Afternoon Nap Research Articles

Sleep preserves subjective and sympathetic emotional response of memories.

Sleep consolidates episodic content of emotional memories. Whether it likewise preserves or, to the contrary, depotentiates the emotional response associated with memory content is unclear, as there is conflicting evidence. In the current study, we investigated the influence of an afternoon nap (2-hr nap opportunity) on emotional responses of memories using multiple simultaneous measures. Young adults viewed 45 negative and 45 neutral pictures before taking a nap (measured with polysomnography) or remaining awake. Following the nap or wake period, participants viewed the same pictures intermixed with novel ones and indicated whether they remembered each picture. Emotional response to each picture was measured at both time points both subjectively, with valence and arousal ratings, and objectively, with recordings of electrodermal activity, electrocardiography, and corrugator supercilii electromyography. Compared to waking, a nap led to preserved subjective valence for negative pictures and preserved/increased skin conductance response in general. On the other hand, heart rate deceleration response decreased over the nap compared to wake interval, and this result was not influenced by picture type. These data suggest that sleep consolidates aspects of both subjective and physiological emotional response associated with episodic memory content. While sympathetic response appears to be preserved over sleep, parasympathetic response may be diminished.

SLEEP AND CARDIAC TACHYARRHYTHMIA: RESULTS FROM THE CROSS-SECTIONAL SLEEP HEART HEALTH STUDY

Despite the well-known relationship between sleep disorders and general cardiovascular risk, few studies have examined sleep quality and quantity at pre-clinical levels in patients with cardiac arrhythmias (CA). Patients with CA are at an elevated risk of stroke, sudden cardiac death, disability, and reduced quality of life. In this study, we sought to elucidate the sleep-related predictors of arrhythmia using both subjective and objective measures of sleep. Baseline, comorbidity, electrocardiogram, and polysomnography data from the Sleep Heart Health Study (age 44-90 y) was available for secondary analysis. ECG data was used to identify participants with cardiac arrhythmias during polysomnography. Exposure variables used included blood oxygen saturation, sleep stages, and tertiles of sleep quality and quantity. Unadjusted and adjusted logistic regression was used to quantify the association between sleep and non-sleep related factors and arrhythmia. Of the 5,804 original SHHS sample, a total of 3,453 participants with complete variables of interest were included in the final analysis (mean age: 68.1 ± 10.6 Years, 54% male, 499 with arrhythmia (Atrial Tachyarrhythmia and Conduction Abnormalities), and 2,954 Controls). At the bivariate level, underweight (BMI < 18.5) (OR: 2.86, 95% CI: 1.1 – 7.2, P < 0.0001), sleep time < 6 hours (OR: 2.58, 1.5-4.3, P < 0.0001), % time in REM sleep ( < 17.6; OR: 1.53, 1.2-1.0, P < 0.0001), sleep efficiency ( < 81; OR: 1.9, 1.5-2.3) and regular afternoon naps (OR:1.8, 1.3-2.4, P < 0.0001) were significantly associated with CA. Once adjusted for age and sex, only % time in REM sleep and minimum oxygen saturation during REM (OR: 0.97, 0.96-0.98, P=0.006, and; OR: 0.97, 0.96-0.98, P=0.001, respectively) were significantly associated with CA. Preliminary analyses suggest differences in sleep stages and oxygen saturation as potential targets for future work in the prevention and early management of CA. Further study is necessary to understand the nature of these relationships using longitudinal data with adjustment for traditional CA markers.

Extended afternoon naps are associated with hypertension in women but not in men

BackgroundThe impact of afternoon napping duration on the risk of hypertension has not been well established, particularly with regards to sex and age differences. ObjectiveTo examine the association between afternoon napping duration and hypertension stratified by sex and age among Chinese adults over 45 years of age. MethodsThe 2011–2012 survey of the China Health and Retirement Longitudinal Study (CHARLS) was used, including 7,980 participants. We conducted logistic regression models in the overall sample, and then stratified by sex and age groups. ResultsMiddle-aged and older women who napped over 90 min were 39% and 54% more likely to have hypertension, respectively; however, the associations were not significant in middle-aged and older men. ConclusionExtended afternoon napping (≥90 min) was associated with hypertension in both the middle-aged women and older women but not in men. Future studies are needed to further examine the association and possible mechanisms.

