Sleep and dreaming in the light of reactive and predictive homeostasis

Dreams are often viewed as fascinating but irrelevant mental epihenomena of the sleeping mind with questionable functional relevance. Despite long hours of oneiric activity, and high individual differences in dream recall, dreams are lost into oblivion. Here, we conceptualize dreaming and dream amnesia as inherent aspects of the reactive and predictive homeostatic functions of sleep. Mental activity during sleep conforms to the interplay of restorative processes and future anticipation, and particularly during the second half of the night, it unfolds as a special form of non-constrained, self-referent, and future-oriented cognitive process. Awakening facilitates constrained, goal-directed prospection that competes for shared neural resources with dream production and dream recall, and contributes to dream amnesia. We present the neurophysiological aspects of reactive and predictive homeostasis during sleep, highlighting the putative role of cortisol in predictive homeostasis and forgetting dreams. The theoretical and methodological aspects of our proposal are discussed in relation to the study of dreaming, dream recall, and sleep-related cognitive processes.

The route to recall a dream: theoretical considerations and methodological implications

The goal of this paper is to shed new light on the relation between dream recall and dream experiences by providing a thorough analysis of the process that leads to dream reports. Three crucial steps of this process will be distinguished: dream production (the generation of a conscious experience during sleep), dream encoding (storing a trace of this experience in memory) and dream retrieval (accessing the memory trace upon awakening). The first part of the paper will assess how major theories think about the relationship between dream reports and these distinct steps. The second part will systematise how trait and state factors affecting dream recall-given different theoretical assumptions-might interact with dream production, encoding and retrieval. Understanding how the distinct steps of dream recall can be modulated by different factors is crucial for getting a better grip on how to acquire information about these steps empirically and for drawing methodological conclusions with regard to the tools dream research relies on to collect subjective data about dream experiences. The third part of the paper will analyse how laboratory reports, logs and retrospective scales interact with the different factors that affect the distinct steps leading to dream reports and will argue that prospective methods provide more direct access to data regarding dream production and encoding than retrospective methods, which-due to their inability to provide systematic control over the factors affecting the retrieval stage-screen-off the variability in the production and the encoding of dreams.

Dreams and nightmares during the pandemic

The pandemic caused by the coronavirus disease 2019 (COVID-19) had a huge impact on public mental health. This was also reflected in dreams. Not only did people start to remember more dreams, but dream content changed as themes like sickness, confinement, and-in the English-speaking world-even bugs began to dominate. This also led to an increase in nightmare frequency. There are various factors that contributed to this change in the dream landscape. Some people have started to sleep more and hereby spend more time in REM sleep, which is known to increase dream recall and further lead to bizarre and vivid dreams. On the other hand, stress and poor mental health had an impact on sleep, and sleep quality thus dropped in many individuals. Poor sleep quality can also lead to an increase in dream recall. Dreams are known to regulate mood, so the rise in dreams and the change in dream content could also reflect a reaction to the overall rise in stress and decline in mental health. Recent studies have shown that as the pandemic progresses, further changes in mental health, dream recall, and dream content arise, but data are still scarce. Further research could help understand the impact the pandemic still has on mental health and dreams, and how this impact is changing over the course of the pandemic.

Dream recall frequency is associated with attention rather than with working memory abilities

Several factors influencing dream recall frequency (DRF) have been identified, but some remain poorly understood. One way to study DRF is to compare cognitive processes in low and high dream recallers (LR and HR). According to the arousal-retrieval model, long-term memory encoding of a dream requires wakefulness while its multisensory short-term memory is still alive. Previous studies showed contradictory results concerning short-term memory differences between LR and HR. It has also been found that extreme DRFs are associated with different electrophysiological traits related to attentional processes. However, to date, there is no evidence for attentional differences between LR and HR at the behavioural level. To further investigate attention and working memory in HR and LR, we used a newly-developed challenging paradigm called "MEMAT" (for MEMory and ATtention), which allows the study of selective attention and working memory interaction during memory encoding of non-verbal auditory stimuli. We manipulated the difficulties of the distractor to ignore and of the memory task. The performance of the two groups were not differentially impacted by working memory load. However, HR were slower and less accurate in the presence of a hard rather than easy to-ignore distractor, while LR were much less impacted by the distractor difficulty. Therefore, we show behavioural evidence towards less resistance to hard-to-ignore distractors in HR. Using a challenging task, we show for the first time, attentional differences between HR and LR at the behavioural level. The impact of auditory attention and working memory on dream recall is discussed.

