1 Day-of-week mood patterns in adolescents considering chronotype, sleep length and sex Juan F. Díaz-Morales *, Zaida Parra-Robledo Facultad de Psicología, Universidad Complutense de Madrid, Spain This is the original version of the manuscript accepted for publication by Elsevier in Personality and Individual Differences on 2021. The final published version of the editorial can be found at: Díaz-Morales, J. F., & Parra-Robledo, Z. (2021). Day-of-week mood patterns in adolescents considering chronotype, sleep length and sex. Personality and Individual Differences, 179, 110951. https://doi.org/10.1016/j.paid.2021.110951 * Corresponding author at: Individual Differences, Work and Social Psychology Department, Facultad de Psicología, Complutense University of Madrid, Campus Somosaguas, s/n, 28223, Pozuelo de Alarcón, Madrid, Spain. E-mail address: jfdiazmo@ucm.es (J.F. Díaz-Morales). Web: https://www.ucm.es/jfdiazmorales/ https://doi.org/10.1016/j.paid.2021.110951 file:///G:/Mi%20unidad/Zaida/evegen_datos_02_diario/Mood%20Sleep%20Crono%20submission/6_PAID/2_response%20PAID_datos_copia/jfdiazmo@ucm.es https://www.ucm.es/jfdiazmorales/ 2 Abstract In this study, the day-of-week effect on positive and negative affect, taking into account the moderating effect of chronotype, sleep length and sex, was analyzed. Ninety-four adolescents (16.1±1.24 years old) attending high schools in the morning filled out diaries to assess the natural day-to-day fluctuations in mood and sleep. Using multilevel modeling, the results indicated that positive affect increased and negative affect decreased over the weekend. Negative affect was moderated by chronotype such that negative affect decreased less over the weekend in evening adolescents. Short sleep length was related to negative affect, and girls reported higher negative affect. Individual differences in weekly mood patterns should be taken into account when designing psychosocial interventions for adolescents. Keywords: day-of-week, mood, chronotype, sleep, sex, diary, individual differences. 3 1. Introduction Morningness/Eveningness (M/E) is considered an individual characteristic regarding the preference for functioning at different times of the day. M/E is a reflection of individual differences in circadian rhythms and manifests itself in a wide range of behaviors, such as sleep habits, mental efficiency, and mood (Adan et al., 2012). Considered as continuous dimensions, Morning, Neither and Evening chronotypes (M-, N- and E-types, respectively) can also be established. Adolescence is a high-risk period for the onset of clinical mood disorders, with an increase in adolescents reporting significant depressive and anxiety symptoms (Lovato & Gradisar, 2014). Biological changes produce a delay in the phase of the circadian rhythms (Hagenauer et al., 2009). Also, important changes in psychological processes such as greater interpersonal sensitivity, social-evaluative concerns, autonomy, conflict with parents, social influences such as more evening activities (e.g., electronic media), greater social opportunities, increased academic responsibilities, and more extracurricular activities make up what has been called the perfect puberty storm (Carskadon, 2011). This convergence of factors during adolescence suggests that it is particularly important to understand the relationships among chronotype, sleep and mood in the natural context of high school (Bauducco et al., 2020). A weekly mood pattern among adults and undergraduate students has been described, with a peak in Positive Affect (PA) observed on the weekend compared to the PA on weekdays (Csikszentmihalyi & Hunter, 2003), so it would be interesting to consider the day-of-week effect on mood comparing school days, constrained by morning school schedules, with the weekend, unconstrained by school schedules. Three reasons could be put forward. First, mood is influenced by genetic and environmental factors and is often associated with sleep and circadian rhythms through a complex and bidirectional relationship (McClung, 2013). 