NREM Sleep Stages Research Articles

Afternoon nap and bright light exposure improve cognitive flexibility post lunch.

Beneficial effects of napping or bright light exposure on cognitive performance have been reported in participants exposed to sleep loss. Nonetheless, few studies investigated the effect of these potential countermeasures against the temporary drop in performance observed in mid-afternoon, and even less so on cognitive flexibility, a crucial component of executive functions. This study investigated the impact of either an afternoon nap or bright light exposure on post-prandial alterations in task switching performance in well-rested participants. Twenty-five healthy adults participated in two randomized experimental conditions, either wake versus nap (n=15), or bright light versus placebo (n=10). Participants were tested on a switching task three times (morning, post-lunch and late afternoon sessions). The interventions occurred prior to the post-lunch session. In the nap/wake condition, participants either stayed awake watching a 30-minute documentary or had the opportunity to take a nap for 30 minutes. In the bright light/placebo condition, participants watched a documentary under either bright blue light or dim orange light (placebo) for 30 minutes. The switch cost estimates cognitive flexibility and measures task-switching efficiency. Increased switch cost scores indicate higher difficulties to switch between tasks. In both control conditions (wake or placebo), accuracy switch-cost score increased post lunch. Both interventions (nap or bright light) elicited a decrease in accuracy switch-cost score post lunch, which was associated with diminished fatigue and decreased variability in vigilance. Additionally, there was a trend for a post-lunch benefit of bright light with a decreased latency switch-cost score. In the nap group, improvements in accuracy switch-cost score were associated with more NREM sleep stage N1. Thus, exposure to bright light during the post-lunch dip, a countermeasure easily applicable in daily life, results in similar beneficial effects as a short nap on performance in the cognitive flexibility domain with possible additional benefits on latency switch-cost scores.

Linear and non-linear interdependence of EEG and HRV frequency bands in human sleep.

The characterisation of functional interdependencies of the autonomic nervous system (ANS) stands an evergrowing interest to unveil electroencephalographic (EEG) and Heart Rate Variability (HRV) interactions. This paper presents a biosignal processing approach as a supportive computational resource in the estimation of sleep dynamics. The application of linear, non-linear methods and statistical tests upon 10 overnight polysomnographic (PSG) recordings, allowed the computation of wavelet coherence and phase locking values, in order to identify discerning features amongst the clinical healthy subjects. Our findings showed that neuronal oscillations θ, α and σ interact with cardiac power bands at mid-to-high rank of coherence and phase locking, particularly during NREM sleep stages.

A Polysomnographic and Clinical Study of Sleep Disorders in Patients with Behçet and Neuro-Behçet Syndrome

Brainstem is the most common site of involvement in neuro-Behçet syndrome (NBS). On the other hand, the critical importance of this anatomical region in the regulation of sleep has been disregarded in the literature. We aimed to investigate the microstructure of sleep in patients with Behçet syndrome (BS) and NBS. Patients were allocated to 2 groups: (1) BS without any neurological involvement and (2) NBS with brainstem lesions only. A control group was also enrolled in this study. The comparison of polysomnographic parameters between all patients (BS and NBS) with the control group showed that sleep onset was longer (p = 0.006), the duration of superficial NREM sleep stage (N2) was significantly longer (p = 0.018), and the respiratory disturbance index was significantly higher (p = 0.034) in patients. Sleep apnea and restless legs syndrome are more commonly observed in BS and NBS. Our findings emphasize the importance of questioning the quality of sleep and its disorders in patients with BS in order to better handle the common somatic complaints in these patients, such as fatigue or daytime sleepiness.