A Novel Approach to Dream Content Analysis Reveals Links Between Learning-Related Dream Incorporation and Cognitive Abilities

Can dreams reveal insight into our cognitive abilities and aptitudes (i.e., "human intelligence")? The relationship between dream production and trait-like cognitive abilities is the foundation of several long-standing theories on the neurocognitive and cognitive-psychological basis of dreaming. However, direct experimental evidence is sparse and remains contentious. On the other hand, recent research has provided compelling evidence demonstrating a link between dream content and new learning, suggesting that dreams reflect memory processing during sleep. It remains to be investigated whether the extent of learning-related dream incorporation (i.e., the semantic similarity between waking experiences and dream content) is related to inter-individual differences in cognitive abilities. The relationship between pre-post sleep memory performance improvements and learning-related dream incorporation was investigated (N = 24) to determine if this relationship could be explained by inter-individual differences in intellectual abilities (e.g., reasoning, short term memory (STM), and verbal abilities). The extent of dream incorporation using a novel and objective method of dream content analysis, employed a computational linguistic approach to measure the semantic relatedness between verbal reports describing the experience on a spatial (e.g., maze navigation) or a motor memory task (e.g., tennis simulator) with subsequent hypnagogic reverie dream reports and waking "daydream" reports, obtained during a daytime nap opportunity. Consistent with previous studies, the extent to which something new was learned was related (r = 0.47) to how richly these novel experiences were incorporated into the content of dreams. This was significant for early (the first 4 dream reports) but not late dreams (the last 4 dream reports). Notably, here, we show for the first time that the extent of this incorporation for early dreams was related (r = 0.41) to inter-individual differences in reasoning abilities. On the other hand, late dream incorporation was related (r = 0.46) to inter-individual differences in verbal abilities. There was no relationship between performance improvements and intellectual abilities, and thus, inter-individual differences in cognitive abilities did not mediate the relationship between performance improvements and dream incorporation; suggesting a direct relationship between reasoning abilities and dream incorporation. This study provides the first evidence that learning-related dream production is related to inter-individual differences in cognitive abilities.

Dream Recall and Sleep Duration: State or Trait Factor

Previous research regarding the relationship between habitual sleep duration and frequency of dream recall yielded conflicting results, whereas experimental manipulation of sleep duration showed a significant effect on dream recall. In this study analyzing both effects simultaneously, the findings from diaries kept by the 196 participants over 28 to 111 days indicated that intra-individual fluctuations in sleep duration were significantly related to dream recall whereas interindividual differences in dream recall were not associated with differences in sleep duration. Sleep inertia might be an explanation for this finding.

Boundaries and Dreams

This article reviews and summarizes a number of published studies as well as recent unpublished work by the authors on Boundaries (Thin vs. Thick Boundaries) and various aspects of dreaming. Analysis of data by groups as well as by individuals demonstrates a surprisingly high correlation between thinness of boundaries and dream recall frequency. Thin boundaries appears to be the only personality measure clearly related to dream recall frequency. We also demonstrate a relationship between thin boundaries and dream content. Dreams of persons with thin boundaries are rated more dream-like, more emotional, and more bizarre in a number of studies. Thin boundaries are correlated especially with Central Image Intensity—a measure that is elevated in powerful dreams, dreams after traumatic events, and dreams after 9/11/01. These results strengthen our view that the “trait” continuum running from very thick boundaries at one end to very thin boundaries at the other is closely related to the “state” continuum running from focused waking activity at one end, through waking fantasy and daydreaming, to dreaming at the other end.

Factors Influencing the Gender Difference in Dream Recall Frequency

The results of the present study confirmed earlier findings of heightened dream recall frequency in women. The variables “meaningfulness of dreams” and “occurrence of problem-solving dreams” significantly reduced the correlation coefficient between gender and dream recall frequency. However, the question of causality remains unsolved. Other variables such as emotional intensity of dreams or nightmare frequency did not affect the relationship between gender and dream recall in a substantial way. Future research should manipulate systematically the variables under consideration, e.g., attitude toward dreams in order to observe the effects of this procedure on dream recall frequency and focus on the development of the gender difference during childhood and adolescence in relation to the child's understanding of the dream phenomenon.

