Nocturnal sleep mediates the relationship between morningness–eveningness preference and the sleep architecture of afternoon naps in university students

RESEARCH ARTICLE Nocturnal sleep mediates the relationship between morningness–eveningness preference and the sleep architecture of afternoon naps in university students Tzu-Yin Lee1, Pi-Chen Chang1, Ing-Jy Tseng2, Min-Huey Chung1,3* 1 School of Nursing, College of Nursing, Taipei Medical University, Taipei, Taiwan, 2 School of Gerontology Health Management, College of Nursing, Taipei Medical University, Taipei, Taiwan, 3 Department of Nursing, Taipei Medical University-Shuang Ho Hospital, New Taipei City, Taiwan a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Lee T-Y, Chang P-C, Tseng I-J, Chung MH (2017) Nocturnal sleep mediates the relationship between morningness–eveningness preference and the sleep architecture of afternoon naps in university students. PLoS ONE 12(10): e0185616. https://doi.org/10.1371/journal.pone.0185616 Editor: Raffaele Ferri, Associazione OASI Maria SS, ITALY Received: July 4, 2017 Accepted: September 15, 2017 Published: October 17, 2017 Copyright: © 2017 Lee et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and available on Figshare at the following link: https://figshare.com/articles/nap_ data_sav/5450152. Funding: This research received no grant from any funding agency in the public, commercial, or notfor-profit sectors. Competing interests: The authors declare that they have no competing interest. * Abstract The present study investigated the parameters of nocturnal sleep that mediate the relationship between morningness–eveningness preference and the sleep architecture of naps in university students. This study had a cross-sectional, descriptive correlational design. The sleep architecture of 52 students invited to take an afternoon nap in the laboratory was recorded. The morningness–eveningness questionnaire (MEQ) was used to evaluate morningness–eveningness preference. An actigraph was used to collect students’ nighttime sleep data in the week preceding the study. Polysomnography was used to measure the sleep architecture of the participants’ naps. After adjustments for potential factors, although the MEQ did not directly correlate with the percentage of sleep stages in naps, the effects of the MEQ on the percentage of Stage 1 sleep, slow-wave sleep, and rapid eye movement sleep; sleep duration; and sleep efficiency of naps were mediated by the total sleep time in the preceding week. This preliminary study suggests that nap quality was affected by morningness–eveningness preference through the mediation of total nocturnal sleep time. Therefore, future studies should be carefully designed to consider nighttime sleep patterns when analyzing the effects of chronotypes on daytime sleep. Introduction Napping refers to a short duration of sleep during the day. A decline in arousal after half a day of activity may compel people to take a break [1]. A 30-min nap was reported to improve participants’ reaction times [2]. Increasing evidence has revealed the physiological and psychological benefits of napping [3–5]. A chronotype is one of the most crucial differences in human circadian preference; it can be defined in terms of the propensity of an individual to engage in activity and sleep at particular times during a 24-h period [6]. “Eveningness” and “morningness” are the two extremes [7]. Studies have extensively examined the association of chronotypes with daytime sleepiness [8, 9], napping behavior [10, 11], and sleep latency during naps [12]. However, data on the possible factors mediating the relationship between chronotypes PLOS ONE | https://doi.org/10.1371/journal.pone.0185616 October 17, 2017 1 / 11 Morningness-eveningness preference and sleep architecture of afternoon naps and the sleep architecture of naps remain scarce. A thorough understanding of the relationship between circadian preference, nocturnal sleep, and daytime sleep could help researchers to control the interaction effect when investigating sleep-related behaviors and gain a comprehensive understanding of individual sleep tendencies when managing patients’ sleep disorders. In contrast to napping, nocturnal sleep is the main sleep period influenced by a person’s circadian rhythm, which significantly correlates with nap quality [13–15]. More sleep fragmentation during nighttime sleep was associated with a higher risk of napping but not nap duration [13]. Significant correlations were observed between weekend nighttime sleep and weekday perceived sleep debt and nap length [14]. Participants who napped for more than 2 h had poorer nighttime sleep quality [15]. Therefore, individual differences in the tendency to nap could be a consequence of disturbed or decreased nighttime sleep. A chronotype strongly correlates with the quality of nighttime sleep. One study indicated that chronotypes more comprehensively explain individual differences in nocturnal sleep than does age [16]. Children of evening type had shorter nocturnal sleep times during weekdays than did other children [17]. Adolescents with eveningness are more likely to have poorer sleep quality and sleep less on weekdays than those with morningness [10, 18]. Adults of morning type had higher sleep efficiency than those of evening type [19]. Compared with participants of morning and intermediate type, those of evening type have reported more time in bed and greater variability in time out of bed [9, 20]. Thus, the relationship between morningness– eveningness preference and the quality and quantity of nighttime sleep has been confirmed. Studies have reported the association between morningness–eveningness preference and nocturnal sleep [20–23], as well as that between nighttime and daytime sleep [13–15], and have indicated that evening-type naps during weekdays are more frequent than morning-type naps [10]. However, few studies have examined the association between morningness–eveningness and the sleep latency of naps [12]. Moreover, people with eveningness have higher sleep debt and therefore suffer from daytime sleepiness; their high sleep tendencies during daytime is a concern [8]. Additional studies are warranted to obtain the details of nocturnal sleep among participants to comprehensively understand the association between morningness–eveningness preference and the sleep architecture of naps. Future studies must determine whether the sleep stages of naps are affected by nocturnal sleep across different chronotypes. Therefore, this study investigated the parameters of nocturnal sleep that mediate the relationship between morningness–eveningness preference and the sleep architecture of naps in university students. Materials and methods Study design and participants This study had a cross-sectional, descriptive cor (...truncated)