Lifestyles and the risk of depression in the “Seguimiento Universidad de Navarra” cohort

BackgroundLifestyles are involved in the pathogenesis of depression and many of these factors can be modified for the potential prevention of depression. Our aim was to assess the association between a healthy-lifestyle score, that includes some less-studied lifestyle indicators, and the risk of depression. MethodsWe followed 14,908 participants initially free of any history of depression in the “Seguimiento Universidad de Navarra” (SUN) cohort. Information was collected biennially from 1999 to December 2016. We calculated a healthy-lifestyle score (0–10 points), previously associated with cardioprotection, by giving one point to each of the following components: never smoking, physical activity (> 20 METs-h/week), Mediterranean diet adherence (≥ 4 points), healthy body mass index (≤ 22 kg/m2), moderate alcohol consumption (women 0.1–5 g/d; men 0.1–10 g/d of ethanol), avoidance of binge drinking (never more than 5 alcoholic drinks in a row), low television exposure (≤ 2 h/d), short afternoon nap (≤ 30 min/day), time spent with friends (>1 h/d) and working at least 40 h/week. ResultsDuring a median follow-up of 10.4 years, we observed 774 new cases of major depression among participants initially free of depression. The highest category (8–10 factors) showed a significant inverse association with a 32% relative risk reduction for depression compared to the lowest category (0–3 factors) (multivariable-adjusted hazard ratio: 0.68; 95% CI:0.49-0.95) (p for trend = 0.010). ConclusionsAdopting a healthy-lifestyle was associated with a lower risk of incident depression in the SUN cohort. This index, including ten simple healthy lifestyle habits, may be useful for a more integrative approach to depression prevention.

Targeted Memory Reactivation during Sleep Elicits Neural Signals Related to Learning Content.

Retrieval of learning-related neural activity patterns is thought to drive memory stabilization. However, finding reliable, noninvasive, content-specific indicators of memory retrieval remains a central challenge. Here, we attempted to decode the content of retrieved memories in the EEG during sleep. During encoding, male and female human subjects learned to associate spatial locations of visual objects with left- or right-hand movements, and each object was accompanied by an inherently related sound. During subsequent slow-wave sleep within an afternoon nap, we presented half of the sound cues that were associated (during wake) with left- and right-hand movements before bringing subjects back for a final postnap test. We trained a classifier on sleep EEG data (focusing on lateralized EEG features that discriminated left- vs right-sided trials during wake) to predict learning content when we cued the memories during sleep. Discrimination performance was significantly above chance and predicted subsequent memory, supporting the idea that retrieval leads to memory stabilization. Moreover, these lateralized signals increased with postcue sleep spindle power, demonstrating that retrieval has a strong relationship with spindles. These results show that lateralized activity related to individual memories can be decoded from sleep EEG, providing an effective indicator of offline retrieval.SIGNIFICANCE STATEMENT Memories are thought to be retrieved during sleep, leading to their long-term stabilization. However, there has been relatively little work in humans linking neural measures of retrieval of individual memories during sleep to subsequent memory performance. This work leverages the prominent electrophysiological signal triggered by lateralized movements to robustly demonstrate the retrieval of specific cued memories during sleep. Moreover, these signals predict subsequent memory and are correlated with sleep spindles, neural oscillations that have previously been implicated in memory stabilization. Together, these findings link memory retrieval to stabilization and provide a powerful tool for investigating memory in a wide range of learning contexts and human populations.

Insomnia and Alcohol Use in Assisted Living

Insomnia and Alcohol Use in Assisted Living

Cognitive Impairments during the Transition to Working at Night and on Subsequent Night Shifts.

Demands of modern society force many work operations into the night when the internal circadian timekeeping system is promoting sleep. The combination of disturbed daytime sleep and circadian misalignment, which is common in overnight shift work, decreases cognitive performance, yet how performance may differ across multiple consecutive nights of shift work is not fully understood. Therefore, the primary aim of this study was to use a simulated night-shift protocol to examine the cognitive performance and ratings of sleepiness and clear-headedness across the hours of a typical daytime shift, a first night shift with an afternoon nap and extended wakefulness, and 2 subsequent overnight shifts. We tested the hypothesis that cognitive performance would be worse on the first night shift as compared with the baseline and subsequent nighttime shifts and that performance during nighttime shifts would be reduced as compared with the baseline daytime shift. Fifteen healthy adults (6 men) were studied in the 6-day in-laboratory protocol. Results showed that working during the night increased subjective sleepiness and decreased clear-headedness and performance on the Psychomotor Vigilance Task (i.e., slower median, fastest and slowest reaction times, and increased attentional lapses), Stroop color word task (decreased number of correct responses and slower median reaction time), and calculation addition performance task (decreased number attempted and correct). Furthermore, we observed limited evidence of sleepiness, clear-headedness, or performance adaptation across subsequent nights of simulated night work. Our findings demonstrate that night-shift work, regardless of whether it is the first night shift with a nap and extended wakefulness or subsequent night shifts, decreases performance and clear-headedness as compared with the day shift.