4 Second, the shift to eveningness among adolescents along with an earlier morning school schedule results in considerable sleep debt on school days and weekend catch-up sleep duration, with E-types reporting worse mood than M-types (Taylor & Hasler, 2018). Third, sex represents a scarcely studied factor in this complex chronotype-sleep- mood relationship, despite a disproportionate number of girls beginning to report anxiety and depression symptoms in early adolescence (Grills-Taquechel et al., 2010). Additionally, the shift to eveningness occurs earlier in girls, coinciding with earlier pubertal development, as has been indicated in different studies (Carskadon, 2011). 1.1. Mood and time of day Mood is an affective state that is not only important for optimal emotional well- being but is also relevant for working memory, creativity, decision making, and immune response. Mood is influenced by internal biological rhythms, as well as by external factors such as social and physical activity, daily work routine, commuting, sleep and eating habits (Salk et al., 2016). PA exists on a continuum, with high PA representing high levels of energy, alertness, and enjoyment and low PA reflecting feelings of sadness and low energy. Conversely, Negative Affect (NA) is a measure of subjective distress, with high NA reflecting feelings of hostility, irritability, and shame and low NA reflecting calmness (Watson et al., 1988). The study of the time-of-day effect on mood in adults has revealed that eveningness was related to lower PA, whereas NA was unrelated to eveningness (Jankowski & Ciarkowska, 2008). The theoretical position is that PA, but not NA, would be closely tied to the individual’s internal biological clock, as PA is known to fluctuate according to the 24-hr cycle (Murray et al., 2002). There is evidence that 5 higher NA and lower PA affect may be related to the shift toward an evening circadian preference observed in adolescents (Díaz-Morales et al., 2015; Dolsen & Harvey, 2018). 1.2. Mood and day of week Adolescents are exposed to varying academic schedules and are constrained by a weekly schedule with permanent social jetlag (school days vs. weekend) that dictates when and how much they sleep (Wittmann et al., 2006). Empirical research using national surveys of the adult population has revealed consistent findings of day-week effects for emotions, revealing happiness or PA peak on weekends (Helliwell & Wang, 2014). These longitudinal studies examined adult participants’ daily mood over several weeks, finding that the weekly profile of mood differs by age, gender, work, partner status (Stone et al., 2012) and religion (Tsai, 2019). On the other hand, chronobiological research has supported the general notion of a weekly mood cycle (Ryan et al., 2010), and some studies have found that day-of-week patterns differ between PA and NA. For instance, Golder & Macy (2011) used the lexical analysis of messages on Facebook and Twitter as a measure of mood and confirmed both the time-of-day and day-of-week effects on PA and NA using large samples of adults. Stone et al. (2012) analyzed demographic and social moderators of the day-of-week effect on mood, finding that weekends were strongly associated with better mood and that women reported considerably greater NA than males. Csikszentmihalyi & Hunter (2003) assert that the social structure of time has an impact on mood, such as the early part of the weekend, with its freedom from work or school, is experienced as liberating. The effect is probably different between adolescents and adults, for whom the week is presumably even more constraining. 6 In the adolescence stage, a series of biological (e.g., sleep phase delay, sex, puberty), social (e.g., morning school hours and family pressure, friends, relationships), physical and technological (e.g., exposure to screen light) factors converge, making their study much more complex. Therefore, the results obtained from adults cannot be automatically applied to adolescents. Generally, adolescents tend to sleep less, go to bed and get up later, and experience greater daytime sleepiness and weekend compensation for sleep shortages on school days (Crowley et al., 2018). Late-night bedtimes combined with early school start times contribute to what is increasingly regarded as an epidemic of sleep deprivation among adolescents (Touitou et al., 2016), which can be more serious for girls. Previous studies have found sex differences in sleep habits among adolescents (Randler et al., 2017). Girls get up earlier and spend less time in bed during school days, while boys get up earlier during the weekend. Girls reported shorter sleep length during school days and longer sleep length over weekends (Moreno et al., 2018), and they experienced greater social jetlag (Collado et al., 2012). These results confirm the sleep catchup that occurs on the weekends during adolescence and the importance of considering sex and weekly effects on sleep habits (Fuligni et al., 2019). 