Athletes underestimate sleep quantity during daytime nap opportunities

ABSTRACT

This study examined the difference between athletes’ self-reported and objective sleep durations during two nap opportunities. Twelve well-trained male soccer players’ sleep durations were assessed using polysomnography and a self-report question during a 60- and 120-min nap opportunity. Participants underestimated sleep compared to objective sleep assessments for both the 60-min nap opportunity (p = 0.004) and 120-min nap opportunity (p = 0.001). Soccer players underestimated their sleep duration by approximately 10 min per hour of nap opportunity. It is yet to be determined if athletes are likely to underestimate sleep duration during their main nighttime sleep period.     

Proprioceptive Feedback and Brain Computer Interface (BCI) Based Neuroprostheses

Brain computer interface (BCI) technology has been proposed for motor neurorehabilitation, motor replacement and assistive technologies. It is an open question whether proprioceptive feedback affects the regulation of brain oscillations and therefore BCI control. We developed a BCI coupled on-line with a robotic hand exoskeleton for flexing and extending the fingers. 24 healthy participants performed five different tasks of closing and opening the hand: (1) motor imagery of the hand movement without any overt movement and without feedback, (2) motor imagery with movement as online feedback (participants see and feel their hand, with the exoskeleton moving according to their brain signals, (3) passive (the orthosis passively opens and closes the hand without imagery) and (4) active (overt) movement of the hand and rest. Performance was defined as the difference in power of the sensorimotor rhythm during motor task and rest and calculated offline for different tasks. Participants were divided in three groups depending on the feedback receiving during task 2 (the other tasks were the same for all participants). Group 1 (n = 9) received contingent positive feedback (participants'' sensorimotor rhythm (SMR) desynchronization was directly linked to hand orthosis movements), group 2 (n = 8) contingent “negative” feedback (participants'' sensorimotor rhythm synchronization was directly linked to hand orthosis movements) and group 3 (n = 7) sham feedback (no link between brain oscillations and orthosis movements). We observed that proprioceptive feedback (feeling and seeing hand movements) improved BCI performance significantly. Furthermore, in the contingent positive group only a significant motor learning effect was observed enhancing SMR desynchronization during motor imagery without feedback in time. Furthermore, we observed a significantly stronger SMR desynchronization in the contingent positive group compared to the other groups during active and passive movements. To summarize, we demonstrated that the use of contingent positive proprioceptive feedback BCI enhanced SMR desynchronization during motor tasks.     

The Effects of a Nap Opportunity in Quiet and Noisy Environments on Driving Performance

While napping has previously been shown to alleviate the effects of sleep loss, before advocating the use of naps in transport accident campaigns it is necessary to consider whether a nap opportunity in a noisy uncomfortable environment can produce the same benefits as a nap opportunity in conditions that are conducive to sleep. To examine this, eight participants drove a driving simulator for 50 min at 11:00 h on three different test days. The simulator used has previously been found to be sensitive to the effects of sleep loss, alcohol consumption, and time of day. All three sessions were conducted after one night of sleep loss. Prior to driving during each session the participants either had a 60 min nap opportunity in a quiet or noisy environment, or no nap opportunity. Driving performance and reaction time while driving were measured, as were subjective sleepiness and ratings of sleep quality. No significant benefits of nap opportunities on driving performance were found. Levels of subjective sleepiness were not affected by the nap opportunity condition; however, sleep was rated as more refreshing and restful after a nap in a quiet environment compared to noisy environment. The measures of effect size reported suggest further research is required to unequivocally test the effects of nap opportunities on driving ability.     