Does sleep predict next-day napping or does napping influence same-day nocturnal sleep? Results of a population-based ecological momentary assessment study

Study objectivesThe temporal relationship between nocturnal sleep and daytime napping has only been assessed in small non-representative samples, and suggests that nocturnal sleep and napping are interdependent, although mixed results exist. In this study, we investigated the temporal relationship between nocturnal sleep and napping (and vice versa). MethodsA population-based sample of middle-aged adults (N = 683, mean age 60.7 [SD 9.5]) completed seven days of ecological momentary assessment reporting sleep and nap characteristics. Multilevel random-effects models were used to assess the temporal relationship between sleep duration and quality, and nap occurrence and duration (and vice versa). ResultsIn sum, 64% of the study population took at least one nap over the course of seven days. Poor subjective sleep quality and shorter sleep duration increased the likelihood and duration of next-day napping. No effect of nap occurrence or duration was found on same-day nocturnal sleep duration and quality. However, when considering the timing of nap, afternoon naps, but not morning or evening naps, decreased same-day nocturnal sleep duration. ConclusionNaps seem to compensate for poor subjective sleep quality, and to some extent for short sleep duration. As only afternoon naps reduced same-day nocturnal sleep duration, timing of the daytime nap seems to matter with respect to same-day nocturnal sleep duration.

Emotional Memory Moderates the Relationship Between Sigma Activity and Sleep-Related Improvement in Affect.

Sleep is essential for regulating mood and affect, and it also consolidates emotional memories. The mechanisms underlying these effects may overlap. Here, we investigated whether the influence of sleep on affect may be moderated by emotional memory consolidation. Young adults viewed 45 negative and 45 neutral pictures before taking an afternoon nap measured with polysomnography. Following the nap period, participants viewed the same pictures intermixed with novel ones and indicated whether they remembered each picture. Affect was measured with the Positive and Negative Affect Schedule (PANAS) at baseline before the initial picture viewing task, immediately following the initial picture viewing task, and following the nap. The ratio of positive to negative affect declined over the task period and recovered over the nap period. When controlling for pre-nap affect, NREM sigma activity significantly predicted post-nap affect. Memory for negative pictures moderated this relationship such that a positive association between sigma activity and affect occurred when memory was low but not when memory was high. These results indicate that emotional memory consolidation influences the relationship between nap physiology and mood.

Napping between scylla and charybdis of N1 and N3: latency to N2 in a brief afternoon nap can be reduced by binaural beating

ABSTRACT Afternoon nap is regarded as a potent behavioral strategy minimizing sleepiness and fatigue. The benefits of afternoon naps require the accumulation of, at least, 3 min of stage 2 sleep. However, there are practical disadvantages of nap longer than 10–15 min, such as greater length of time consumed by the nap, appearance of slow wave sleep causing greater sleep inertia right after the nap, and possible detrimental impact of such nap on subsequent nocturnal sleep. We previously found that a binaural beat treatment that is a dichotic presentation of two almost equivalent pure tones with slightly different frequencies led to a reduction of latency to stage 2 sleep. To replicate this result and to examine whether such reduction leads, in turn, to the earlier appearance of slow wave sleep, we asked 23 and 21 healthy volunteers to nap in the afternoon for 30 and 20 min, respectively. Almost half of volunteers showed latency to stage 2 longer than 17 min, but most of them responded to the treatment by its reduction. The following occurrence of slow wave sleep reduced level of alertness self-assessed right after the nap. We concluded that latency to stage 2 sleep might be experimentally challenged by binaural beating.

Differential effects of split and continuous sleep on neurobehavioral function and glucose tolerance in sleep-restricted adolescents.

