NREM)-rapid eye movement (REM) sleep balance, sleep efficiency] and microstructural (e.g., hierarchically nested slow waves and spindles) characteristics that contribute to a variety of functions. This complexity necessitates precise targeting strategies, often employing real-time brain state-dependent stimulation, to modulate specific sleep-related processes effectively. In this review, we summarize the functions of sleep and the available noninvasive tools for its modulation, addressing key methodological challenges and providing recommendations for best practices in sleep neuromodulation.

Study Objectives:Sleep spindles are potential biomarkers for memory decline in aging. However, significant within-person variability in spindle attributes complicates their utility in predicting cognitive deterioration. This study aimed to uncover distinct spindle types and their relevance to memory decline using exploratory, data-driven clustering. Methods:Polysomnography was collected from younger (n = 43, ages 20-45 years) and older cognitively healthy adults (n = 34, ages 60-81 years). Clustering analysis was performed using multiple features and spatiotemporal context, irrespective of participant age. Results:Resulting clusters were hierarchically defined by the sleep stage, slow oscillation concurrence, and hemisphere. Stage N3 spindles (15%; predominantly coinciding with slow oscillations) formed a distinct group, followed by N2 spindles coinciding with slow oscillations (27%). Remaining N2 spindles were categorized into unilateral (41%) and bilateral clusters (17%). In older adults, there was a lower proportion of N2 bilateral spindles and a higher proportion of N2 spindles concurrent with slow oscillations. Lower proportion of N2 bilateral spindles was associated with better composite memory performance in younger adults, whereas higher spindle power, regardless of cluster belonging, was associated with reduced memory benefit from sleep compared with wakefulness. Conclusions:Our results indicate differing expression of spatiotemporal spindle clusters in older age, as well as intertwined dynamics between spindle propagation, slow oscillation concurrence, and frequency shifts in aging. In addition, spindle heterogeneity aligned with global sleep stage dynamics. These results emphasize the interconnectedness of spindle activity with overall sleep patterns, underscoring the importance of spatiotemporal context within and across sleep stages.

