Learning, Memory, and Sleep in Humans

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.

Emotional scene remembering: A combination of disturbing and facilitating effects of emotion?

An emotion-induced memory trade-off effect is frequently reported when participants have to memorize complex items that include both neutral and emotional features. This bias corresponds to better remembering of central emotional information accompanied by poor performance related to neutral background information. Although the trade-off effect has been mainly associated with attentional bias toward emotional content, findings suggest that other non-attentional cognitive processes could also be involved. The aim of this work was to assess whether emotional effects would be reported apart from their influence on attentional processing in an immediate delay memory task. Three studies were conducted. In Study 1, manipulation of the diffusion quality of emotional content allowed us to select focal emotional pictures vs. diffuse emotional pictures, which prevented attentional focus. The two studies that followed consisted of a recognition task of low- and high-complexity pictures in which we used partial visual cues during the test that could display either the emotional elements (i.e., central patch cues, Study 2) or the peripheral elements (i.e., peripheral patch cues, Study 3) of the focal emotional pictures. Results from Studies 2 and 3 replicated traditional trade-off effects only for high-complexity pictures. In addition, diffuse emotional pictures were associated with lower memory performance than were neutral pictures, suggesting that emotion features could both disturb and enhance (via their attentional effect) encoding processes.

Snoozing: an examination of a common method of waking

Study Objectives

Snoozing was defined as using multiple alarms to accomplish waking, and considered as a method of sleep inertia reduction that utilizes the stress system. Surveys measured snoozing behavior including who, when, how, and why snoozing occurs. In addition, the physiological effects of snoozing on sleep were examined via wearable sleep staging and heart rate (HR) activity, both over a long time scale, and on the days that it occurs. We aimed to establish snoozing as a construct in need of additional study.

Methods

A novel survey examined snoozing prevalence, how snoozing was accomplished, and explored possible contributors and motivators of snoozing behavior in 450 participants. Trait- and day-level surveys were combined with wearable data to determine if snoozers sleep differently than nonsnoozers, and how snoozers and nonsnoozers differ in other areas, such as personality.

Results

57% of participants snoozed. Being female, younger, having fewer steps, having lower conscientiousness, having more disturbed sleep, and being a more evening chronotype increased the likelihood of being a snoozer. Snoozers had elevated resting HR and showed lighter sleep before waking. Snoozers did not sleep less than nonsnoozers nor did they feel more sleepiness or nap more often.

Conclusions

Snoozing is a common behavior associated with changes in sleep physiology before waking, both in a trait- and state-dependent manner, and is influenced by demographic and behavioral traits. Additional research is needed, especially in detailing the physiology of snoozing, its impact on health, and its interactions with observational studies of sleep.

Relationship Between Attitudes and Beliefs About Sleep, Sleep Disturbance, and Pain Interference in Patients With Spinal Pain

OBJECTIVES

Sleep impairments are a strong predictor of pain, making sleep a potential interest when treating patients with spine pain. Typical beliefs about the importance of sleep in patients seeking care for spinal pain are unknown. The purpose of this study was to describe the beliefs and attitudes about sleep in patients seeking care for spinal pain and to examine the relationships between dysfunctional beliefs and attitudes about sleep (DBAS), disordered sleep, and pain interference.

MATERIALS AND METHODS

This cross-sectional study included patients presenting to physical therapy with spine pain. Participants completed questionnaires including demographics, medical history, pain interference (pain, enjoyment, and general activity), DBAS-16, and sleep-related impairment (Patient-Reported Outcome Measurement Information System). Correlations were calculated between DBAS-16 scores and measures of sleep quality/quantity, and a generalized linear model was used to investigate the predictive ability of DBAS-16 scores on pain interference.

RESULTS

The mean DBAS-16 score was 4.22 (SD=2.03), with 52.5% of participants having DBAS. There was a strong relationship between DBAS-16 and Patient-Reported Outcome Measurement Information System ( rs =0.7; P <0.001). For every point higher score on the DBAS-16, pain interference scores increased by approximately half a point (B=0.46; 95% CI 0.33, 0.59, 1.80; P <0.001).

DISCUSSION

These results highlight a strong relationship between beliefs and attitudes about sleep and measures of sleep quality/quantity and a linear association with pain interference scores. These findings provide a rationale for targeting beliefs and attitudes about sleep when managing pain-related symptoms in patients seeking care for spine pain.