Study ObjectivesMany adolescents are exposed to sleep restriction on school nights. We assessed how different apportionment of restricted sleep (continuous vs. split sleep) influences neurobehavioral function and glucose levels.MethodsAdolescents, aged 15–19 years, were evaluated in a dormitory setting using a parallel-group design. Following two baseline nights of 9-hour time-in-bed (TIB), participants underwent either 5 nights of continuous 6.5-h TIB (n = 29) or 5-hour nocturnal TIB with a 1.5-hour afternoon nap (n = 29). After two recovery nights of 9-hour TIB, participants were sleep restricted for another three nights. Sleep was assessed using polysomnography (PSG). Cognitive performance and mood were evaluated three times per day. Oral glucose tolerance tests (OGTT) were conducted on mornings after baseline sleep, recovery sleep, and the third day of each sleep restriction cycle.ResultsThe split sleep group had fewer vigilance lapses, better working memory and executive function, faster processing speed, lower level of subjective sleepiness, and more positive mood, even though PSG-verified total sleep time was less than the continuous sleep group. However, vigilance in both sleep-restricted groups was inferior to adolescents in a prior sample given 9-hour nocturnal TIB. During both cycles of sleep restriction, blood glucose during the OGTT increased by a greater amount in the split sleep schedule compared with persons receiving 6.5-hour continuous sleep.ConclusionsIn adolescents, modest multinight sleep restriction had divergent negative effects on cognitive performance and glucose levels depending on how the restricted sleep was apportioned. They are best advised to obtain the recommended amount of nocturnal sleep.Trial registration https://clinicaltrials.gov/ct2/show/NCT03333512

Trait-like characteristics of sleep EEG power spectra in adolescents across sleep opportunity manipulations.

The electroencephalographic power spectra of non‐rapid eye movement sleep in adults demonstrate trait‐like consistency within participants across multiple nights, even when prior sleep deprivation is present. Here, we examined the extent to which this finding applies to adolescents who are habitually sleep restricted on school‐days and sleep longer on weekends. We evaluated 78 adolescents across three sleep restriction groups who underwent different permutations of adequate sleep (9 hr time‐in‐bed), sleep restriction (5 hr time‐in‐bed), afternoon naps (1 hr afternoon) and recovery sleep (9 hr time‐in‐bed) that simulate behaviour on school‐days and weekends. The control group comprised a further 22 adolescents who had 9 hr of sleep opportunity each night. Intra‐class correlation coefficients showed moderate to almost perfect within‐subject stability in electroencephalographic power spectra across multiple nights in both sleep restriction and control groups, even when changes to sleep macrostructure were observed. While nocturnal intra‐class correlation metrics were lower in the low‐frequency and spindle frequency bins in the sleep restriction compared with the control group, hierarchical clustering measures could still identify multi‐night electroencephalographic spectra as originating from the same individual. The trait‐like characteristics of electroencephalographic spectra from an adolescent remain identifiable despite the disruptive effects of multi‐night sleep restriction to sleep architecture.

Whole-Night Continuous Rocking Entrains Spontaneous Neural Oscillations with Benefits for Sleep and Memory

Sensory processing continues during sleep and can influence brain oscillations. We previously showed that a gentle rocking stimulation (0.25Hz), during an afternoon nap, facilitates wake-sleep transition and boosts endogenous brain oscillations (i.e., EEG spindles and slow oscillations [SOs]). Here, we tested the hypothesis that the rhythmic rocking stimulation synchronizes sleep oscillations, a neurophysiological mechanism referred to as "neural entrainment." We analyzed EEG brain responses related to the stimulation recorded from 18 participants while they had a full night of sleep on a rocking bed. Moreover, because sleep oscillations are considered of critical relevance for memory processes, we also investigated whether rocking influences overnight declarative memory consolidation. We first show that, compared to a stationary night, continuous rocking shortened the latency to non-REM (NREM) sleep and strengthened sleep maintenance, as indexed by increased NREM stage 3 (N3) duration and fewer arousals. These beneficial effects were paralleled by an increase in SOs and in slow and fast spindles during N3, without affecting the physiological SO-spindle phase coupling. We then confirm that, during the rocking night, overnight memory consolidation was enhanced and also correlated with the increase in fast spindles, whose co-occurrence with the SO up-state is considered to foster cortical synaptic plasticity. Finally, supporting the hypothesis that a rhythmic stimulation entrains sleep oscillations, we report a temporal clustering of spindles and SOs relative to the rocking cycle. Altogether, these findings demonstrate that a continuous rocking stimulation strengthens deep sleep via the neural entrainment of intrinsic sleep oscillations.