Journal of Sleep ResearchEarly View e14160 LETTER TO THE EDITOR Probing the embodiment of sleep functions: Insights from cardiac responses to word-induced relaxation during sleep Matthieu Koroma, Corresponding Author Matthieu Koroma [email protected] orcid.org/0000-0003-2269-7841 Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Fund for Scientific Research FNRS, Brussels, Belgium Sleep & Chronobiology Laboratory, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Correspondence Matthieu Koroma and Athena Demertzi, Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging Center, University of Liège, Allée du 6 Août, 8 (B30), 4000 Sart Tilman, Liège, Belgium. Email: [email protected] and [email protected] Contribution: Conceptualization, ​Investigation, Writing - original draft, Methodology, Validation, Visualization, Writing - review & editing, Project administration, Formal analysis, Software, Data curation, Resources, Funding acquisition, SupervisionSearch for more papers by this authorJonas Beck, Jonas Beck orcid.org/0000-0002-4637-5911 Swiss Sleep House Bern, Department of Neurology, Inselspital, Bern, Switzerland Contribution: ​Investigation, Validation, Formal analysis, Data curation, Visualization, Writing - review & editing, Methodology, ResourcesSearch for more papers by this authorChristina Schmidt, Christina Schmidt orcid.org/0000-0002-9867-012X Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Fund for Scientific Research FNRS, Brussels, Belgium Sleep & Chronobiology Laboratory, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Psychology & Neuroscience of Cognition (PsyNCog), University of Liège, Liège, Belgium Contribution: Conceptualization, Writing - original draft, Methodology, Validation, Visualization, Project administration, Formal analysis, Supervision, Writing - review & editing, Funding acquisitionSearch for more papers by this authorBjörn Rasch, Björn Rasch orcid.org/0000-0001-7607-3415 Department of Psychology, University of Fribourg, Fribourg, Switzerland Contribution: Data curation, Resources, Project administration, Formal analysis, Visualization, Validation, Funding acquisition, ​Investigation, Conceptualization, Supervision, Methodology, Writing - review & editingSearch for more papers by this authorAthena Demertzi, Corresponding Author Athena Demertzi [email protected] orcid.org/0000-0001-8021-3759 Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Fund for Scientific Research FNRS, Brussels, Belgium Sleep & Chronobiology Laboratory, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Psychology & Neuroscience of Cognition (PsyNCog), University of Liège, Liège, Belgium Correspondence Matthieu Koroma and Athena Demertzi, Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging Center, University of Liège, Allée du 6 Août, 8 (B30), 4000 Sart Tilman, Liège, Belgium. Email: [email protected] and [email protected] Contribution: Supervision, Conceptualization, ​Investigation, Validation, Visualization, Writing - review & editing, Formal analysis, Project administration, Resources, Data curation, Methodology, Writing - original draft, Funding acquisitionSearch for more papers by this author Matthieu Koroma, Corresponding Author Matthieu Koroma [email protected] orcid.org/0000-0003-2269-7841 Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Fund for Scientific Research FNRS, Brussels, Belgium Sleep & Chronobiology Laboratory, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Correspondence Matthieu Koroma and Athena Demertzi, Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging Center, University of Liège, Allée du 6 Août, 8 (B30), 4000 Sart Tilman, Liège, Belgium. Email: [email protected] and [email protected] Contribution: Conceptualization, ​Investigation, Writing - original draft, Methodology, Validation, Visualization, Writing - review & editing, Project administration, Formal analysis, Software, Data curation, Resources, Funding acquisition, SupervisionSearch for more papers by this authorJonas Beck, Jonas Beck orcid.org/0000-0002-4637-5911 Swiss Sleep House Bern, Department of Neurology, Inselspital, Bern, Switzerland Contribution: ​Investigation, Validation, Formal analysis, Data curation, Visualization, Writing - review & editing, Methodology, ResourcesSearch for more papers by this authorChristina Schmidt, Christina Schmidt orcid.org/0000-0002-9867-012X Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Fund for Scientific Research FNRS, Brussels, Belgium Sleep & Chronobiology Laboratory, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Psychology & Neuroscience of Cognition (PsyNCog), University of Liège, Liège, Belgium Contribution: Conceptualization, Writing - original draft, Methodology, Validation, Visualization, Project administration, Formal analysis, Supervision, Writing - review & editing, Funding acquisitionSearch for more papers by this authorBjörn Rasch, Björn Rasch orcid.org/0000-0001-7607-3415 Department of Psychology, University of Fribourg, Fribourg, Switzerland Contribution: Data curation, Resources, Project administration, Formal analysis, Visualization, Validation, Funding acquisition, ​Investigation, Conceptualization, Supervision, Methodology, Writing - review & editingSearch for more papers by this authorAthena Demertzi, Corresponding Author Athena Demertzi [email protected] orcid.org/0000-0001-8021-3759 Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Fund for Scientific Research FNRS, Brussels, Belgium Sleep & Chronobiology Laboratory, GIGA-CRC In Vivo Imaging, GIGA Institute, University of Liège, Liège, Belgium Psychology & Neuroscience of Cognition (PsyNCog), University of Liège, Liège, Belgium Correspondence Matthieu Koroma and Athena Demertzi, Physiology of Cognition Lab, GIGA-CRC In Vivo Imaging Center, University of Liège, Allée du 6 Août, 8 (B30), 4000 Sart Tilman, Liège, Belgium. Email: [email protected] and [email protected] Contribution: Supervision, Conceptualization, ​Investigation, Validation, Visualization, Writing - review & editing, Formal analysis, Project administration, Resources, Data curation, Methodology, Writing - original draft, Funding acquisitionSearch for more papers by this author First published: 14 February 2024 https://doi.org/10.1111/jsr.14160Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Open Research DATA AVAILABILITY STATEMENT The data that support the findings of this study are openly available in OSF at https://osf.io/jn7ar/, reference number doi:10.17605/OSF.IO/JN7AR. REFERENCES Azzalini, D., Rebollo, I., & Tallon-Baudry, C. (2019). Visceral signals shape brain dynamics and cognition. Trends in Cognitive Sciences, 23(6), 488–509. https://doi.org/10.1016/j.tics.2019.03.007 10.1016/j.tics.2019.03.007 PubMedWeb of Science®Google Scholar Beck, J., Loretz, E., & Rasch, B. (2021). Exposure to relaxing words during sleep promotes slow-wave sleep and subjective sleep quality. Sleep, 44(11), zsab148. https://doi.org/10.1093/sleep/zsab148 10.1093/sleep/zsab148 PubMedWeb of Science®Google Scholar Chouchou, F., & Desseilles, M. (2014). Heart rate variability: A tool to explore the sleeping brain? 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IntroductionThe HGSHS:A is one of the most commonly used measures of hypnotic suggestibility. However, this test suffers from low feasibility due to a time requirement exceeding 1 h, and from a questionable representation of the normal population. Recently, a short version of HGSHS-5:G was developed and published, and now the first results are available. The scope of this investigation was to verify the assumption of equally positioned and normally distributed scores, resulting in equally sized suggestibility groups in a number of different studies with full or short versions of HGSHS, and to compare the results of the 11-item score with the 5-item score, the latter being calculated from either the full version or the short version test.MethodsData from 21 studies with testing for HGSHS were analyzed, 15 using the HGSHS:A full version and six using the HGSHS-5:G short version, for a total of 2,529 data sets. Position and distribution of both the 11-item score and the 5-item score were tested. Linear regression analysis was used to compare the two scores, as well as cross-table and weighted Cohen’s kappa to determine the match of grouping into low and high suggestibility. To evaluate contributing factors to the observed differences in the study results, a multifactorial analysis of variance was performed.ResultsIn the different studies, position and distribution of scores, as well as group sizes for low and high suggestibles, varied. All score distributions were found to be non-normal and shifted to the right from the middle score; the shift was more extensive with the 11-item score. The correlation between both scores calculated from full version tests was moderate (R2 = 0.69), as was the match of suggestibility grouping (κ = 0.58). Studies using the short version involving less student-dominated populations showed sufficient agreement with the full version, but lower scores were caused by an increase in the zero score.ConclusionA normal population is not represented in most applications of HGSHS, and grouping into low and high suggestibles varies, mainly due to different positions of score distributions. A direct comparison of full and short versions of HGSHS tested in the same subjects is still missing.