Sleep and incubation: Using problem reactivation during sleep to study forgetting fixation and unconscious processing during sleep incubation

When people are stuck on a problem, they sometimes benefit from an incubation period -a break from working on the problem. Anecdotes and empirical evidence suggest that sleeping during incubation is useful, but the mechanisms remain poorly understood. We examined how targeted memory reactivation during sleep, which boosts next-day solving, relates to forgetting fixation, a well-supported explanation of awake incubation. In evening sessions, participants attempted puzzles, while a unique sound cue played during each puzzle. Half the time, puzzles included fixating information reinforcing an incorrect representation. Later, during deep sleep, sounds associated with half of participants' previously unsolved puzzles were presented. The sounds should strengthen puzzle memories and reduce forgetting of the fixating information. In morning solving, overnight cueing reliably interacted with fixating information: participants solved numerically more cued than uncued puzzles, but only when puzzles included fixating information. These results suggest that additional processing occurred beyond simple fixation forgetting.

The association of changes of sleep architecture related to donepezil: A systematic review and meta-analysis

BACKGROUND

Donepezil had been recognized to have impact on sleep quality in demented patients. However, there was insufficient evidences about the actual effect of donepezil in the sleep architectures. Our meta-analysis aimed to evaluate the changes of sleep architectures related to donepezil use.

METHODS

Followed the PRISMA2020 and AMSTAR2 guidelines, electronic search had been performed on the databases of PubMed, Embase, ScienceDirect, ClinicalKey, Cochrane CENTRAL, ProQuest, Web of Science, and ClinicalTrials.gov. The outcome measurement was changes of sleep parameters detected by polysomnography. A random-effects meta-analysis was conducted.

RESULTS

Total twelve studies had been involved. The percentage of REM sleep would significantly increase after donepezil treatment (Hedges' g = 0.694, p < 0.001). Compared to placebo/controls, subjects with donepezil would had significantly increased percentage of REM sleep stage (Hedges' g = 0.556, p = 0.018). Furthermore, donepezil was also associated with the decreased stage 2 sleep percentage, sleep efficiency, or total sleep time in different analysis conditions.

CONCLUSION

Our meta-analysis provided detailed changes of sleep architectures related to donepezil treatment. Further larger sample size studies with stricter control of potential moderators are needed to clarify these issues.

Higher post-encoding cortisol benefits the selective consolidation of emotional aspects of memory

Emotional experiences create durable memory traces in the brain, especially when these memories are consolidated in the presence of stress hormones such as cortisol. Although some research suggests cortisol elevation can increase long-term memory for emotional relative to neutral content, the impact of stress and cortisol on the consolidation of emotional and neutral aspects of memories when they are part of the same experience remains unknown. Here, after encoding complex scenes consisting of negative or neutral objects placed on neutral backgrounds, participants were exposed to a psychosocial stressor (or matched control condition) in order to examine the impact of stress and cortisol on early consolidation processes. The next day, once cortisol levels had returned to baseline, specific and gist recognition memory were tested separately for objects and backgrounds. Results indicate that while there was a numerical increase in memory for negative objects in the stress group, higher endogenous cortisol concentrations were specifically associated with decreased memory for the neutral backgrounds originally paired with negative objects. Moreover, across all participants, cortisol levels were positively correlated with the magnitude of the emotional memory trade-off effect. Specifically, while memory for negative objects was preserved, elevated cortisol during early consolidation was associated with decreased memory for neutral backgrounds that were initially paired with negative objects. These memory effects were observed in both the stricter specific measure of memory and the less conservative measure of gist memory. Together, these findings suggest that rather than influencing all aspects of an experience similarly, elevated cortisol during early consolidation selectively preserves what is most emotionally salient and adaptive to remember while allowing the loss of memory for less important neutral information over time.

Overnight sleep benefits both neutral and negative direct associative and relational memory

Strong evidence suggests that sleep plays a role in memory consolidation, which involves both stabilizing memory into long-term storage as well as integrating new information into existing stores. The current study investigated consolidation, across a day of wakefulness or night of sleep, of emotional and neutral directly learned visual paired associates (A-B/B-C pairs) as well as formation of memory for relational pairs formed via overlapping learned components (A-C pairs). Participants learned 40 negative and 40 neutral face-object pairs followed by a baseline test in session 1 either in the morning or evening. They then spent a 12-hour retention period during which participants either went about their normal day or spent the night in the sleep lab. During session 2, participants completed a surprise test to assess their memory for relational pairs (A-C) as well as memory for direct associates (A-B/B-C). As hypothesized, the results demonstrated that a 12-hour retention period predominantly spent asleep, compared to awake, benefited memory for both relational and direct associative memory. However, contrary to the hypothesis that emotional salience would promote preferential consolidation, sleep appeared to benefit both negative and neutral information similarly for direct associative and relational memories, suggesting that sleep may interact with other factors affecting encoding (e.g., depth of encoding) to benefit direct and relational associative memory. As one of the few studies examining the role of nocturnal sleep and emotion on both direct and relational associative memory, our findings suggest key insights into how overnight sleep consolidates these different forms of memory.