Adolescent sleep restriction effects on cognition and mood.

Adolescents throughout the world do not obtain adequate sleep. A recent proliferation of experimental and quasi-experimental studies has considerably clarified the relationships between sleep loss and neurobehavioral function suggested by earlier epidemiological and cross-sectional studies. These new studies concur in finding that multiple successive nights of restricted sleep can impair multiple cognitive and affective functions. These effects cumulate from night to night, may not fully recover after weekend recovery sleep and may even be compounded by re-exposure to sleep restriction. An hour long afternoon nap reduces sleepiness in addition to improving vigilance, memory encoding and mood without interfering with nocturnal sleep when the latter is shortened. However, this does not detract from the point that adolescents require approximately 9h of sleep per night for optimal neurobehavioral function, a message that more need to embrace.

Healthy Lifestyle and Incidence of Metabolic Syndrome in the SUN Cohort.

We assessed the relationship between a healthy lifestyle and the subsequent risk of developing metabolic syndrome. The “Seguimiento Universidad de Navarra” (SUN) Project is a prospective cohort study, focused on nutrition, lifestyle, and chronic diseases. Participants (n = 10,807, mean age 37 years, 67% women) initially free of metabolic syndrome were followed prospectively for a minimum of 6 years. To evaluate healthy lifestyle, nine habits were used to derive a Healthy Lifestyle Score (HLS): Never smoking, moderate to high physical activity (>20 MET-h/week), Mediterranean diet (≥4/8 adherence points), moderate alcohol consumption (women, 0.1–5.0 g/day; men, 0.1–10.0 g/day), low television exposure (<2 h/day), no binge drinking (≤5 alcoholic drinks at any time), taking a short afternoon nap (<30 min/day), meeting up with friends >1 h/day, and working at least 40 h/week. Metabolic syndrome was defined according to the harmonizing definition. The association between the baseline HLS and metabolic syndrome at follow-up was assessed with multivariable-adjusted logistic regressions. During follow-up, we observed 458 (4.24%) new cases of metabolic syndrome. Participants in the highest category of HLS adherence (7–9 points) enjoyed a significantly reduced risk of developing metabolic syndrome compared to those in the lowest category (0–3 points) (adjusted odds ratio (OR) = 0.66, 95% confidence interval (CI) = 0.47–0.93). Higher adherence to the Healthy Lifestyle Score was associated with a lower risk of developing metabolic syndrome. The HLS may be a simple metabolic health promotion tool.

Targeted memory reactivation during sleep to strengthen memory for arbitrary pairings

A powerful way to investigate memory consolidation during sleep utilizes acoustic stimulation to reactivate memories. In multiple studies, Targeted Memory Reactivation (TMR) using sounds associated with prior learning improved later memory, as in recalling locations where objects previously appeared. In the present experiment, we examined whether a variant of the same technique could strengthen memory for the locations of pairs of objects. Each sound was naturally connected to one object from each pair, but we hypothesized that both memories could be improved with TMR. We first asked participants to memorize each of 50 pairs of objects by associating the two objects with each other and with the sound of one of the objects (e.g., cat-meow). Next, objects were presented in unique locations on a grid. Participants learned these locations in an adaptive procedure. During an afternoon nap, 25 of the sounds were quietly presented. In memory tests given twice before and twice after the nap, participants heard the sound for each object pair and were asked to recall the name of the second object and the locations of both objects. Forgetting scores were calculated using the mean difference between pre-nap and post-nap spatial recall errors. We found less forgetting after the nap for cued compared to non-cued objects. Additionally, the extent of forgetting tended to be similar for the two members of each pair, but only for cued pairs. Results thus substantiate the potential for sounds to reactivate spatial memories during sleep and thereby improve subsequent recall performance, even for multiple objects associated with a single sound and when participants must learn a novel sound-object association.

