Nina Landmann

The reorganisation of memory during sleep

Sleep after learning promotes the quantitative strengthening of new memories. Less is known about the impact of sleep on the qualitative reorganisation of memory, which is the focus of this review. Studies have shown that, in the declarative system, sleep facilitates the abstraction of rules (schema formation), the integration of knowledge into existing schemas (schema integration) and creativity that requires the disbandment of existing patterns (schema disintegration). Schema formation and integration might primarily benefit from slow wave sleep, whereas the disintegration of a schema might be facilitated by rapid eye movement sleep. In the procedural system, sleep fosters the reorganisation of motor memory. The neural mechanisms of these processes remain to be determined. Notably, emotions have been shown to modulate the sleep-related reorganisation of memories. In the final section of this review, we propose that the sleep-related reorganisation of memories might be particularly relevant for mental disorders. Thus, sleep disruptions might contribute to disturbed memory reorganisation and to the development of mental disorders. Therefore, sleep-related interventions might modulate the reorganisation of memories and provide new inroads into treatment.

Sleep restriction over several days does not affect long-term recall of declarative and procedural memories in adolescents

OBJECTIVES There is broad evidence that sleep as opposed to waking facilitates the consolidation of both declarative and procedural memory. The current study addressed the question whether different extents of sleep restriction after learning would impair long-term memory consolidation in adolescents. METHODS Eighty-eight healthy adolescents were randomized to five different sleep protocols with 9, 8, 7, 6 or 5 h of time in bed for four consecutive nights under controlled conditions that excluded daytime sleep. Declarative (word-pair task) and procedural memory (mirror tracing task) encoding was assessed prior to the sleep restriction protocol. Recall was assessed after two recovery nights following the sleep protocol and 4 weeks later. RESULTS Sleep diaries and actigraphy data demonstrated that the participants closely followed the sleep protocols. There were no differences in demographic parameters or memory encoding at baseline. In contrast to the initial prediction, restriction of nocturnal sleep over four consecutive nights had no significant impact on declarative or procedural memory consolidation. Polysomnographic monitoring after sleep restriction demonstrated a high preservation of the amount of slow wave sleep in the restricted conditions. CONCLUSIONS The results suggest that adolescents show a high resilience of memory consolidation to substantial sleep curtailment across four nights that might be promoted by increased sleep intensity under conditions of sleep restriction.

REM sleep and memory reorganization: Potential relevance for psychiatry and psychotherapy

Sleep can foster the reorganization of memory, i.e. the emergence of new memory content that has not directly been encoded. Current neurophysiological and behavioral evidence can be integrated into a model positing that REM sleep particularly promotes the disintegration of existing schemas and their recombination in the form of associative thinking, creativity and the shaping of emotional memory. Particularly, REM sleep related dreaming might represent a mentation correlate for the reconfiguration of memory. In a final section, the potential relevance for psychiatry and psychotherapy is discussed.

The Timing of Learning before Night-Time Sleep Differentially Affects Declarative and Procedural Long-Term Memory Consolidation in Adolescents

Sleep after learning has been shown to foster the consolidation of new memories. However, fundamental questions on the best timing of learning before night-time sleep persist. We tested the hypothesis that learning directly prior to night-time sleep compared to 7.5 hrs prior to night-time sleep provides better conditions for the consolidation of declarative and procedural memories. Fifty healthy female adolescents (aged 16–17 years) were trained on a declarative word-pair and a procedural finger-tapping task at 3 pm (afternoon group, n = 25) or at 9 pm (evening group, n = 25), followed by a sleep laboratory night. Retrieval was assessed 24 hours and 7 days after initial training. Subjects trained in the afternoon showed a significantly elevated retention rate of word-pairs compared to subjects trained in the evening after 24 hours, but not after 7 days. In contrast, off-line gains in finger-tapping performance were significantly higher in subjects trained in the evening compared to those trained in the afternoon after both retention intervals. The observed enhanced consolidation of procedural memories after training in the evening fits to current models of sleep-related memory consolidation. In contrast, the higher retention of declarative memories after encoding in the afternoon is surprising, appeared to be less robust and needs further investigation.

The effect of sleep-specific brain activity versus reduced stimulus interference on declarative memory consolidation.

Studies suggest that the consolidation of newly acquired memories and underlying long‐term synaptic plasticity might represent a major function of sleep. In a combined repeated‐measures and parallel‐group sleep laboratory study (active waking versus sleep, passive waking versus sleep), we provide evidence that brief periods of daytime sleep (42.1 ± 8.9 min of non‐rapid eye movement sleep) in healthy adolescents (16 years old, all female), compared with equal periods of waking, promote the consolidation of declarative memory (word‐pairs) in participants with high power in the electroencephalographic sleep spindle (sigma) frequency range. This observation supports the notion that sleep‐specific brain activity when reaching a critical dose, beyond a mere reduction of interference, promotes synaptic plasticity in a hippocampal‐neocortical network that underlies the consolidation of declarative memory.

Prolonged Sleep under Stone Age Conditions

STUDY OBJECTIVES We report on a unique experiment designed to investigate the impact of prehistoric living conditions on sleep-wake behavior. METHODS A group of five healthy adults were assessed during life in a Stone Age-like settlement over two months. RESULTS The most notable finding was that nocturnal time in bed and estimated sleep time, as measured by actigraphy, markedly increased during the experimental period compared to the periods prior to and following the experiment. These increases were primarily driven by a phase-advance shift of sleep onset. Subjective assessments of health and functioning did not reveal any relevant changes across the study. CONCLUSIONS Our observations provide further evidence for the long-held belief that the absence of modern living conditions is associated with an earlier sleep phase and prolonged sleep duration. COMMENTARY A commentary on this article appears in this issue on page 723.