Is dream recall underestimated by retrospective measures and enhanced by keeping a logbook? An empirical investigation

In a recent review, Aspy, Delfabbro, and Proeve (2015) highlighted the tendency for retrospective measures of dream recall to yield substantially lower recall rates than logbook measures, a phenomenon they termed the retrospective-logbook disparity. One explanation for this phenomenon is that retrospective measures underestimate true dream recall. Another explanation is that keeping a logbook tends to enhance dream recall. The present study provides a thorough empirical investigation into the retrospective-logbook disparity using a range of retrospective and logbook measures and three different types of logbook. Retrospective-logbook disparities were correlated with a range of variables theoretically related to the retrospective underestimation effect, and retrospective-logbook disparities were greater among participants that reported improved dream recall during the logbook period. These findings indicate that dream recall is underestimated by retrospective measures and enhanced by keeping a logbook. Recommendations for the use of retrospective and logbook measures of dream recall are provided.

Is dream recall underestimated by retrospective measures and enhanced by keeping a logbook? A review

There are two methods commonly used to measure dream recall in the home setting. The retrospective method involves asking participants to estimate their dream recall in response to a single question and the logbook method involves keeping a daily record of one's dream recall. Until recently, the implicit assumption has been that these measures are largely equivalent. However, this is challenged by the tendency for retrospective measures to yield significantly lower dream recall rates than logbooks. A common explanation for this is that retrospective measures underestimate dream recall. Another is that keeping a logbook enhances it. If retrospective measures underestimate dream recall and if logbooks enhance it they are both unlikely to reflect typical dream recall rates and may be confounded with variables associated with the underestimation and enhancement effects. To date, this issue has received insufficient attention. The present review addresses this gap in the literature.

Factors of Home Dream Recall and Nightmare Frequency in a Non-Student Sample

Home dream recall frequencies and nightmare frequencies show great inter-individual differences. Most of the studies trying to explain these differences, however, studied young participants, so these findings might not be true for persons older than 25 years. The present study investigated the relationship between dream recall, nightmare frequency, age, gender, sleep parameters, stress, and subjective health in a community-based sample ( N = 455) with a mean age of about 55 years. Some of the factors that have been shown to be associated with dream recall and nightmare frequency were also associated with these variables in non-student sample like frequency of nocturnal awakenings, current stress, and tiredness during the day. We were not able to replicate the effect of sex-role orientation on dream recall and nightmare frequency, supporting the idea that age might mediate the effect of daytime variables on dream recall and nightmare frequency. As nightmare frequency was related to sleep quality, stress, health problems, and tiredness during the day, it would be desirable that clinicians include a question about nightmares in their anamneses.

Dream recall frequency: impact of prospective measures and motivational factors

Significant individual differences exist in dream recall frequency (DRF) but some variance is likely attributable to instrument choice in measuring DRF. Three hundred and fifty eight participants estimated their weekly DRF and recorded their dreams in either a narrative log (n = 165) or checklist log (n = 193) for 2-5 weeks. There was an early peak in DRF within the first week of both types of prospective logs after which DRF remained relatively stable. Although the two groups did not differ in their estimated DRF, significantly fewer dreams were reported per week on the narrative logs and only checklist logs yielded significantly higher DRF than participants' questionnaire estimates. The interactions between DRF measures did not vary across groups with low, medium or high baseline levels of DRF. Keeping a dream log does not necessarily increase DRF and narrative logs' time consuming nature can impact subjects' motivation to report all of their dreams over time.

The Continuity between Waking Mood and Dream Emotions: Direct and Second-Order Effects

Emotions are an integral part of dreaming. From the viewpoint of the continuity hypothesis, two areas are of interest: how are dream emotions affected by day-time mood and how do dream emotions affect subsequent waking life. Seventy-four participants kept a diary over a 2-week period recording daytime mood, emotional valence of daytime events, and their dreams. Confirming earlier findings, the present study demonstrated direct effects of waking life on dreams and of dream emotions on waking-life. Positive and negative effects of daytime events on dreams were reported equally often (i.e., emotional intensity and not emotional valence is important in determining whether daytime events affect dreams). An interesting finding was a second-order effect; that is, the intensity of the negative effects of daytime events on dream content predicts—in addition to emotional intensity of the dream—the effect of that dream on daytime mood. The findings fit in within the framework of the continuity hypothesis, and pursuing this approach might shed further light on the function of dreaming.