Targeted memory reactivation (TMR) is an effective technique to enhance sleep-associated memory consolidation. The successful reactivation of memories by external reminder cues is typically accompanied by an event-related increase in theta oscillations, preceding better memory recall after sleep. However, it remains unclear whether the increase in theta oscillations is a causal factor or an epiphenomenon of successful TMR. Here, we used transcranial alternating current stimulation (tACS) to examine the causal role of theta oscillations for TMR during non-rapid eye movement (non-REM) sleep. Thirty-seven healthy participants learned Dutch–German word pairs before sleep. During non-REM sleep, we applied either theta-tACS or control-tACS (23 Hz) in blocks (9 min) in a randomised order, according to a within-subject design. One group of participants received tACS coupled with TMR time-locked two seconds after the reminder cue (time-locked group). Another group received tACS in a continuous manner while TMR cues were presented (continuous group). Contrary to our predictions, we observed no frequency-specific benefit of theta-tACS coupled with TMR during sleep on memory performance, neither for continuous nor time-locked stimulation. In fact, both stimulation protocols blocked the TMR-induced memory benefits during sleep, resulting in no memory enhancement by TMR in both the theta and control conditions. No frequency-specific effect was found on the power analyses of the electroencephalogram. We conclude that tACS might have an unspecific blocking effect on memory benefits typically observed after TMR during non-REM sleep.

Theta oscillations support memory formation, but their exact contribution to the communication between prefrontal cortex (PFC) and the hippocampus is unknown. We tested the functional relevance of theta oscillations as a communication link between both areas for memory formation using transcranial alternating current stimulation (tACS). Healthy, young participants learned two lists of Dutch-German word pairs and retrieved them immediately and with a 30-min delay. In the encoding group (N = 30), tACS was applied during the encoding of list 1. List 2 was used to test stimulation aftereffects. In the retrieval group (N = 23), we stimulated during the delayed recall. In both groups, we applied tACS bilaterally at prefrontal and tempo-parietal sites, using either individualized theta frequency or 15 Hz (as control), according to a within-subject design. Stimulation with theta-tACS did not alter overall learning performance. An exploratory analysis revealed that immediate recall improved when word-pairs were learned after theta-tACS (list 2). Applying theta-tACS during retrieval had detrimental effects on memory. No changes in the power of the respective frequency bands were observed. Our results do not support the notion that impacting the communication between PFC and the hippocampus during a task by bilateral tACS improves memory. However, we do find evidence that direct stimulation had a trend for negatively interfering effects during immediate and delayed recall. Hints for beneficial effects on memory only occurred with aftereffects of the stimulation. Future studies need to further examine the effects during and after stimulation on memory formation.