Sleep architecture following treatment of patients with middle-of-the-night insomnia with buffered sublingual zolpidem compared to placebo

Aside from inducing and maintaining sleep, hypnotic medications also impact sleep stages. Historically, the barbiturates were REM suppressants, while the benzodiazepines were Stage 3–4 (SWS) suppressants. The newer benzodiazepine receptor agonists like zolpidem increase stage 2 and latency to REM. Previous research, however, was conducted utilizing bedtime dosing. Across the night, there are dramatic changes in sleep stages, with the first half being primarily enriched with SWS, and the last half with REM sleep. A buffered zolpidem sublingual formulation (ZST) is approved to be taken in the middle of the night (MOTN) by patients with MOTN insomnia (and at least 4 h of bedtime remaining). The question then arises regarding possible sleep stage effects following MOTN ZST dosing taking pharmacologic and circadian influences into consideration. A double-blind, placebo-controlled 3- way cross-over sleep laboratory polysomnography study conducted with 1.75 and 3.5 mg buffered zolpidem tartrate sublingual tablets (ZST) demonstrated efficacy, compared to placebo, on latency to persistent sleep (LPS) when administered to patients with primary insomnia whose chief complaint was MOTN awakenings with difficulty returning to sleep. 58 Female and 24 male patients were randomized. Patients were dosed with 3.5 mg, 1.75 mg or placebo 4 h after lights out, kept awake for 30 min, then returned to bed for 4 h. Sleep stages were scored centrally by standardized criteria in 30 s epochs for 4 h predose and 4 h postdose. The mean of each sleep parameter was calculated from the postdose periods of the 2 treatment nights. Subjective ratings of sleep quality and morning alertness were also obtained. LPS, total sleep time, and sleep efficiency all improved significantly postdose with both ZST doses compared to placebo. Postdose, there was a dose-dependent change in NREM sleep. The percent of light stage 1 decreased, while the % and minutes of deeper stages 2 and 3–4 increased. While REM sleep minutes were stable, % REM declined with a minor dose- dependent increase in REM latency. The overall improvements in sleep quality and daytime alertness reported by patients on mornings post ZST dosing may be related to the differential effects in NREM sleep stages. In summary, sleep after MOTN dosing with ZST included all sleep stages, including SWS typically not observed in the latter half of the night. These findings may relate to the improved ratings of sleep quality and alertness. Funding was provided by Transcept Pharmaceuticals, Inc. and Purdue Pharma LP .

Cardiac autonomic modulation and sleepiness: Physiological consequences of sleep deprivation due to 40 h of prolonged wakefulness

The autonomic nervous system (ANS) is modulated by sleep and wakefulness. Noninvasive assessment of cardiac ANS with heart rate variability (HRV) analysis is a window for monitoring malfunctioning of cardiovascular autonomic modulation due to sleep deprivation.This study represents the first investigation of dynamic ANS effects and of electrophysiological and subjective sleepiness, in parallel, during 40h of prolonged wakefulness under constant routine (CR) conditions. In eleven young male healthy subjects, ECG, EEG, EOG, and EMG chin recordings were performed during baseline sleep, during 40h of sleep deprivation, and during recovery sleep. After sleep deprivation, slow-wave sleep and sleep efficiency increased, whereas HRV – global variability and HRV sympathovagal balance – was reduced (all p<0.05). Sleep-stage-dependent analysis revealed reductions in the sympathovagal balance only for NREM sleep stages (all p<0.05). Comparison of the daytime pattern of CR day one (CR baseline) with that of CR day two (CR sleep deprivation) disclosed an increase in subjective sleepiness, in the amount of unintended sleep, and in HRV sympathovagal balance, with accompaniment by increased EEG alpha attenuation (all p<0.05). Circadian rhythm analysis revealed the strongest influence on heart rate, with less influence on HRV sympathovagal balance. Hour-by-hour analysis disclosed the difference between CR sleep deprivation and CR baseline for subjective sleepiness at almost every single hour and for unintended sleep particularly in the morning and afternoon (both p<0.05). These findings indicate that 40h of prolonged wakefulness lead in the following night to sleep-stage-dependent reduction in cardiac autonomic modulation. During daytime, an increased occurrence of behavioral and physiological signs of sleepiness was accompanied by diminished cardiac autonomic modulation. The observed changes are an indicator of autonomic stress due to sleep deprivation — which, if chronic, could potentially increase cardiovascular risk.