The effects of a nap opportunity in quiet and noisy environments on driving performance

While napping has previously been shown to alleviate the effects of sleep loss, before advocating the use of naps in transport accident campaigns it is necessary to consider whether a nap opportunity in a noisy uncomfortable environment can produce the same benefits as a nap opportunity in conditions that are conducive to sleep. To examine this, eight participants drove a driving simulator for 50 min at 11:00 h on three different test days. The simulator used has previously been found to be sensitive to the effects of sleep loss, alcohol consumption, and time of day. All three sessions were conducted after one night of sleep loss. Prior to driving during each session the participants either had a 60 min nap opportunity in a quiet or noisy environment, or no nap opportunity. Driving performance and reaction time while driving were measured, as were subjective sleepiness and ratings of sleep quality. No significant benefits of nap opportunities on driving performance were found. Levels of subjective sleepiness were not affected by the nap opportunity condition; however, sleep was rated as more refreshing and restful after a nap in a quiet environment compared to noisy environment. The measures of effect size reported suggest further research is required to unequivocally test the effects of nap opportunities on driving ability.     

Effect of daytime nap on consolidation of declarative memory in humans

We studied effects of a daytime nap (1 hour) with including only NREM sleep on performance of declarative memory task (60 semantically unrelated word pairs) and general functional state. During training, procedure of learning of 30 word pairs was presented once, and that of the other 30 pairs was repeated twice. Strength of the task acquisition was tested. Subjects participated in two experiments: basic and control one. After learning participants either took a nap (basic experiment) or kept awake looking movies (control experiment). In 4.5 hours after the training session all the subjects were retested. As compared to the subjects who stayed awake during the training-retesting interval, subjects who had a NREM nap demonstrated enhanced performance. Concerning the strength of task acquisition, sleep-dependent performance was observed only for the word pairs learned once. Naps did not affect the functional state assessed by the reaction time dynamics and psychological testing.     

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.     

The effects of extended nap periods on cognitive,physiological and subjective responses under simulated night shift conditions

Extended nap opportunities have been effective in maintaining alertness in the context of extended night shifts (+12?h). However, there is limited evidence of their efficacy during 8-h shifts. Thus, this study explored the effects of extended naps on cognitive, physiological and perceptual responses during four simulated, 8-h night shifts. In a laboratory setting, 32 participants were allocated to one of three conditions. All participants completed four consecutive, 8-h night shifts, with the arrangements differing by condition. The fixed night condition worked from 22h00 to 06h00, while the nap early group worked from 20h00 to 08h00 and napped between 00h00 and 03h20. The nap late group worked from 00h00 to 12h00 and napped between 04h00 and 07h20. Nap length was limited to 3 hours and 20 minutes. Participants performed a simple beading task during each shift, while also completing six to eight test batteries roughly every 2?h. During each shift, six test batteries were completed, in which the following measures were taken. Performance indicators included beading output, eye accommodation time, choice reaction time, visual vigilance, simple reaction time, processing speed and object recognition, working memory, motor response time and tracking performance. Physiological measures included heart rate and tympanic temperature, whereas subjective sleepiness and reported sleep length and quality while outside the laboratory constituted the self reported measures. Both naps reduced subjective sleepiness but did not alter the circadian and homeostatic-related changes in cognitive and physiological measures, relative to the fixed night condition. Additionally, there was evidence of sleep inertia following each nap, which resulted in transient reductions in certain perceptual cognitive performance measures. The present study suggested that there were some benefits associated with including an extended nap during 8-h night shifts. However, the effects of sleep inertia need to be effectively managed to ensure that post-nap alertness and performance is maintained.     

The Impact of Different Visual Feedbacks in User Training on Motor Imagery Control in BCI