Targeted Memory Reactivation During Sleep Improves Next-Day Problem Solving

Many people have claimed that sleep has helped them solve a difficult problem, but empirical support for this assertion remains tentative. The current experiment tested whether manipulating information processing during sleep impacts problem incubation and solving. In memory studies, delivering learning-associated sound cues during sleep can reactivate memories. We therefore predicted that reactivating previously unsolved problems could help people solve them. In the evening, we presented 57 participants with puzzles, each arbitrarily associated with a different sound. While participants slept overnight, half of the sounds associated with the puzzles they had not solved were surreptitiously presented. The next morning, participants solved 31.7% of cued puzzles, compared with 20.5% of uncued puzzles (a 55% improvement). Moreover, cued-puzzle solving correlated with cued-puzzle memory. Overall, these results demonstrate that cuing puzzle information during sleep can facilitate solving, thus supporting sleep's role in problem incubation and establishing a new technique to advance understanding of problem solving and sleep cognition.

Interactive effects of stress reactivity and rapid eye movement sleep theta activity on emotional memory formation

Sleep and stress independently enhance emotional memory consolidation. In particular, theta oscillations (4-7 Hz) during rapid eye movement (REM) sleep increase coherence in an emotional memory network (i.e., hippocampus, amygdala, and prefrontal cortex) and enhance emotional memory. However, little is known about how stress during learning might interact with subsequent REM theta activity to affect emotional memory. In the current study, we examined whether the relationship between REM theta activity and emotional memory differs as a function of pre-encoding stress exposure and reactivity. Participants underwent a psychosocial stressor (the Trier Social Stress Task; n = 32) or a comparable control task (n = 32) prior to encoding. Task-evoked cortisol reactivity was assessed by salivary cortisol rise from pre- to post-stressor, and participants in the stress condition were additionally categorized as high or low cortisol responders via a median split. During incidental encoding, participants studied 150 line drawings of negative, neutral, and positive images, followed by the complete color photo. All participants then slept overnight in the lab with polysomnographic recording. The next day, they were given a surprise recognition memory task. Results showed that memory was better for emotional relative to neutral information. Critically, these findings were observed only in the stress condition. No emotional memory benefit was observed in the control condition. In stressed participants, REM theta power significantly predicted memory for emotional information, specifically for positive items. This relationship was observed only in high cortisol responders. For low responders and controls, there was no relationship between REM theta and memory of any valence. These findings provide evidence that elevated stress at encoding, and accompanying changes in neuromodulators such as cortisol, may interact with theta activity during REM sleep to promote selective consolidation of emotional information.

Screen viewing behavior and sleep duration among children aged 2 and below

Background

Few studies have investigated the association between screen viewing (SV) and sleep duration among young children. This study aims to examine the association between total and device-specific SV and sleep duration among children aged 2 and below.

Methods

We conducted a cross-sectional study of 714 Singaporean children aged 2 years and below. Parents were recruited during routine well-child clinic visits from two national polyclinics. In Singapore, all parents visit well-child clinics with their children at regular intervals for routine check-ups and vaccinations. Socio-demographic characteristics, duration of total and device-specific SV, and sleep duration were reported by parents via interviewer-administered questionnaires. Multiple linear regression analysis was used to assess associations between various types of SV and sleep duration, adjusted for socio-demographic variables. Due to significant interaction between SV and age, stratified analyses for children aged less than 6 months and those aged 7–24 months were performed.

Results

The prevalence of daily SV among children was 53.1%; 28.3% in children up to 6 months and 73.8% in children aged 7 to 24 months. TV viewing was reported for 44.3% of all children and mobile device SV for 30.1%. Children’s average sleep duration was 13.9 (SD = 3.5) hours daily and younger children had longer sleep duration than older ones (up to 6 months: mean = 15.6 h, SD = 3.9; 7–24 months: mean = 12.4 h, SD = 2.2; P < 0.01). In the regression analysis among all children, each 1 h per day increment in total SV was significantly associated with 0.26 h shorter sleep duration with similar significant associations for TV (β = − 0.28 h, 95%CI: -0.50, − 0.06) and mobile devices (β = − 0.35 h, 95%CI: -0.61, − 0.09). Stratified analysis revealed significantly greater reductions in sleep with higher SV among children aged 6 months and below (β = − 0.73 h, 95%CI: -1.12, − 0.34), while associations were weaker in older children (β = − 0.13 h, 95% CI: -0.24, − 0.01).