Sleep Transitions in Hypocretin-Deficient Narcolepsy

Narcolepsy is characterized by instability of sleep-wake, tonus, and rapid eye movement (REM) sleep regulation. It is associated with severe hypothalamic hypocretin deficiency, especially in patients with cataplexy (loss of tonus). As the hypocretin neurons coordinate and stabilize the brain's sleep-wake pattern, tonus, and REM flip-flop neuronal centers in animal models, we set out to determine whether hypocretin deficiency and/or cataplexy predicts the unstable sleep-wake and REM sleep pattern of the human phenotype. We measured the frequency of transitions in patients with narcolepsy between sleep-wake states and to/from REM and NREM sleep stages. Patients were subdivided by the presence of +/- cataplexy and +/- hypocretin-1 deficiency. Sleep laboratory studies conducted from 2001-2011. In total 63 narcolepsy patients were included in the study. Cataplexy was present in 43 of 63 patients and hypocretin-1 deficiency was present in 37 of 57 patients. Hypocretin-deficient patients with narcolepsy had a significantly higher frequency of sleep-wake transitions (P = 0.014) and of transitions to/from REM sleep (P = 0.044) than patients with normal levels of hypocretin-1. Patients with cataplexy had a significantly higher frequency of sleep-wake transitions (P = 0.002) than those without cataplexy. A multivariate analysis showed that transitions to/from REM sleep were predicted mainly by hypocretin-1 deficiency (P = 0.011), whereas sleep-wake transitions were predicted mainly by cataplexy (P = 0.001). In human narcolepsy, hypocretin deficiency and cataplexy are both associated with signs of destabilized sleep-wake and REM sleep control, indicating that the disorder may serve as a human model for the sleep-wake and REM sleep flip-flop switches.

Sleep and Anesthesia - Common mechanisms of action.

Author(s): Vacas, Susana; Kurien, Philip; Maze, Mervyn | Abstract: Appropriate sleep hygiene is crucial for repair in states of disease and injury and in restoring function especially in the central nervous and immune systems. Anesthetics have different action targets and ultimately different consequences. The effects of anesthetics on circadian rhythm possibly also lead to immune deregulation. α2-adrenergic agonists converge on sleep pathways within the brainstem and are associated with changes in neuronal activity similar to those seen in deeper stages of NREM sleep. Thoughtful attention must be made in selecting an anesthetic agent that best mimics natural sleep. Future studies will further elucidate the benefits of dexmedetomidine as a good anesthetic/sedative candidate. © 2013 Elsevier Inc.

1161 – Generalized partial directed coherence provides a new description of the brain functional connectivity across sleep stages

Objectives Cerebral activity during can be approached by the measurement of functional activity in healthy young people. Functional activity reflects the statistical dependencies among near and distant brain regions by generalized partial directed coherence (GPDC). Methods Sleep stages of 9 healthy young subjects without somatic, psychiatric and sleep disorders were defined in accordance with the Rechtschaffen and Kales criteria (Wake, stages 1 to 4 and REM sleep). In regard of specific sleep EEG frequencies (delta, theta, alpha, sigma and beta), the first 20 epochs of each sleep stage were used for analysis. GPDC was applied between each electrode and each other of the 19 EEG electrodes. Results In comparison to wake and REM sleep, GPDC showed increased interdependencies across each NREM sleep stage. Lower sleep EEG frequencies increased these changes. Moreover, during slow EEG rhythms, differences observed across stages were more pronounced. In addition, differences between bands were more marked during wake and sleep stages 1 and 2. None differences were observed for GPDC values between both sides of human brain. In the right hemisphere as well as in the left one, GPDC is similar between sleep stages in delta frequency while GPDC differed from wake to other sleep stages in theta, alpha, sigma and beta EEG frequencies. Conclusions A different mapping in the three classical sleep states (Wake, NREM and REM sleep) is showed by generalized partial directed coherence in sleep EEG frequencies. Additional studies investigating the directional information flow should be performed to complete our results.