The challenges of research into brain–computer interfaces (BCI) include significant individual differences in learning pace and in the effective operation of BCI devices. The use of neurofeedback training is a popular method of improving the effectiveness BCI operation. The purpose of the present study was to determine to what extent it is possible to improve the effectiveness of operation of sensorimotor rhythm-based brain–computer interfaces (SMR-BCI) by supplementing user training with elements modifying the characteristics of visual feedback. Four experimental groups had training designed to reinforce BCI control by: visual feedback in the form of dummy faces expressing emotions (Group 1); flashing the principal elements of visual feedback (Group 2) and giving both visual feedbacks in one condition (Group 3). The fourth group participated in training with no modifications (Group 4). Training consisted of a series of trials where the subjects directed a ball into a basket located to the right or left side of the screen. In Group 1 a schematic image a face, placed on the controlled object, showed various emotions, depending on the accuracy of control. In Group 2, the cue and targets were flashed with different frequency (4 Hz) than the remaining elements visible on the monitor. Both modifications were also used simultaneously in Group 3. SMR activity during the task was recorded before and after the training. In Group 3 there was a significant improvement in SMR control, compared to subjects in Group 2 and 4 (control). Differences between subjects in Groups 1, 2 and 4 (control) were insignificant. This means that relatively small changes in the training procedure may significantly impact the effectiveness of BCI control. Analysis of behavioural data acquired from all participants at training showed greater effectiveness in directing the object towards the right side of the screen. Subjects with the greatest improvement in SMR control showed a significantly lower difference in the accuracy of rightward and leftward movement than others.     

Daytime napping after a night of sleep loss decreases sleepiness, improves performance, and causes beneficial changes in cortisol and interleukin-6 secretion

Sleep loss has been associated with increased sleepiness, decreased performance, elevations in inflammatory cytokines, and insulin resistance. Daytime napping has been promoted as a countermeasure to sleep loss. To assess the effects of a 2-h midafternoon nap following a night of sleep loss on postnap sleepiness, performance, cortisol, and IL-6, 41 young healthy individuals (20 men, 21 women) participated in a 7-day sleep deprivation experiment (4 consecutive nights followed by a night of sleep loss and 2 recovery nights). One-half of the subjects were randomly assigned to take a midafternoon nap (1400-1600) the day following the night of total sleep loss. Serial 24-h blood sampling, multiple sleep latency test (MSLT), subjective levels of sleepiness, and psychomotor vigilance task (PVT) were completed on the fourth (predeprivation) and sixth days (postdeprivation). During the nap, subjects had a significant drop in cortisol and IL-6 levels (P < 0.05). After the nap they experienced significantly less sleepiness (MSLT and subjective, P < 0.05) and a smaller improvement on the PVT (P < 0.1). At that time, they had a significant transient increase in their cortisol levels (P < 0.05). In contrast, the levels of IL-6 tended to remain decreased for approximately 8 h (P = 0.1). We conclude that a 2-h midafternoon nap improves alertness, and to a lesser degree performance, and reverses the effects of one night of sleep loss on cortisol and IL-6. The redistribution of cortisol secretion and the prolonged suppression of IL-6 secretion are beneficial, as they improve alertness and performance.     

Effects of increased training load on vagal-related indexes of heart rate variability: a novel sleep approach

There is little doubt that moderate training improves cardiac vagal activity and thus has a cardioprotective effect against lethal arrhythmias. Our purpose was to learn whether a higher training load would further increase this beneficial effect. Cardiac autonomic control was inferred from heart rate variability (HRV) and analyzed in three groups of young subjects (24.5 +/- 3.0 yr) with different training states in a period free of stressful stimuli or overload. HRV was analyzed in 5-min segments during slow-wave sleep (SWS, a parasympathetic state that offers high electrocardiographic stationarity) and compared with data collected during quiet waking periods in the morning. Sleep parameters, fatigue, and stress levels checked by questionnaire were identical for all three groups with no signs of overtraining in the highly trained (HT) participants. During SWS, a significant (P <0.05) increase in absolute and normalized vagal-related HRV indexes was observed in moderately trained (MT) individuals compared with sedentary (Sed) subjects; this increase did not persist in HT athletes. During waking periods, most of the absolute HRV indexes indistinctly increased in MT individuals compared with controls (P < 0.05) but did not increase in HT athletes. Normalized spectral HRV indexes did not change significantly among the three groups. Heart rate was similar for MT and Sed subjects but was significantly (P <0.05) lower in HT athletes under both recording conditions. These results indicate that SWS discriminates the state of sympathovagal balance better than waking periods. A moderate training load is sufficient to increase vagal-related HRV indexes. However, in HT individuals, despite lower heart rate, vagal-related HRV indexes return to Sed values even in the absence of competition, fatigue, or overload.     