2. Current study The aim of this study was to analyze the relationships between chronotype, sleep length and mood measured during the school week, given that weekly cycles are driven by organization of social life rather than biological clocks. We were interested in analyzing the day-of-week effect on PA and NA, taking into account the moderating effects of chronotype, sleep length and sex. According to previous results on time-of- day and day-of-week effects on mood, PA is expected to increase over the weekend, shorter sleep length is expected to be associated with NA, morning adolescents are expected to report higher PA and girls are expected to report higher NA. Daily diaries to 7 capture the natural occurrence of day-to-day fluctuations in mood were used, increasing ecological validity and reducing recall bias. These hypotheses were tested using multilevel modeling, allowing us to examine the within-day mood effect and between- subjects differences in chronotype, sleep length and sex. 3. Methods 3.1. Participants A total of 94 high school adolescents (42 boys and 52 girls) aged 14 to 20 years (16.1±1.24) participated in the study. We screened a total of 174 adolescents to select a subsample of definite M- and E-types (N = 111; mean age: 16.1 ± 1.17 years old). Of them, 17 cases were removed because they partially completed the mood diary. All participants attended three public high schools. Census track data indicated that these adolescents resided primarily in lower-middle- to upper-middle-income families. None of the participants were on pharmacological treatment or taking hormonal contraception. The board of directors at the schools authorized the study after obtaining the parents’ permission. Participation was voluntary, unpaid and anonymous. 3.2. Variables and measures Chronotype The Morningness-Eveningness Scale for Children (MESC; Carskadon et al., 1993) was used to assess M/E orientation in adolescents. The MESC is composed of ten questions written in a language style that allows children and adolescents to understand the questions. Outcome scores ranged from 10 (eveningness) to 43 (morningness). In the present study, the reliability of the scale was α = 0.68 (Cronbach’s alpha), very similar to obtained in previous studies (Díaz-Morales, 2015). Sleep diary 8 Every day before going to sleep, adolescents filled out a diary answering these specific questions: “What time did you go to bed last night?” and “What time did you wake up this morning?” These questions are typically used in sleep diary studies, and responses are moderately correlated with sleep estimates obtained from more objective methods, such as actigraphy. From these answers, time in bed as a proxy for week sleep length was calculated. Mood diary Additionally, the diary included the 20-item Positive and Negative Affect Schedule (PANAS; Watson et al., 1988) as a measure of PA and NA. Adolescents were asked to rate each item based on how they felt during the day on a 10-point Likert scale from 1 “not at all” to 10 “extremely”. Composite scores are calculated by summing 10 items each for PA and NA, with higher scores indicating higher levels of PA and NA. The scale’s reliability and construct validity have been supported by previous studies ( López-Gómez et al., 2015). In the present study, the reliability was excellent PA (α = 0.88) and NA (α = 0.89). 