Topography-specific spindle frequency changes in Obstructive Sleep Apnea

BackgroundSleep spindles, as detected on scalp electroencephalography (EEG), are considered to be markers of thalamo-cortical network integrity. Since obstructive sleep apnea (OSA) is a known cause of brain dysfunction, the aim of this study was to investigate sleep spindle frequency distribution in OSA. Seven non-OSA subjects and 21 patients with OSA (11 mild and 10 moderate) were studied. A matching pursuit procedure was used for automatic detection of fast (≥13Hz) and slow (<13Hz) spindles obtained from 30min samples of NREM sleep stage 2 taken from initial, middle and final night thirds (sections I, II and III) of frontal, central and parietal scalp regions.ResultsCompared to non-OSA subjects, Moderate OSA patients had higher central and parietal slow spindle percentage (SSP) in all night sections studied, and higher frontal SSP in sections II and III. As the night progressed, there was a reduction in central and parietal SSP, while frontal SSP remained high. Frontal slow spindle percentage in night section III predicted OSA with good accuracy, with OSA likelihood increased by 12.1%for every SSP unit increase (OR 1.121, 95% CI 1.013 - 1.239, p=0.027).ConclusionsThese results are consistent with diffuse, predominantly frontal thalamo-cortical dysfunction during sleep in OSA, as more posterior brain regions appear to maintain some physiological spindle frequency modulation across the night. Displaying changes in an opposite direction to what is expected from the aging process itself, spindle frequency appears to be informative in OSA even with small sample sizes, and to represent a sensitive electrophysiological marker of brain dysfunction in OSA.

Survival analysis indicates that age-related decline in sleep continuity occurs exclusively during NREM sleep

A common complaint of older persons is disturbed sleep, typically characterized as an inability to return to sleep after waking. As every sleep episode (i.e., time in bed) includes multiple transitions between wakefulness and sleep (which can be subdivided into rapid eye movement [REM] sleep and non-REM [NREM] sleep), we applied survival analysis to sleep data to determine whether changes in the “hazard” (duration-dependent probability) of awakening from sleep and/or returning to sleep underlie age-related sleep disturbances. The hazard of awakening from sleep—specifically NREM sleep—was much greater in older than in young adults. We found, however, that when an individual had spontaneously awakened, the probability of falling back asleep was not greater in young persons. Independent of bout length, the number of transitions between NREM and REM sleep stages relative to number of transitions to wake was approximately 6 times higher in young than older persons, highlighting the difficulty in maintaining sleep in older persons. Interventions to improve age-related sleep complaints should thus target this change in awakenings.