The Memory Experiences and Dreams Questionnaire (MED-Q): A Validated Measure of Dream Remembering

Dream recall has been investigated via various means, often relying upon self-report judgments. Such methods, as well as individual differences and cognitive correlates of dream recall, rarely acknowledge models, theory, or empirical work concerning waking memory. Study 1 presents the development and psychometric validation of the Memory Experiences and Dreaming Questionnaire (MED-Q)—a 30-item measure of autobiographical and dream memory sensations incorporating items on dreaming, sensory experiences, and autobiographical remembering behaviors. It produces a single score and can be broken down into its constituent factors: “awareness of dreaming,” “daydreaming,” “dream sensations upon waking,” “déjà-states,” “comprehensibility of dream content,” and “senses.” Study 2 demonstrates the validity of the MED-Q as compared with dream report indices of dream detail. The MED-Q measures the extent to which a person engages with their dream memories through both frequency and subjective characteristic ratings. It is therefore novel in emphasizing the context of autobiographical memory for dreaming.

Measuring nightmare and bad dream frequency: impact of retrospective and prospective instruments

Studies on nightmare frequency have yielded inconsistent results. We compared the frequency of nightmares and bad dreams obtained with retrospective methods (annual and monthly estimates) and with two types of prospective measures (narrative and checklist logs). Four hundred and eleven participants completed retrospective estimates of nightmare and bad dream frequency and recorded their dreams in either narrative or checklist logs for 2-5 weeks. When measured prospectively with narrative logs, nightmare frequency was marginally higher than the 1-year estimate (P = 0.057) but not significantly different from the 1-month estimate (P > 0.05). Prospective bad dream frequency was significantly greater than the two retrospective estimates (ps < 0.0005). There were no significant differences in the frequency of nightmares and bad dreams reported prospectively with narrative versus checklist logs (ps > 0.05). However, checklist logs yielded a significantly greater number of everyday dreams per week (P < 0.0001). Taken together, the results provide partial support for the idea that when compared to daily logs, retrospective self-reports significantly underestimate current nightmare and bad dream frequency. Prospective studies of dream recall and nightmare frequency should take into account the type of log used, its duration, and the participants' level of motivation over time.

Dream Recall, Dream Length, and Sleep Duration: State or Trait Factor

Longer sleep duration should be related to dream recall (longer REM sleep), but previous findings regarding the relationship between habitual sleep duration and dream recall frequency have been conflicting, whereas experimental manipulation of sleep duration yielded a significant effect. The analysis of diaries kept by the 444 participants over 14 days indicated that intra-individual fluctuations and interindividual differences in sleep duration affect dream recall.

Gender differences in dream recall: a meta-analysis

Many studies have reported gender differences in dream recall. Data from 175 independent studies have been included in the analyses. Overall, estimated effect sizes in five age groups of healthy persons differed significantly from zero. Variables like measurement method and publication year did not affect the gender difference but age groups showed different effect sizes. The smallest effect size was found for children (0.097), the largest for adolescents (0.364), whereas the three adult groups ranged from 0.242 to 0.270. The findings suggest that the age-dependent gender differences in dream recall might be explained by gender-specific 'dream socialization'. Longitudinal studies in this area, however, are still lacking.

Dreaming during anaesthesia in adult patients

Dreaming during anaesthesia is defined as any recalled experience (excluding awareness) that occurred between induction of anaesthesia and the first moment of consciousness upon emergence. Dreaming is a commonly-reported side-effect of anaesthesia. The incidence is higher in patients who are interviewed immediately after anaesthesia (approximately 22%) than in those who are interviewed later (approximately 6%). A minority of dreams, which include sensory perceptions obtained during anaesthesia, provide evidence of near-miss awareness. These patients may have risk factors for awareness and this type of dreaming may be prevented by depth of anaesthesia monitoring. Most dreaming however, occurs in younger, fitter patients, who have high home dream recall, who receive propofol-based anaesthesia and who emerge rapidly from anaesthesia. Their dreams are usually short and pleasant, are related to work, family and recreation, are not related to inadequate anaesthesia and probably occur during recovery. Dreaming is a common, fascinating, usually pleasant and harmless phenomenon.