3.3. Procedure and data analysis Participants completed the MESC and demographic questions during the usual school schedule (8:30–14:30 h) in groups of approximately 25–30 adolescents. Chronotype groups were established using 23/28 MESC values corresponding to the 25/75 percentiles (Díaz-Morales, 2015). The distinction of chronotypes (morning vs. evening groups) is appropriate when the research design is based on repeated measures, since it facilitates data analysis, interpretation and presentation (Jankowski, 2013). In the second phase, the chronotype groups selected completed paper-pencil diaries at night before going to sleep. Every night, they took 5 to 10 min to complete one page of the diaries that included questions about sleep questions and mood. Although typically 9 sleep diaries are completed after waking up to reduce recall bias, bedtime timing was chosen because questions about mood were referring to the passed day. An email was sent in the evening as a reminder to fill in the diary and to answer questions in case the participants had doubts. Multivariate analysis of covariance (MANCOVA) was used to analyze sex and chronotype effects on PA and NA during school days and weekend, controlling for sleep length. Partial eta-squared (ηp 2) was used as a measure of effect size (low: below 0.05; medium: 0.06-0.13; high: above 0.14). Multilevel modeling with maximum likelihood estimation and robust standard errors was conducted to account for repeated day-of-week observations on PA and NA among adolescents. Day-of-week and sleep length (within-individual effects) at Level 1 and sex and chronotype (between- individual effects) at Level 2 were considered predictors of PA and NA. To test the moderation hypothesis, we added interaction of day-of-week with sex, chronotype and sleep length. The statistical analyses were conducted using SPSS version 25 software (IBM Corporation, Armonk, NY). 4. Results 4.1. Between-person analysis Preliminary correlational analysis rejected any possible effect of age on chronotype, sleep length and mood. The results revealed a significant sex effect on PA and NA during the weekend (F1,86 = 6.03, p < 0.01, ηp 2 = 0.066 and F1,86 = 3.93, p < 0.05, ηp 2 = 0.044, respectively), such that girls reported lower PA and higher NA over the weekend (M±SD; PA: 45.02±12.81 vs. 52.48±13.55; and NA: 19.84±14.94 vs. 14.10±10.90, respectively). Additionally, the sex effect on NA was significant during school days (F1,86 = 6.99, p < 0.01, ηp 2 = 0.075), such that girls reported higher NA on 10 school days (27.45±14.41 vs. 20.25±12.64, respectively). The effects of chronotype as main factor and sleep length as a covariate were not significant. Table 1: Means and Standard Deviations on Sleep Length, Positive and Negative Affect (PA and NA) by day-of-week, sex and chronotype Monday Tuesday Wednesday Thursday Friday Saturday Sunday Sleep length Total 7.24 (1.2) 7.06 (1.2) 7.29 (1.3) 7.12 (1.1) 7.07 (1.6) 8.51 (1.6) 8.61 (1.8) Boys 7.17 (1) 7.16 (1) 7.45 (1.3) 7.01 (1.2) 6.99 (1.7) 8.71 (1.9) 8.29 (1.6) Girls 7.3 (1.3) 6.99 (1.4) 7.15 (1.3) 7.2 (1.1) 7.13 (1.5) 8.36 (1.3) 8.86 (1.9) Evening 7.12 (1.2) 6.88 (1.3) 7.18 (1.3) 6.93 (1) 7.06 (1.9) 8.74 (1.4) 8.64 (1.7) Morning 7.35 (1.1) 7.21 (1.1) 7.38 (1.4) 7.27 (1.2) 7.07 (1.4) 8.32 (1.7) 8.58 (1.8) PA Total 43.57 (16.3) 44.26 (15.5) 43.36 (15) 47.3 (13.4) 50.29 (14) 52.37 (16.3) 50.87 (14.3) Boys 48.99 (14.2) 45.73 (14.7) 46.82 (15.3) 51.41 (12.2) 52.23 (12.5) 61.77 (8.6) 57.96 (9.8) Girls 39.24 (16.8) 43.08 (16.1) 40.59 (14.3) 44.01 (13.6) 48.75 (15) 44.86 (17.1) 45.19 (14.8) Evening 42.03 (13.7) 42.93 (14.8) 43.21 (14.2) 45.96 (10) 49.7 (12) 50.23 (17.2) 47.82 (14) Morning 44.92 (18.4) 45.42 (16.2) 43.49 (15.8) 48.47 (15.8) 50.81 (15.6) 54.25 (15.3) 53.53 (14) NA Total 22.89 (17.4) 24.81 (15.8) 26.11 (17.5) 25.85 (19) 21.74 (16.6) 14.96 (14.1) 19.64 (15.1) Boys 19.36 (16.5) 22.12 (16.2) 21.8 (16.8) 19.53 (14) 18.74 (13.2) 11.64 (10.6) 16.58 (13.6) Girls 25.71 (17.7) 26.97 (15.4) 29.57 (17.5) 30.9 (20.9) 24.14 (18.7) 17.61 (16) 22.08 (15.9) Evening 23.08 (17.1) 22.44 (14.5) 24.12 (17.9) 26.35 (19.1) 21.27 (15.9) 15.93 (14.6) 21.09 (17.1) Morning 22.71 (17.9) 26.9 (16.8) 27.86 (17.2) 25.41 (19) 22.15 (17.3) 14.11 (13.8) 18.37 (13.2) Note: n = 94; Boys = 42; Girls = 52; Evening types = 43; Morning types = 51. Sleep Length is expressed in decimals. 