Are Complexity Metrics Reliable in Assessing HRV Control in Obese Patients During Sleep?

Obesity is associated with cardiovascular mortality. Linear methods, including time domain and frequency domain analysis, are normally applied on the heart rate variability (HRV) signal to investigate autonomic cardiovascular control, whose imbalance might promote cardiovascular disease in these patients. However, given the cardiac activity non-linearities, non-linear methods might provide better insight. HRV complexity was hereby analyzed during wakefulness and different sleep stages in healthy and obese subjects. Given the short duration of each sleep stage, complexity measures, normally extracted from long-period signals, needed be calculated on short-term signals. Sample entropy, Lempel-Ziv complexity and detrended fluctuation analysis were evaluated and results showed no significant differences among the values calculated over ten-minute signals and longer durations, confirming the reliability of such analysis when performed on short-term signals. Complexity parameters were extracted from ten-minute signal portions selected during wakefulness and different sleep stages on HRV signals obtained from eighteen obese patients and twenty controls. The obese group presented significantly reduced complexity during light and deep sleep, suggesting a deficiency in the control mechanisms integration during these sleep stages. To our knowledge, this study reports for the first time on how the HRV complexity changes in obesity during wakefulness and sleep. Further investigation is needed to quantify altered HRV impact on cardiovascular mortality in obesity.     

Age-related changes in the circadian and homeostatic regulation of human sleep

The reduction of electroencephalographic (EEG) slow-wave activity (SWA) (EEG power density between 0.75-4.5 Hz) and spindle frequency activity, together with an increase in involuntary awakenings during sleep, represent the hallmarks of human sleep alterations with age. It has been assumed that this decrease in non-rapid eye movement (NREM) sleep consolidation reflects an age-related attenuation of the sleep homeostatic drive. To test this hypothesis, we measured sleep EEG characteristics (i.e., SWA, sleep spindles) in healthy older volunteers in response to high (sleep deprivation protocol) and low sleep pressure (nap protocol) conditions. Despite the fact that the older volunteers had impaired sleep consolidation and reduced SWA levels, their relative SWA response to both high and low sleep pressure conditions was similar to that of younger persons. Only in frontal brain regions did we find an age-related diminished SWA response to high sleep pressure. On the other hand, we have clear evidence that the circadian regulation of sleep during the 40 h nap protocol was changed such that the circadian arousal signal in the evening was weaker in the older study participants. More sleep occurred during the wake maintenance zone, and subjective sleepiness ratings in the late afternoon and evening were higher than in younger participants. In addition, we found a diminished melatonin secretion and a reduced circadian modulation of REM sleep and spindle frequency-the latter was phase-advanced relative to the circadian melatonin profile. Therefore, we favor the hypothesis that age-related changes in sleep are due to weaker circadian regulation of sleep and wakefulness. Our data suggest that manipulations of the circadian timing system, rather than the sleep homeostat, may offer a potential strategy to alleviate age-related decrements in sleep and daytime alertness levels.     

Effectiveness of emWave Biofeedback in Improving Heart Rate Variability Reactivity to and Recovery from Stress

The current study examined the efficacy of heart rate variability (HRV) biofeedback using emWave, a publicly available biofeedback device, to determine whether training affected physiological tone and stress responses. Twenty-seven individuals aged 18–30 years were randomized to a treatment or no-treatment control group. Treatment participants underwent 4–8 sessions of emWave intervention, and all participants attended pre-treatment and post-treatment assessment sessions during which acute stressors were administered. Physiological data were collected at rest, during stress, and following stress. emWave treatment did not confer changes in tonic measures of HRV or in HRV recovery following stress. However, treatment participants exhibited higher parasympathetic responses (i.e., pNN50) during stress presentations at the post-treatment session than their control counterparts. No treatment effects were evident on self-reported measures of stress, psychological symptoms, or affect. Overall, results from the current study suggest that the emWave may confer some limited treatment effects by increasing HRV during exposure to stress. Additional development and testing of the emWave treatment protocol is necessary before it can be recommended for regular use in clinical settings, including the determination of what physiological changes are clinically meaningful during HRV biofeedback training.     