Conclusions

This study provides evidence for a substantial association between longer SV and shorter sleep duration among very young children. These associations appeared stronger among children aged 6 months and below as compared with those aged 7 to 24 months. Further studies are warranted to confirm our findings.

Electronic supplementary material

The online version of this article (10.1186/s12889-018-6385-6) contains supplementary material, which is available to authorized users.

Do different salience cues compete for dominance in memory over a daytime nap?

Information that is the most salient and important for future use is preferentially preserved through active processing during sleep. Emotional salience is a biologically adaptive cue that influences episodic memory processing through interactions between amygdalar and hippocampal activity. However, other cues that influence the importance of information, such as the explicit direction to remember or forget, interact with the inherent salience of information to determine its fate in memory. It is unknown how sleep-based processes selectively consolidate this complex information. The current study examined the development of memory for emotional and neutral information that was either cued to-be-remembered (TBR) or to-be-forgotten (TBF) across a daytime period including either napping or wakefulness. Baseline memory revealed dominance of the TBR cue, regardless of emotional salience. As anticipated, napping was found to preserve memory overall significantly better than remaining awake. Furthermore, we observed a trending interaction indicating that napping specifically enhanced the discrimination between the most salient information (negative TBR items) over other information. We found that memory for negative items was positively associated with the percentage of SWS obtained during a nap. Furthermore, the magnitude of the difference in memory between negative TBR items and negative TBF items increased with greater sleep spindle activity. Taken together, our results suggest that although the cue to actively remember or intentionally forget initially wins out, active processes during sleep facilitate the competition between salience cues to promote the most salient information in memory.

Relationships Among Nightly Sleep Quality, Daily Stress, and Daily Affect

Objectives.

We explored the prospective, microlevel relationship between nightly sleep quality (SQ) and the subsequent day's stress on positive (PA) and negative affect (NA) as well as the moderating relationships between nightly SQ, subsequent stress, and subsequent PA on NA. We investigated whether age moderated these relationships.

Method.

We collected 56 days of sleep, stress, and affect data using daily diary questionnaires (N = 552). We used multilevel modeling to assess relationships at the between- and within-person levels.

Results.

Daily increases in SQ and decreases in stress interacted to predict higher daily PA and lower daily NA. Better SQ in older adults enhanced the benefits of PA on the stress-NA relationship more during times of low stress, whereas better sleep in younger adults enhanced the benefits of PA more during times of high stress. Between-person effects were stronger predictors of well-being outcomes than within-person variability.

Discussion.

The combination of good SQ and higher PA buffered the impact of stress on NA. The moderating impact of age suggests that sleep and stress play different roles across adulthood. Targeting intervention and prevention strategies to improve SQ and enhance PA could disrupt the detrimental relationship between daily stress and NA.

The role of sleep in cognitive processing: focusing on memory consolidation

Research indicates that sleep promotes various cognitive functions, such as decision-making, language, categorization, and memory. Of these, most work has focused on the influence of sleep on memory, with ample work showing that sleep enhances memory consolidation, a process that stores new memories in the brain over time. Recent psychological and neurophysiological research has vastly increased understanding of this process. Such work not only suggests that consolidation relies on plasticity-related mechanisms that reactivate and stabilize memory representations, but also that this process may be experimentally manipulated by methods that target which memory traces are reactivated during sleep. Furthermore, aside from memory storage capabilities, memory consolidation also appears to reorganize and integrate memories with preexisting knowledge, which may facilitate the discovery of underlying rules and associations that benefit other cognitive functioning, including problem solving and creativity. WIREs Cogn Sci 2017, 8:e1433. doi: 10.1002/wcs.1433 For further resources related to this article, please visit the WIREs website.