Absorption, psychological boundaries and attitude towards dreams as correlates of dream recall: two decades of research seen through a meta-analysis

Many studies have reported positive correlations between dream recall frequency (DRF) and measures of absorption, psychological boundaries and attitude towards dreams. A majority of these studies, however, have relied exclusively on retrospective measures of DRF even though daily dream logs are generally considered to be more direct and valid measures of DRF. The first goal of the present meta-analysis was to evaluate the effect sizes of three variables (absorption, psychological boundaries and attitude towards dreams) as correlates of DRF. The second goal was to evaluate if these effect sizes varied as a function of how DRF was operationalized (i.e. retrospective measure versus dream log). Data from 24 studies were included in the analyses. For each of the three variables investigated, correlations with retrospective measures of DRF were of greater magnitude than those obtained with daily logs. These results indicate that scores on measures of absorption and psychological boundaries are not related to DRF per se, but rather to people's tendency to retrospectively underestimate or overestimate their DRF, while attitude towards dreams is related both to DRF per se and to people's retrospective estimation bias. Implications of these findings for dream research are discussed.

Nightmares and oxygen desaturations: is sleep apnea related to heightened nightmare frequency?

In the 19th century, several authors held the view that nightmares are caused by oxygen shortage. The present study was designed to study nightmare frequency in patients with obstructive sleep apnea syndrome and its relationship to respiratory parameters. A brief questionnaire was administered to 323 patients with sleep apnea syndrome before their first laboratory night. The reduction in nightmare frequency in the sleep apnea group was explained by the reduced dream recall frequency. Despite some illustrative examples of a correlation between oxygen desaturation and dream content, the respiratory parameters as measures of sleep apnea syndrome severity did not correlate substantially with nightmare frequency. Psychiatric comorbidity and an intake of psychotropic medication were associated with heightened nightmare frequency in this sample. It must be concluded that the oxygen hypothesis did not play a major role in explaining the occurrence of nightmares. As this might be partly explained by adaptation to the nightly desaturation periods, it will be fruitful to apply experimental procedures that interrupt airflow during (rapid eye movement) REM sleep for short periods in a systematic way without the knowledge of the sleeper and to then study their effects on dream content. Some patients reported a correlation between daytime stressors and nightmares, which is in line with modern etiological models of nightmares.

Rhinal-hippocampal connectivity determines memory formation during sleep

Compared with waking state attention, volition and semantic processing play a minor role during sleep. Thus, investigating declarative memory formation during sleep may allow us to isolate mnemonic core processes. The most feasible approach to memory formation during sleep is the analysis of dream memories. Lesion and imaging studies have demonstrated that encoding of declarative memories, i.e. consciously accessible events and facts, depends on operations within the rhinal cortex and the hippocampus, two substructures of the medial temporal lobe. Successful memory formation is accompanied by a transient rhinal-hippocampal interaction. Consequently, the ability to memorize dreams may be related to mediotemporal connectivity. Therefore, we recorded EEG during sleep from rhinal and hippocampal depth electrodes implanted in 12 epilepsy patients (eight women, mean age 41.1 +/- 6.4 years). They were awakened during rapid eye movement sleep (REM) and asked to recall their dream. Via coherence analyses we show that rhinal-hippocampal connectivity values are approximately twice as large for patients with good dream recall versus those patients with poor recall. This suggests that rhinal-hippocampal connectivity is a key factor in determining declarative memory formation.

Dream recall frequency and sleep quality of patients with restless legs syndrome

The present study investigated the dream recall frequency and the pattern of influencing factors of patients with restless legs syndrome in comparison with healthy controls. The patients' dream recall frequency did not differ from that of healthy controls. Dream recall, however, was negatively associated with the number of periodic leg movements with arousal (PLMAI). Subjective estimates of sleep quality or feeling of being refreshed in the morning, on the other hand, did not correlate with the PLMAI index. Whereas subjective sleep parameters were related to dream recall frequency in healthy controls, no substantial relationships were found in the patient group, except for the positive correlation between sleep latency and dream recall frequency. The results of the present study can not be interpreted as clear evidence for the arousal-retrieval model of dream recall; it seems plausible that other factors, e.g. the functional state of the brain, are of importance in explaining dream recall in this patient group.