Large-Scale Assessment of a Fully Automatic Co-Adaptive Motor Imagery-Based Brain Computer Interface

In the last years Brain Computer Interface (BCI) technology has benefited from the development of sophisticated machine leaning methods that let the user operate the BCI after a few trials of calibration. One remarkable example is the recent development of co-adaptive techniques that proved to extend the use of BCIs also to people not able to achieve successful control with the standard BCI procedure. Especially for BCIs based on the modulation of the Sensorimotor Rhythm (SMR) these improvements are essential, since a not negligible percentage of users is unable to operate SMR-BCIs efficiently. In this study we evaluated for the first time a fully automatic co-adaptive BCI system on a large scale. A pool of 168 participants naive to BCIs operated the co-adaptive SMR-BCI in one single session. Different psychological interventions were performed prior the BCI session in order to investigate how motor coordination training and relaxation could influence BCI performance. A neurophysiological indicator based on the Power Spectral Density (PSD) was extracted by the recording of few minutes of resting state brain activity and tested as predictor of BCI performances. Results show that high accuracies in operating the BCI could be reached by the majority of the participants before the end of the session. BCI performances could be significantly predicted by the neurophysiological indicator, consolidating the validity of the model previously developed. Anyway, we still found about 22% of users with performance significantly lower than the threshold of efficient BCI control at the end of the session. Being the inter-subject variability still the major problem of BCI technology, we pointed out crucial issues for those who did not achieve sufficient control. Finally, we propose valid developments to move a step forward to the applicability of the promising co-adaptive methods.     

Young women with major depression live on higher homeostatic sleep pressure than healthy controls

There is mounting evidence for the involvement of the sleep-wake cycle and the circadian system in the pathogenesis of major depression. However, only a few studies so far focused on sleep and circadian rhythms under controlled experimental conditions. Thus, it remains unclear whether homeostatic sleep pressure or circadian rhythms, or both, are altered in depression. Here, the authors aimed at quantifying homeostatic and circadian sleep-wake regulatory mechanisms in young women suffering from major depressive disorder and healthy controls during a multiple nap paradigm under constant routine conditions. After an 8-h baseline night, 9 depressed women, 8 healthy young women, and 8 healthy older women underwent a 40-h multiple nap protocol (10 short sleep-wake cycles) followed by an 8-h recovery night. Polysomnographic recordings were done continuously, and subjective sleepiness was assessed. In order to measure circadian output, salivary melatonin samples were collected during scheduled wakefulness, and the circadian modulation of sleep spindles was analyzed with reference to the timing of melatonin secretion. Sleep parameters as well as non-rapid eye movement (NREM) sleep electroencephalographic (EEG) spectra were determined for collapsed left, central, and right frontal, central, parietal, and occipital derivations for the night and nap-sleep episodes in the frequency range .75-25 Hz. Young depressed women showed higher frontal EEG delta activity, as a marker of homeostatic sleep pressure, compared to healthy young and older women across both night sleep episodes together with significantly higher subjective sleepiness. Higher delta sleep EEG activity in the naps during the biological day were observed in young depressed women along with reduced nighttime melatonin secretion as compared to healthy young volunteers. The circadian modulation of sleep spindles between the biological night and day was virtually absent in healthy older women and partially impaired in young depressed women. These data provide strong evidence for higher homeostatic sleep pressure in young moderately depressed women, along with some indications for impairment of the strength of the endogenous circadian output signal involved in sleep-wake regulation. This finding may have important repercussions on the treatment of the illness as such that a selective suppression of EEG slow-wave activity could promote acute mood improvement.     