STUDY OBJECTIVES:Voluntary sleep restriction is a common phenomenon in industrialized societies aiming to increase time spent awake and thus productivity. We explored how restricting sleep to a radically polyphasic schedule affects neural, cognitive, and endocrine characteristics. METHODS:Ten young healthy participants were restricted to one 20-minute nap opportunity at the end of every 4 hours (i.e. six sleep episodes per 24 hours) without any extended core sleep window, which resulted in a cumulative sleep amount of just 2 hours per day (i.e. ~20 minutes per bout). RESULTS:All but one participant terminated this schedule during the first month. The remaining participant (a 25-year-old male) succeeded in adhering to a polyphasic schedule for five out of the eight planned weeks. Cognitive and psychiatric measures showed modest changes during polyphasic as compared to monophasic sleep, while in-blood cortisol or melatonin release patterns and amounts were apparently unaltered. In contrast, growth hormone release was almost entirely abolished (>95% decrease), with the residual release showing a considerably changed polyphasic secretional pattern. CONCLUSIONS:Even though the study was initiated by volunteers with exceptional intrinsic motivation and commitment, none of them could tolerate the intended 8 weeks of the polyphasic schedule. Considering the decreased vigilance, abolished growth hormone release, and neurophysiological sleep changes observed, it is doubtful that radically polyphasic sleep schedules can subserve the different functions of sleep to a sufficient degree.

The first night in an unfamiliar environment is marked by reduced sleep quality and changes in sleep architecture. This so-called first-night effect (FNE) is well established for two consecutive nights and lays the foundation for including an adaptation night in sleep research to counteract FNEs. However, adaptation nights rarely happen immediately before experimental nights, which raises the question of how sleep adapts over nonconsecutive nights. Furthermore, it is yet unclear, how environmental familiarity and hemispheric asymmetry of slow-wave sleep (SWS) contribute to the explanation of FNEs. To address this gap, 45 healthy participants spent two weekly separated nights in the sleep laboratory. In a separate study, we investigated the influence of environmental familiarity on 30 participants who spent two nonconsecutive nights in the sleep laboratory and two nights at home. Sleep was recorded by polysomnography. Results of both studies show that FNEs also occur in nonconsecutive nights, particularly affecting wake after sleep onset, sleep onset latency, and total sleep time. Sleep disturbances in the first night happen in both familiar and unfamiliar environments. The degree of asymmetric SWS was not correlated with the FNE but rather tended to vary over the course of several nights. Our findings suggest that nonconsecutive adaptation nights are effective in controlling for FNEs, justifying the current practice in basic sleep research. Further research should focus on trait- and fluctuating state-like components explaining interhemispheric asymmetries.

IntroductionHypnotizability is conceptualized as a stable personality trait describing the ability to respond to suggestions given under hypnosis. Hypnotizability is a key factor in explaining variance in the effects of hypnotic suggestions on behavior and neural correlates, revealing robust changes mostly in high hypnotizable participants. However, repeated experience and training have been discussed as possible ways to increase willingness, motivation, and ability to follow hypnotic suggestions, although their direct influence on hypnotizability are still unclear. Additionally, it is important whether hypnotizability can be assessed reliably online.MethodsWe investigated the influence of the degree of experience with hypnosis and the presentation mode (online versus live) on the stability of hypnotizability in two groups of 77 and 102 young, healthy students, respectively. The first group was tested twice with the Harvard Group Scale of Hypnotic Susceptibility (HGSHS) after two weeks. During this period, participants either repeatedly listened to a hypnosis or trained on a progressive muscle relaxation or served as waitlist control group. In the secondgroup, participants performed both an online or offline version of the HGSHS, with varying time intervals (1–6 weeks).ResultsContrary to our expectations, hypnotizability declined from the first to second assessment in the first group. The reductionwas most prominent in initially highly hypnotizable subjects and independent of the experience intervention. We observed a similar reduction of hypnotizability in the second group, independent of presentation modality. The reduction was again driven by initially highly hypnotizable subjects, while the scores of low hypnotizable subjects remained stable. The presentation modality (online vs. offline) did not influence HGSHS scores, but the test–retest reliability was low to moderate (rtt = 0.44).DiscussionOur results favor the conclusion that generally, hypnotizability is a relatively stable personality trait which shows no major influence of preexperience or modality of assessment. However, particularly highly hypnotizable subjects are likely to experience a decline in hypnotizability in a retest. The role of the concrete assessment tool, psychological factors, and interval length are discussed. Future studies should replicate the experiments in a clinical sample which might have higher intrinsic motivation of increasing responsiveness toward hypnotic interventions or be more sensitive to presentation mode.

The active system consolidation theory assumes that sleep between encoding and retrieval promotes memory consolidation. In the present study, we cued new memories during slow-wave (SWS) or rapid eye movements (REM) sleep stages by presenting an instrumental music stimuli that had been previously presented during a learning session. In a within-subjects design, 18 participants slept for three nonconsecutive nights (cue during SWS, cue during REM, and no cue during control night) and were trained in a visuo-spatial memory task. The administration of cue during SWS produced better memory accuracy in comparison with REM and the control condition.