4.2. Within-person analysis Descriptive statistics on weekly sleep length, PA, and NA by day-of-week, sex and chronotype are presented in Table 1. As seen in Table 2 and Figure 1, the day-of- week effect on PA indicated a progressive increase from Monday to Sunday, and boys reported higher PA than girls. When moderators were included in the model, no interaction was significant. On the other hand, the day-of-week effect on NA indicated a progressive decrease from Monday to Sunday; girls reported higher NA than boys, and shorter sleep duration was related to higher NA. When moderators were included in the model, there was a significant cross-level interaction between day-of-week and 11 chronotype such that negative affect decreased less over the weekend in evening adolescents (see Table 2 and Figure 2). Table 2: Day-of-week, sex, chronotype and weekly sleep length and interactions predicting Positive and Negative Affect. Positive Affect Negative Affect Variables b SE t b SE t Intercept 40.76 2.09 19.48*** 29.49 2.32 12.70*** Day-of-week 0.90 0.22 3.97*** -0.85 0.23 -3.67*** Sex 6.68 2.34 2.85** -8.95 2.64 -3.40** Chronotype -0.80 2.30 -0.35 -1.61 2.58 -0.63 Weekly sleep length 0.34 0.38 0.89 -1.39 0.41 -3.36** Day-of-week * sex -0.38 0.44 -0.86 0.31 0.45 0.70 Day-of-week * chronotype -0.37 0.43 -0.84 0.92 0.45 2.05* Day-of-week * weekly sleep length -0.01 0.15 0.10 0.15 0.16 0.93 Note: *** p < 0.001; ** p < 0.01; * p < 0.05; Sex: 0 = boys; 1= girls; Chronotype: 0 = evening; 1 = morning; Day-of-week: 0 = Monday to 6 = Sunday Figure 1: Positive Affect according day-of-week by sex, controlling for sleep length. 30 35 40 45 50 55 60 Mon Tues Wen Thu Fri Sat Sun Boys Girls 12 Figure 2: Negative Affect according day-of-week by chronotype, controlling for sleep length. 5. Discussion Adolescence is associated with major changes in sleep, chronotype and mood, a complex constellation of processes that are critical to development. However, current knowledge of how sleep, sex and chronotype are associated with PA and NA throughout the school week in adolescents is scarce, even though the pattern of mood over the course of a week has fascinated people for a long time. In adolescents attending school in the morning, PA increases and NA decreases from school days to the weekend. The lowest PA was reported on Wednesday, and each day afterwards, PA increased slightly, reaching its peak on Saturday. In contrast, the NA peak was Wednesday and progressively decreased to its lowest value on Saturday. This weekly NA pattern was moderated by chronotype, such that NA decreased less over the weekend in evening adolescents. As predicted, short sleep length was related to NA, and girls reported higher NA. The between-person analysis also confirmed that girls reported higher NA. 5 10 15 20 25 30 35 Mon Tues Wen Thu Fri Sat Sun Evening Morning 13 One possible explanation of the increase in PA over the weekend could be the greater opportunities the weekend provides for time with friends or family. The weekend, characterized by freedom from school, could be experienced as liberating and rewarding. Kennedy-Moore et al., (1992) showed that the greater relative frequency of desirable events and the lower frequency of undesirable events on weekends than on weekdays could partially explain the weekend effects. This hypothesis could provide a possible explanation of the findings in boys, who further increase their positive affect during the weekend. Previous studies have indicated that among the reasons to get up during the weekend, boys reported playing sports such as soccer (Collado et al., 2012). Additionally, these activities could be very rewarding for boys, and they would also be more exposed to natural light, which is considered the most powerful environmental cue (zeitgeber) that entrains circadian rhythm to the light/dark cycle (Crowley et al., 2014). Comforting activities and the degree of exposure to light could both be factors implicated in the increase in positive affect in boys during the weekend. These variables could be analyzed in future studies. Csikszentmihalyi and Hunter (2003) showed that students spending more time in school and participating in social activities are happier than those who spend less time on these activities. Additionally, Helliwell and Wang (2014) indicated that the extent and quality of social connections on the weekend compared to those during the week could explain the weekend effect on emotions, especially positive emotions. They found that weekend effects are larger for males than females, which is similar to the result obtained in the present study, revealing an increase in PA during the weekend in