EEG microstates of wakefulness and NREM sleep

EEG-microstates exploit spatio-temporal EEG features to characterize the spontaneous EEG as a sequence of a finite number of quasi-stable scalp potential field maps. So far, EEG-microstates have been studied mainly in wakeful rest and are thought to correspond to functionally relevant brain-states. Four typical microstate maps have been identified and labeled arbitrarily with the letters A, B, C and D. We addressed the question whether EEG-microstate features are altered in different stages of NREM sleep compared to wakefulness. 32-channel EEG of 32 subjects in relaxed wakefulness and NREM sleep was analyzed using a clustering algorithm, identifying the most dominant amplitude topography maps typical of each vigilance state. Fitting back these maps into the sleep-scored EEG resulted in a temporal sequence of maps for each sleep stage. All 32 subjects reached sleep stage N2, 19 also N3, for at least 1 min and 45 s. As in wakeful rest we found four microstate maps to be optimal in all NREM sleep stages. The wake maps were highly similar to those described in the literature for wakefulness. The sleep stage specific map topographies of N1 and N3 sleep showed a variable but overall relatively high degree of spatial correlation to the wake maps (Mean: N1 92%; N3 87%). The N2 maps were the least similar to wake (mean: 83%). Mean duration, total time covered, global explained variance and transition probabilities per subject, map and sleep stage were very similar in wake and N1. In wake, N1 and N3, microstate map C was most dominant w.r.t. global explained variance and temporal presence (ratio total time), whereas in N2 microstate map B was most prominent. In N3, the mean duration of all microstate maps increased significantly, expressed also as an increase in transition probabilities of all maps to themselves in N3. This duration increase was partly--but not entirely--explained by the occurrence of slow waves in the EEG. The persistence of exactly four main microstate classes in all NREM sleep stages might speak in favor of an in principle maintained large scale spatial brain organization from wakeful rest to NREM sleep. In N1 and N3 sleep, despite spectral EEG differences, the microstate maps and characteristics were surprisingly close to wakefulness. This supports the notion that EEG microstates might reflect a large scale resting state network architecture similar to preserved fMRI resting state connectivity. We speculate that the incisive functional alterations which can be observed during the transition to deep sleep might be driven by changes in the level and timing of activity within this architecture.

Neurophysiological Two-Channel Polysomnographic Device in the Diagnosis of Sleep Apnea

Our objective was to evaluate a portable device (Somté, Compumedics, Australia), which incorporates 2 neurophysiological channels (electroencephalography and electrooculography) with cardiorespiratory monitoring for the diagnosis of obstructive sleep apnea (OSA). Full polysomnography (PSG) and Somté recordings were simultaneously performed in 68 patients with suspected OSA. Data were analyzed blindly by 2 scorers. A good agreement between methods in sleep efficiency was observed (68.8% [18.4] with PSG vs 68% [19.1] with Somté [p: n.s.] for scorer 1, and 67.5% [19.1] vs 68.4% [18.5; p: n.s.] for scorer 2). The apnea-hypopnea index (AHI) obtained with Somté was lower than with PSG: 19 (17.8) vs 21.7 (19) (p < 0.001) for scorer 1, and 16.6 (16.7) vs 20 (18.8) (p < 0.001) for scorer 2. The sensitivity of Somté for a PSG-AHI > 5 was 91% for scorer 1 and 90% for scorer 2, while specificity was 77% and 90%, respectively. The areas under the receiver operating curve for different PSG-AHI cutoff points (≥ 5, ≥ 15, and ≥ 30) were 0.81, 0.90, and 0.86, respectively, for scorer 1, and 0.90, 0.88, and 0.83 for scorer 2. These data suggest that Somté is an effective device to identify sleep and respiratory variables in patients with suspected OSA.

Functional and Effective Connectivity in Healthy Human EEG across Sleep Stages.

Functional and Effective Connectivity in Healthy Human EEG across Sleep Stages.

NREM Sleep Stage Transitions Control Ultradian REM Sleep Rhythm

The cyclic sequence of NREM and REM sleep, the so-called ultradian rhythm, is a highly characteristic feature of sleep. However, the mechanisms responsible for the ultradian REM sleep rhythm, particularly in humans, have not to date been fully elucidated. We hypothesize that a stage transition mechanism is involved in the determination of the ultradian REM sleep rhythm. Ten healthy young male volunteers (AGE: 22 ± 4 years, range 19-31 years) spent 3 nights in a sleep laboratory. The first was the adaptation night, and the second was the baseline night. On the third night, the subjects received risperidone (1 mg tablet), a central serotonergic and dopaminergic antagonist, 30 min before the polysomnography recording. We measured and investigated transition probabilities between waking, REM, and NREM sleep stages (N1, N2, and N3) within the REM-onset intervals, defined as the intervals between the onset of one REM period and the beginning of the next, altered by risperidone. We also calculated the transition intensity (i.e., instantaneous transition rate) and examined the temporal pattern of transitions within the altered REM-onset intervals. We found that when the REM-onset interval was prolonged by risperidone, the probability of transitions from N2 to N3 was significantly increased within the same prolonged interval, with a significant delay and/or recurrences of the peak intensity of transitions from N2 to N3. These results suggest that the mechanism governing NREM sleep stage transitions (from light to deep sleep) plays an important role in determining ultradian REM sleep rhythms.