Beneficial effects of a daytime nap on verbal memory in adolescents

This study aimed to examine the sleep-dependent memory consolidation of verbal declarative memory in Chinese adolescents in a naturalistic experimental setting. Thirty-nine healthy boarding school students (ages 15–18, 70% female) were randomized to either a one-hour afternoon nap or wake group between the baseline and the retest sessions of three verbal declarative memory tasks: a Prose Stories Recall task, a Word Pair Associates task, and Rey Auditory Verbal Learning Test. Results showed that the nap group performed better than the no-nap group on both the Prose Stories Recall task and the Word Pair Associates task, but not on list learning. Our findings suggest that napping is beneficial to verbal declarative memory in adolescents, providing ecologically-valid empirical support for the sleep-dependent memory consolidation hypothesis using a napping paradigm in participants' naturalistic habitat. Our results demonstrate the potential importance of napping as a practical mnemonic intervention/compensatory strategy for student populations.

Effects of a Short Daytime Nap on Shooting and Sprint Performance in High-Level Adolescent Athletes.

To investigate the sport-specific performance effect of a brief afternoon nap on high-level Asian adolescent student-athletes who were habitually short sleepers. Participants were randomly assigned to a nap or nonnap (reading) condition. In the first study, 12 male shooters (13.8 [1.0]y) performed a shooting assessment (20 competition shots) with heart-rate variability monitored during the assessment. In the second study, 19 male track-and-field athletes (14.8 [1.1]y) performed a 20-m sprint-performance assessment. Subjective measures of sleepiness and alertness were obtained in both studies. The brief nap had no effect on any measure of shooting performance (P > .05) or autonomic function (P > .05) in shooters. However, the fastest 20-m sprint times increased significantly (P < .05) from 3.385 (0.128) s to 3.411 (0.143) s, with mean 2-m times trending toward significance (P < .1) among the track-and-field athletes. No significant differences were observed in any other measures. The results of the research indicate varying effects of naps between sport-specific performance measures. Napping had no effect on shooting performance, whereas a negative effect existed in 20-m sprint performance, potentially due to sleep inertia. Considering these findings, some caution is warranted when advocating naps for adolescent athletes.

The influence of an afternoon nap on the endurance performance of trained runners

The effectiveness of a nap as a recovery strategy for endurance exercise is unknown and therefore the present study investigated the effect of napping on endurance exercise performance. Eleven trained male runners completed this randomised crossover study. On two occasions, runners completed treadmill running for 30 min at 75% ⩒O2max in the morning, returning that evening to run for 20 min at 60% ⩒O2max, and then to exhaustion at 90% ⩒O2max. On one trial, runners had an afternoon nap approximately 90 min before the evening exercise (NAP) whilst on the other, runners did not (CON). All runners napped (20 ± 10 min), but time to exhaustion (TTE) was not improved in all runners (NAP 596 ± 148 s vs. CON 589 ± 216 s, P = .83). Runners that improved TTE after the nap slept less at night than those that did not improve TTE (night-time sleep 6.4 ± 0.7 h vs. 7.5 ± 0.4 h, P < .01). Furthermore, night-time sleep predicted change in TTE, indicating that runners sleeping least at night improved TTE the most after the nap compared to CON (r2 = −0.76, P = .001). In runners that improved TTE, ratings of perceived exertion (RPE) were lower during the TTE on NAP than CON compared to runners that did not improve (−0.4 ± 0.6 vs. 0 ± 0, P = .05). Reduced exercising sense of effort (RPE) may account for the improved TTE after the nap. In conclusion, a short afternoon nap improves endurance performance in runners that obtain less than 7 h night-time sleep.

Effect of a relatively long afternoon nap on autonomous nervous activity, sleep architecture, and subjective sleep quality

Post‐lunch sleepiness and deterioration in performance, or ‘post‐lunch dip’, is considered a part of the biological rhythm. A short nap is a promising countermeasure to the post‐lunch dip. Compared to the extensive studies on the effects of a short nap, there is limited research on the physiological impact of a relatively long nap such as 60 min or more in the afternoon. Forty‐three male university students with normal sleep–wake cycles were assigned to two groups: group 1 slept for 6 h from 0:00 to 6:00, and group 2 napped for 2 h from 13:15 to 15:15. There was no prominent difference in the subjective quality of sleep between the groups. This was supported by the lack of any significant difference in sleep efficiency and latency between the groups. Additionally, the heart rate after the afternoon nap was significantly higher than the baseline level before nap (overshoot), whereas the heart rate in the morning was the same as recorded the night before. The relatively long afternoon nap may be comparable with a night's sleep in terms of sleep quality, whereas it may enhance greater autonomic arousal after awakening than that in the morning. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.