boys. Another hypothesis would be related to the quality of relationships at school and the extent to which classmates are friends and school activities are enjoyable. There should be less of a difference over the weekend since one is simply changing one set of 14 friends for another. However, if the atmosphere of interpersonal relationships at school is polluted, then we should expect to find significantly more positive emotions and fewer negative emotions during weekends. The better the social context of the school, the smaller differences there are between weekday and weekend emotions. This hypothesis would assume that school days and weekends are not very different for girls in terms of the quality of interpersonal relationships. This could be related to the greater sensitivity of girls to the judgments of others, which has been proposed as one of the relevant factors in the onset of depression during adolescence (Rudolph, 2009). NA decreased less over the weekend in evening adolescents. Longitudinal studies have highlighted how eveningness during adolescence predicted the occurrence of depressive episodes 1 year later (Haraden et al., 2017), and evening depressive patients reported higher NA on weekends, whereas morning depressive patients reported lower NA (Brückmann et al., 2020). Additionally, epidemiological research indicates that the prevalence of depression and anxiety is higher among adolescent girls than boys (Salk et al., 2016), suggesting that the stage of adolescence is a key period for the onset of possible affective disorders. The fact that NA decreases less during the weekend in evening adolescents could be considered another risk factor for the development of possible mental health problems, given that the weekend would not be providing leisure and enjoyment. Doane & Zeiders (2014) found that within-person increases in momentary NA during three typical consecutive weekdays of university students were associated with increases in cortisol. Perceived discrimination and social support from friends moderated the relation between NA and cortisol, so these processes could be analyzed considering the weekly mood patterns of NA and PA reported in the present study. 15 According to the previous results obtained among adults, sleep length and chronotype are related to PA and NA to different extents, but the results obtained in the present study indicated that the day of the week, chronotype and sex are also implicated in this complex relationship. Clearly, the social structure of time at high school has an impact on the mood of adolescents, and individual differences in sex and chronotype act as moderators in a different way. There are several limitations to this study. The small sample size limits the generalizability of the findings. Future studies could replicate these findings with large samples of adolescents to test whether sex and chronotype, along with the other variables raised, moderate day-of-week effects on PA and NA. Second, we cannot determine the sequential nature of the effects between mood, sleep and chronotype, so a longitudinal design could be used to test these sequential relationships in order to test also the effect of events happened during the day. Third, personality as a trait should have been previously measured, and the menstrual cycle was not controlled. Fourth, sleep length evaluated using a sleep diary could have been overestimated and it didn’t allow us to evaluate how long it takes a person to fall asleep. Finally, mood follows a diurnal pattern, showing more depressive moods at night, which was the moment chosen to perform the mood assessment, which could lead to a bias toward a more negative mood (see Díaz-Morales et al, 2015). However, the study has several strengths. For example, both sexes participated in the study, so we could compare sex effects, and repeated measures of mood and sleep habits were considered, avoiding recall bias. Of course, several processes likely influence the weekly mood pattern, such as personality traits, valuation of others, interpersonal relationships, family rules, or mass media influence. The findings extend the current literature by beginning to elucidate sex and chronotype effects on the weekly 16 mood pattern. 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