Increasing Explicit Sequence Knowledge by Odor Cueing during Sleep in Men but not Women

Sleep consolidates newly acquired memories. Beyond stabilizing memories, sleep is thought to reorganize memory representations such that invariant structures, statistical regularities and even new explicit knowledge are extracted. Whereas increasing evidence suggests that the stabilization of memories during sleep can be facilitated by cueing with learning-associated stimuli, the effect of cueing on memory reorganization is less well understood. Here we asked whether olfactory cueing during sleep enhances the generation of explicit knowledge about an implicitly learned procedural memory task. Subjects were trained on a serial reaction time task (SRTT) containing a hidden 12-element sequence in the presence of an odor. During subsequent sleep, half of the subjects were re-exposed to the odor during periods of slow wave sleep (SWS), while the other half received odorless vehicle. In the next morning, subjects were tested on their explicit knowledge about the underlying sequence in a free recall test and a generation task. Although odor cueing did not significantly affect overall explicit knowledge, differential effects were evident when analyzing male and female subjects separately. Explicit sequence knowledge, both in free recall and the generation task, was enhanced by odor cueing in men, whereas women showed no cueing effect. Procedural skill in the SRTT was not affected by cueing, neither in men nor in women. These findings suggest that olfactory memory reactivation can increase explicit knowledge about implicitly learned information, but only in men. Hormonal differences due to menstrual cycle phase and/or hormonal contraceptives might explain the lacking effect in women.

Prospection and emotional memory: how expectation affects emotional memory formation following sleep and wake

Successful prospective memory is necessarily driven by an expectation that encoded information will be relevant in the future, leading to its preferential placement in memory storage. Like expectation, emotional salience is another type of cue that benefits human memory formation. Although separate lines of research suggest that both emotional information and information explicitly expected to be important in the future benefit memory consolidation, it is unknown how expectation affects the processing of emotional information and whether sleep, which is known to maximize memory consolidation, plays a critical role. The purpose of this study was to investigate how expectation would impact the consolidation of emotionally salient content, and whether this impact would differ across delays of sleep and wake. Participants encoded scenes containing an emotionally charged negative or neutral foreground object placed on a plausible neutral background. After encoding, half of the participants were informed they would later be tested on the scenes (expected condition), while the other half received no information about the test (unexpected condition). At recognition, following a 12-h delay of sleep or wakefulness, the scene components (objects and backgrounds) were presented separately and one at a time, and participants were asked to determine if each component was old or new. Results revealed a greater disparity for memory of negative objects over their paired neutral backgrounds for both the sleep and wake groups when the memory test was expected compared to when it was unexpected, while neutral memory remained unchanged. Analyzing each group separately, the wake group showed a threefold increase in the magnitude of this object/background trade-off for emotional scenes when the memory test was expected compared to when it was unexpected, while those who slept performed similarly across conditions. These results suggest that emotional salience and expectation cues interact to benefit emotional memory consolidation during a delay of wakefulness. The sleeping brain, however, may automatically tag emotionally salient information as important, such that explicit instruction of an upcoming memory test does not further improve memory performance.

Enhanced synchronization of gamma activity between frontal lobes during REM sleep as a function of REM sleep deprivation in man

Studies have shown that synchrony or temporal coupling of gamma activity is involved in processing and integrating information in the brain. Comparing rapid eye movement (REM) sleep to waking and non-REM (NREM) sleep, interhemispheric temporal coupling is higher, but lower between the frontal and posterior association areas of the same hemisphere. However, the homeostatic response of REM sleep temporal coupling after selective REM sleep deprivation (REMD) has not been studied. This study proposed exploring the effect of one night of selective REMD on the temporal coupling of cortical gamma activity during recovery REM sleep. Two groups of healthy subjects were subjected to either REMD by awakening them at each REM sleep onset, or to NREM sleep interruptions. Subjects slept four consecutive nights in the laboratory: first for adaptation, second as baseline, third for sleep manipulation, and fourth for recovery. Interhemispheric and intrahemispheric EEG correlations were analyzed during tonic REM (no eye movements) for the first three REM sleep episodes during baseline sleep, and recovery sleep after one night of selective REMD. Temporal coupling between frontal lobes showed a significant homeostatic rebound that increased during recovery REM sleep relative to baseline and controls. Results showed a rebound in temporal coupling between the two frontal lobes after REM sleep deprivation, indicating that the enhanced gamma temporal coupling that occurs normally during REM sleep has functional consequences.

CONCLUSION

results suggest that synchronized activity during REM sleep may play an important role in integrating and reprocessing information.