Voluntarily Produced Increases in Heart Rate Variability Modulate Autonomic Effects of Endotoxin Induced Systemic Inflammation: An Exploratory Study

Exposure of healthy people to lipopolysaccharide (LPS; endotoxin) produces a pro-inflammatory response, subjective symptoms, and decreased heart rate variability (HRV). Given the efficacy of HRV biofeedback (BF) for treating asthma, the large autonomic effects of HRV BF, and the link between vagus nerve activity and inflammation, we hypothesized that HRV BF would dampen the acute manifestations of systemic inflammation induced by LPS challenge. Healthy participants age 18-40 were randomly assigned to four-one-hour training sessions of either HRV BF (n = 6) or a control 15/min paced breathing condition (n = 5) prior to acute experimentally induced LPS exposure. Participants were coached to do the procedures for 10 min each at five hourly time points after LPS injection, and then 2 h later. Subjective symptoms, HRV parameters, and plasma cytokine levels were measured at each time point, 2 h afterward, and the following morning. Participants were able to perform the procedures both during four pre-exposure training sessions and while experiencing LPS-induced symptoms. The HRV BF group showed significant attenuation of the LPS-induced decline in HRV for the 6 h following LPS exposure, suggesting that HRV BF decreased autonomic dysfunction produced by LPS-induced inflammation. HRV BF also reduced symptoms of headache and eye sensitivity to light, but did not affect LPS-induced levels of pro-inflammatory cytokines or symptoms of nausea, muscle aches, or feverishness. Further evaluation of HRV BF appears to be warranted among patients with inflammatory conditions.     

Post-Sleep Inertia Performance Benefits of Longer Naps in Simulated Nightwork and Extended Operations

Operational settings involving shiftwork or extended operations require periods of prolonged wakefulness, which in conjunction with sleep loss and circadian factors, can have a negative impact on performance, alertness, and workplace safety. Napping has been shown to improve performance and alertness after periods of prolonged wakefulness and sleep loss. Longer naps may not only result in longer-lasting benefits but also increase the risk of sleep inertia immediately upon waking. The time course of performance after naps of differing durations is thus an important consideration in weighing the benefits and risks of napping in workplace settings. The objective of this study was to evaluate the effectiveness of nap opportunities of 20, 40, or 60 min for maintaining alertness and performance 1.5–6 h post-nap in simulated nightwork (P1) or extended operations (P2). Each protocol included 12 participants in a within-subjects design in a controlled laboratory environment. After a baseline 8 h time-in-bed, healthy young males (P1 mean age 25.1 yr; P2 mean age 23.2 yr) underwent either ≈20 h (P1) or ≈30 h (P2) of sleep deprivation on four separate occasions, followed by nap opportunities of 0, 20, 40, and 60 min. Sleep on the baseline night and during the naps was recorded polysomnographically. During the nap opportunities, sleep onset latency was short and sleep efficiency was high. A greater proportion of slow-wave sleep (SWS) was obtained in nap opportunities of 40 and 60 min compared with 20 min. Rapid eye movement (REM) sleep occurred infrequently. A subjective sleepiness rating (Karolinska Sleepiness Scale, KSS), 2-Back Working Memory Task (WMT), and Psychomotor Vigilance Task (PVT) were completed 1.5, 2, 2.5, 3, 4, 5, and 6 h post-nap. The slowest 10% of PVT responses were significantly faster after 40 and 60 min naps compared with a 20 min (P1) or no (P2) nap. There were significantly fewer PVT lapses after 40 and 60 min naps compared with no nap (P2), and after 60 min naps compared with 20 min naps (P1). Participants felt significantly less sleepy and made more correct responses and fewer omissions on the WMT after 60 min naps compared with no nap (P2). Subjective sleepiness and WMT performance were not related to the amount of nap-time spent in SWS. However, PVT response speed was significantly slower when time in SWS was <10 min compared with 20–29.9 min. In conclusion, in operationally relevant scenarios, nap opportunities of 40 and 60 min show more prolonged benefits 1.5–6 h post-nap, than a 20 min or no nap opportunity. Benefits were more apparent when the homeostatic pressure for sleep was high and post-nap performance testing occurred across the afternoon (P2). For sustained improvement in cognitive performance, naps of 40–60 min are recommended. (Author correspondence: t.l.signal@massey.ac.nz)     