NREM sleep staging using WAVCNS index

Visual scoring of 30-s epochs of sleep data is not always adequate to show the dynamic structure of sleep in sufficient details. It is also prone to considerable inter- and intra-rater variability. Moreover, it involves considerable training and experience, and is very tedious, time-consuming, labor-intensive and costly. Hence, automatic sleep staging is needed to overcome these limitations. Since naturally occurring NREM sleep and anesthesia have been reported to possess various underlying neurophysiological similarities, EEG-based depth-of-anesthesia monitors have started to penetrate into sleep research. This study investigates the ability of WAV(CNS) index (as implemented in NeuroSENSE depth-of-anesthesia monitor) to detect NREM sleep stages and wake state for full overnight PSG data. Full overnight PSG sleep data, obtained from 24 adolescents, was scored by a registered PSG technologist for different sleep stages. Retrospective analysis was performed on a single frontal channel using the WAV(CNS) algorithm. Non-parametric descriptive statistics were used to examine the relationship between WAV(CNS) index and sleep stages. A strong correlation (ρ=0.9458) was found between the WAV(CNS) index and NREM sleep stages, with WAV(CNS) index values decreasing with increasing sleep stages. Moreover, there was no significant overlap between different NREM sleep stages as classified by the WAV(CNS) index, which was able to significantly differentiate (P < 0.001) between all pairs of Awake and different NREM stages. This study demonstrates that changes in the depth of natural NREM sleep are reflected sensitively by changes in the WAV(CNS) index. Hence, WAV(CNS) index may serve as an automatic real-time indicator of depth of natural sleep with high temporal resolution, and can possibly be of great use for automated sleep staging in routine/postoperative somnographic studies.

Slow eye movements distribution during nocturnal sleep

Objective To assess the distribution across nocturnal sleep of slow eye movements (SEMs). Methods We evaluated SEMs distribution in the different sleep stages, and across sleep cycles in nocturnal recordings of 10 healthy women. Sleep was scored according to standard criteria, and the percentage of time occupied by the SEMs was automatically detected. Results SEMs were differently represented during sleep stages with the following order: wakefulness after sleep onset (WASO): 61%, NREM sleep stage 1: 54%, REM sleep: 43%, NREM sleep stage 2: 21%, NREM sleep stage 3: 7%, and NREM sleep stage 4: 3% ( p < 0.0001). There was no difference between phasic and tonic REM sleep. SEMs progressively decreased across the NREM sleep cycles (38%, 15%, 13% during NREM sleep stage 2 in the first three sleep cycles, p = 0.006), whereas no significant difference was found for REM, NREM sleep stage 1, slow-wave sleep and WASO. Conclusions Our findings confirm that SEMs are a phenomenon typical of the sleep onset period, but are also found in REM sleep. The nocturnal evolution of SEMs during NREM sleep stage 2 parallels the homeostatic process underlying slow-wave sleep. Significance SEMs are a marker of sleepiness and, potentially, of sleep homeostasis.