The role of sleep in human declarative memory consolidation

Through a variety of methods, researchers have begun unraveling the mystery of why humans spend one-third of their lives asleep. Though sleep likely serves multiple functions, it has become clear that the sleeping brain offers an ideal environment for solidifying newly learned information in the brain. Sleep , which comprises a complex collection of brain states, supports the consolidation of many different types of information. It not only promotes learning and memory stabilization, but also memory reorganization that can lead to various forms of insightful behavior. As this chapter will describe, research provides ample support for these crucial cognitive functions of sleep . Focusing on the declarative memory system in humans, we review the literature regarding the benefits of sleep for both neutral and emotionally salient declarative memory. Finally, we discuss the literature regarding the impact of sleep on emotion regulation.

A Bird's Eye View of Sleep-Dependent Memory Consolidation

How new experiences are solidified into long-lasting memories is a central question in the study of brain and behavior. One of the most intriguing discoveries in memory research is that brain activity during sleep helps to transform newly learned information and skills into robust memories. Though the first experimental work linking sleep and memory was conducted 90 years ago by Jenkins and Dallenbach, the case for sleep-dependent memory consolidation has only garnered strong support in the last decade. Recent studies in humans provide extensive behavioral, imaging, and polysomnographic data supporting sleep consolidation of a broad range of memory tasks. Likewise, studies in a few animal model systems have elucidated potential mechanisms contributing to sleep consolidation such as neural reactivation and synaptic homeostasis. Here, we present an overview of sleep-dependent memory consolidation, focusing on how investigations of sleep and learning in birds have complemented the progress made in mammalian systems by emphasizing a strong connection between behavior and physiology. We begin by describing the behavioral approach that has been utilized to demonstrate sleep consolidation in humans. We then address neural reactivation in the rodent hippocampal system as a putative mechanism of sleep consolidation. Next, we discuss the role of sleep in the learning and maintenance of song in zebra finches. We note that while both the rodent and zebra finch systems provide evidence for sleep-dependent memory changes in physiology and behavior, neither duplicates the pattern of changes most commonly observed in humans. Finally, we present a recently developed model of sleep consolidation involving auditory classification learning in European starlings , which has the potential to connect behavioral evidence of sleep consolidation as developed in humans with underlying neural mechanisms observable in animals.

Sleep and cortisol interact to support memory consolidation

Separate lines of research have demonstrated that rises in cortisol can benefit memory consolidation, as can the occurrence of sleep soon after encoding. For the first time, we demonstrate that pre-learning cortisol interacts with sleep to benefit memory consolidation, particularly for negative arousing items. Resting cortisol levels during encoding were positively correlated with subsequent memory, but only following a period of sleep. There was no such relation following a period of wakefulness. Using eye tracking, we further reveal that for negative stimuli, this facilitative effect may arise because cortisol strengthens the relationship between looking time at encoding and subsequent memory. We suggest that elevated cortisol may "tag" attended information as important to remember at the time of encoding, thus enabling sleep-based processes to optimally consolidate salient information in a selective manner. Neuroimaging data suggest that this optimized consolidation leads to a refinement of the neural processes recruited for successful retrieval of negative stimuli, with the retrieval of items attended in the presence of elevated cortisol and consolidated over a night of sleep associated with activity in the amygdala and vmPFC.

Memory for semantically related and unrelated declarative information: the benefit of sleep, the cost of wake

Numerous studies have examined sleep's influence on a range of hippocampus-dependent declarative memory tasks, from text learning to spatial navigation. In this study, we examined the impact of sleep, wake, and time-of-day influences on the processing of declarative information with strong semantic links (semantically related word pairs) and information requiring the formation of novel associations (unrelated word pairs). Participants encoded a set of related or unrelated word pairs at either 9am or 9pm, and were then tested after an interval of 30 min, 12 hr, or 24 hr. The time of day at which subjects were trained had no effect on training performance or initial memory of either word pair type. At 12 hr retest, memory overall was superior following a night of sleep compared to a day of wakefulness. However, this performance difference was a result of a pronounced deterioration in memory for unrelated word pairs across wake; there was no sleep-wake difference for related word pairs. At 24 hr retest, with all subjects having received both a full night of sleep and a full day of wakefulness, we found that memory was superior when sleep occurred shortly after learning rather than following a full day of wakefulness. Lastly, we present evidence that the rate of deterioration across wakefulness was significantly diminished when a night of sleep preceded the wake period compared to when no sleep preceded wake, suggesting that sleep served to stabilize the memories against the deleterious effects of subsequent wakefulness. Overall, our results demonstrate that 1) the impact of 12 hr of waking interference on memory retention is strongly determined by word-pair type, 2) sleep is most beneficial to memory 24 hr later if it occurs shortly after learning, and 3) sleep does in fact stabilize declarative memories, diminishing the negative impact of subsequent wakefulness.