Age‐related Changes in the Circadian and Homeostatic Regulation of Human Sleep

The reduction of electroencephalographic (EEG) slow‐wave activity (SWA) (EEG power density between 0.75–4.5 Hz) and spindle frequency activity, together with an increase in involuntary awakenings during sleep, represent the hallmarks of human sleep alterations with age. It has been assumed that this decrease in non‐rapid eye movement (NREM) sleep consolidation reflects an age‐related attenuation of the sleep homeostatic drive. To test this hypothesis, we measured sleep EEG characteristics (i.e., SWA, sleep spindles) in healthy older volunteers in response to high (sleep deprivation protocol) and low sleep pressure (nap protocol) conditions. Despite the fact that the older volunteers had impaired sleep consolidation and reduced SWA levels, their relative SWA response to both high and low sleep pressure conditions was similar to that of younger persons. Only in frontal brain regions did we find an age‐related diminished SWA response to high sleep pressure. On the other hand, we have clear evidence that the circadian regulation of sleep during the 40 h nap protocol was changed such that the circadian arousal signal in the evening was weaker in the older study participants. More sleep occurred during the wake maintenance zone, and subjective sleepiness ratings in the late afternoon and evening were higher than in younger participants. In addition, we found a diminished melatonin secretion and a reduced circadian modulation of REM sleep and spindle frequency—the latter was phase‐advanced relative to the circadian melatonin profile. Therefore, we favor the hypothesis that age‐related changes in sleep are due to weaker circadian regulation of sleep and wakefulness. Our data suggest that manipulations of the circadian timing system, rather than the sleep homeostat, may offer a potential strategy to alleviate age‐related decrements in sleep and daytime alertness levels.     

Delayed Onset of a Daytime Nap Facilitates Retention of Declarative Memory

Background

Learning followed by a period of sleep, even as little as a nap, promotes memory consolidation. It is now generally recognized that sleep facilitates the stabilization of information acquired prior to sleep. However, the temporal nature of the effect of sleep on retention of declarative memory is yet to be understood. We examined the impact of a delayed nap onset on the recognition of neutral pictorial stimuli with an added spatial component.

Methodology/Principal Findings

Participants completed an initial study session involving 150 neutral pictures of people, places, and objects. Immediately following the picture presentation, participants were asked to make recognition judgments on a subset of “old”, previously seen, pictures versus intermixed “new” pictures. Participants were then divided into one of four groups who either took a 90-minute nap immediately, 2 hours, or 4 hours after learning, or remained awake for the duration of the experiment. 6 hours after initial learning, participants were again tested on the remaining “old” pictures, with “new” pictures intermixed.

Conclusions/Significance

Interestingly, we found a stabilizing benefit of sleep on the memory trace reflected as a significant negative correlation between the average time elapsed before napping and decline in performance from test to retest (p = .001). We found a significant interaction between the groups and their performance from test to retest (p = .010), with the 4-hour delay group performing significantly better than both those who slept immediately and those who remained awake (p = .044, p = .010, respectively). Analysis of sleep data revealed a significant positive correlation between amount of slow wave sleep (SWS) achieved and length of the delay before sleep onset (p = .048). The findings add to the understanding of memory processing in humans, suggesting that factors such as waking processing and homeostatic increases in need for sleep over time modulate the importance of sleep to consolidation of neutral declarative memories.