The effects of experimental sleep fragmentation on cognitive processing

Objective The primary objective of this study was to characterize the association between cyclic alternating pattern (CAP) and neurocognitive performance in a group of normal subjects before and after two nights of experimentally-induced sleep fragmentation. Subjects and methods Fifteen healthy subjects underwent one night of uninterrupted and two sequential nights of experimental sleep fragmentation achieved by auditory and mechanical stimuli. Eight subjects were re-examined using a similar paradigm with three nights of uninterrupted sleep. Sleep was polygraphically recorded and CAP analysis was performed for all recordings. A battery of neurocognitive tests was performed for spatial attention, inhibition of return, mental rotation, and Stroop color word test in the afternoon following the first and third night of sleep under fragmented and non-fragmented conditions. Results With sleep fragmentation, the percentage of slow-wave sleep was dramatically reduced and there was a twofold increase in total CAP rate across all NREM sleep stages. Moreover, the number of all CAP A subtypes/hour of sleep (index) was significantly increased. Total CAP rate during the non-fragmented night correlated with reaction times. Similarly, the percentages of A1 and A3 subtypes were negatively and positively correlated with reaction times, respectively. Of the neurocognitive test battery, however, only values obtained from some subtests of the mental rotation test showed a significant improvement after sleep fragmentation. Conclusions The results of this study suggest that CAP A1 subtypes are associated with higher cognitive functioning, whereas CAP A3 subtypes are associated with lower cognitive functioning in young healthy subjects. The lack of cognitive functioning impairment after sleep fragmentation may be due to persistence and even enhancement of transient slow-wave activity contained in CAP A1 subtypes which also caused a significant enhancement of the EEG power spectrum in the lower frequencies.

Discovery of 1-[3-(4-Bromo-2-methyl-2H-pyrazol-3-yl)-4-methoxyphenyl]-3-(2,4-difluorophenyl)urea (Nelotanserin) and Related 5-Hydroxytryptamine2A Inverse Agonists for the Treatment of Insomnia

Insomnia affects a growing portion of the adult population in the U.S. Most current therapeutic approaches to insomnia primarily address sleep onset latency. Through the 5-hydroxytryptamine(2A) (5-HT(2A)) receptor, serotonin (5-HT) plays a role in the regulation of sleep architecture, and antagonists/inverse-agonists of 5-HT(2A) have been shown to enhance slow wave sleep (SWS). We describe here a series of 5-HT(2A) inverse-agonists that when dosed in rats, both consolidate the stages of NREM sleep, resulting in fewer awakenings, and increase a physiological measure of sleep intensity. These studies resulted in the discovery of 1-[3-(4-bromo-2-methyl-2H-pyrazol-3-yl)-4-methoxyphenyl]-3-(2,4-difluorophenyl)urea (Nelotanserin), a potent inverse-agonist of 5-HT(2A) that was advanced into clinical trials for the treatment of insomnia.