Sleep Strengthens but does Not Reorganize Memory Traces in a Verbal Creativity Task.

STUDY OBJECTIVES Sleep after learning promotes the quantitative strengthening of new memories. Less is known about the impact of sleep on the qualitative reorganization of memory content. This study tested the hypothesis that sleep facilitates both memory strengthening and reorganization as indexed by a verbal creativity task. METHODS Sixty healthy university students (30 female, 30 male, 20-30 years) were investigated in a randomized, controlled parallel-group study with three experimental groups (sleep, sleep deprivation, daytime wakefulness). At baseline, 60 items of the Compound Remote Associate (CRA) task were presented. At retest after the experimental conditions, the same items were presented again together with 20 new control items to disentangle off-line incubation from online performance effects. RESULTS Sleep significantly strengthened formerly encoded memories in comparison to both wake conditions (improvement in speed of correctly resolved items). Offline reorganization was not enhanced following sleep, but was enhanced following sleep-deprivation in comparison to sleep and daytime wakefulness (solution time of previously incubated, newly solved items). Online performance did not differ between the groups (solution time of new control items). CONCLUSIONS The results support the notion that sleep promotes the strengthening, but not the reorganization, of newly encoded memory traces in a verbal creativity task. Future studies are needed to further determine the impact of sleep on different types of memory reorganization, such as associative thinking, creativity and emotional memory processing, and potential clinical translations, such as the augmentation of psychotherapy through sleep interventions.

Declarative virtual water maze learning and emotional fear conditioning in primary insomnia

Healthy sleep restores the brains ability to adapt to novel input through memory formation based on activity‐dependent refinements of the strength of neural transmission across synapses (synaptic plasticity). In line with this framework, patients with primary insomnia often report subjective memory impairment. However, investigations of memory performance did not produce conclusive results. The aim of this study was to further investigate memory performance in patients with primary insomnia in comparison to healthy controls, using two well‐characterized learning tasks, a declarative virtual water maze task and emotional fear conditioning. Twenty patients with primary insomnia according to DSM‐IV criteria (17 females, three males, 43.5 ± 13.0 years) and 20 good sleeper controls (17 females, three males, 41.7 ± 12.8 years) were investigated in a parallel‐group study. All participants completed a hippocampus‐dependent virtual Morris water maze task and amygdala‐dependent classical fear conditioning. Patients with insomnia showed significantly delayed memory acquisition in the virtual water maze task, but no significant difference in fear acquisition compared with controls. These findings are consistent with the notion that memory processes that emerge from synaptic refinements in a hippocampal–neocortical network are particularly sensitive to chronic disruptions of sleep, while those in a basic emotional amygdala‐dependent network may be more resilient.

Sleep-Related Interventions to Improve Psychotherapy

Mental disorders are among the most prevalent and impairing disorders worldwide. According to the WHO (World Health Organization), about a third of the population will suffer from a mental disorder across the lifespan. Current guidelines include psychotherapy, or at least psychoeducation, as a first-line treatment component for all mental disorders. Psychotherapy represents an interpersonal process that involves the modification of cognition, emotion and behavior. As such, it can, at least in part, be conceptualized as the quantitative strengthening and qualitative reorganization of novel and more adaptive memory representations. Given that sleep substantially modulates learning, memory and underlying neural refinements, this chapter centers on the idea that sleep-related interventions can be used to augment the treatment effects of psychotherapy. The first part identifies basic memory processes with particular relevance for psychotherapy. The second part evaluates the potential of sleep-related interventions prior to and after psychotherapy, as well as the modulation of distinct aspects of sleep to augment these memory processes in psychotherapy and discusses further directions.

Brief periods of NREM sleep do not promote early offline gains but subsequent on-task performance in motor skill learning

HighlightsSleep modulates motor learning.Brief periods of NREM sleep do not promote early offline gains in motor learning.Brief periods of NREM sleep improve subsequent on‐task performance in motor learning.Our findings inform the optimization of sleep‐wake schedules for motor performance.Future work is needed to clarify sleep’s impact on training‐induced performance gains.Future work is needed to specify sleep’s impact on detrimental on‐task processes. ABSTRACT Sleep modulates motor learning, but its detailed impact on performance curves remains to be fully characterized. This study aimed to further determine the impact of brief daytime periods of NREM sleep on ‘offline’ (task discontinuation after initial training) and ‘on‐task’ (performance within the test session) changes in motor skill performance (finger tapping task). In a mixed design (combined parallel group and repeated measures) sleep laboratory study (n = 17 ‘active’ wake vs. sleep, n = 19 ‘passive’ wake vs. sleep), performance curves were assessed prior to and after a 90 min period containing either sleep, active or passive wakefulness. We observed a highly significant, but state‐ (that is, sleep/wake)‐independent early offline gain and improved on‐task performance after sleep in comparison to wakefulness. Exploratory curve fitting suggested that the observed sleep effect most likely emerged from an interaction of training‐induced improvement and detrimental ‘time‐on‐task’ processes, such as fatigue. Our results indicate that brief periods of NREM sleep do not promote early offline gains but subsequent on‐task performance in motor skill learning.