Life Goes on in Dreams

WHAT GOES THROUGH OUR MINDS WHILE WE FALL ASLEEP? AT SLEEP ONSET, STUDENTS PREPARING FOR EXAMS COMMONLY RE-EXPERIENCE SOME OF the material they have been revising during the day; lovers have pervasive thoughts about their romantic partner; software developers may see bits of programming codes unfold in their mind's eye. These anecdotal observations suggest that, during the transition from wakefulness to sleep, elements from daytime activities may spontaneously re-emerge in dream-like mentation also called hypnagogic imagery. In a behavioral study published in this issue of SLEEP, Wamsley and colleagues1 provide support for the existence of such spontaneous cognitive “replay” at sleep onset. In this study, healthy young participants were trained on a highly motivating visuomotor game–the downhill skiing game Alpine Racer II. In this simulated skiing game, the player stands on a movable platform while holding two ski poles. The player's goal is to use his/her body to move the platform left or right to steer and thus improve the downhill skiing of an “avatar skier” presented on a computer screen. After training on the Alpine Racer, 30% of spontaneous mentation collected at different times during pre-sleep wakefulness and light NREM sleep (up to 300 sec after sleep onset) contained imagery (of any modality, 24%) or thoughts (6%) related to the skiing game. Wamsley et al.1 also found that imagery directly related to training on the game (unambiguous representations of the Alpine Racer or of skiing) declined across time. This time-course was paralleled by a tendency for game-related incorporations to become more abstracted from the original experience. These findings do not only provide empirical evidence for spontaneous memory replay during wakefulness and light NREM sleep (stages 1 and 2), but they show that reports of subjective experience offer valuable information about cognitive processes across changing brain states.2 Consistent with animal neurophysiology data,3 human imaging studies have demonstrated that brain activity associated with waking experiences may be reactivated during subsequent sleep, and can thus contribute to memory consolidation and improved performance.4–7 However, although incorporation of the Alpine Racing game was high in the Wamsley study, suggesting abundant memory replay at sleep onset, no relationship was found between introspective measures of memory replay and later improvement in performance on the game. Whether dreaming of some tasks or stimuli may enhance subsequent memory or performance, and whether dreaming actually relates to neural reactivation subsequent to learning, are fundamental questions that still remain unanswered.8–10 The popular belief that dreams reflect waking-life activities—the “continuity hypothesis of dreaming”—has found strong empirical support in the field of dream research.11–13 The recent study by Wamsley et al.1 appears to extent the validity of the continuity hypothesis to hypnagogic imagery and offers new insights into which daytime experiences may be good candidates for cognitive replay. High levels of incorporation of Alpine Racer are most plausibly related to the strong motivational and attentional involvement of the player during the game. Consistent with this interpretation, a few participants who only observed those playing Alpine Racer also incorporated elements of the game into their sleep-onset mentation, at rates similar to the participants who were actively engaged in the game. These effects and their time-course suggest that novelty may be a critical factor for the selection of material to be mentally replayed (Figure 1). Moreover, many baseline night reports incorporated thought or imagery related to the game (compared to a control set of sleep-onset mentation reports), indicating that the mere anticipation of the task could trigger prospective memory processes that emerged at sleep onset. It is tempting to speculate that hypnagogic imagery may contribute to the integration of recent experiences with long-term memories and future goals. How this is achieved at the cognitive and neurophysiological levels is a fascinating and challenging topic for future investigations. Figure 1 Impact of novelty on dream experience. Five days after seeing a new model of an airplane in a newspaper (original photograph on the left), the dreamer reported a very similar airplane in a dream. The drawings (on the right) that accompanied the dream ... Notably, while consciousness dissipates, waking concerns may sometimes also prevent us from falling asleep. For example, pre-sleep cognitive activity has been implicated in the maintenance of sleep-onset insomnia.14,15 Therefore, manipulating hypnagogic imagery at sleep onset may also have important clinical implications for the management of sleep disorders.

Cognitive Replay of Visuomotor Learning at Sleep Onset: Temporal Dynamics and Relationship to Task Performance

Studies of neural activity in animals and humans suggest that experiences are "replayed" in cortical and hippocampal networks during NREM sleep. Here, we examine whether memory reactivation in sleeping humans might also be evident within reports of concomitant subjective experience (i.e., dreaming). Participants were trained on an engaging visuomotor learning task across a period of one or more days, and sleep onset mentation was collected at variable intervals using the "Nightcap" home-monitoring device. Verbal reports of sleep onset mentation were obtained either at the beginning of the night, or following 2 h of initial sleep. Data were collected in participants' home environments, via the Nightcap monitoring system, and at The Center for Sleep and Cognition, Beth Israel Deaconess Medical Center, Boston MA. 43 healthy, medication-free college students (16 males, age 18-25 years). N/A. The learning task exerted a powerful, direct effect on verbal reports of mentation during light NREM sleep (stages 1 and 2). On post-training nights, a full 30% of all verbal reports were related to the task. The nature of this cognitive "replay" effect was altered with increasing durations of sleep, becoming more abstracted from the original experience as time into sleep increased. These observations are interpreted in light of memory consolidation theory, and demonstrate that introspective reports can provide a valuable window on cognitive processing in the sleeping brain.