It is well-established that participating in physical activity (PA) is associated with better mental and psychosocial health among adolescents [1,2]. Specifically, regularly participating in PA is inversely associated with depression, anxiety, stress, and behavioral issues while favorably associated with selfesteem and cognitive functioning [2]. PA is also associated with better sleep quality among adolescents [3,4].

Despite the potential benefits of PA on psychological well-being and sleep among adolescents, PA levels among South Korean (Korean thereafter) adolescents are low compared to their peers in other countries [5,6]. For example, in the Global Matrix 3.0 where report cards on children and youth PA from 49 countries were compared, overall PA among Korean children and adolescents were graded “F”, with only 5.8% of them engaging in 60 minutes of moderate- to vigorous-intensity physical activity (MVPA) daily [6]. In addition, adherence to the World Health Or-ganization's PA guidelines (i.e., accumulating at least 60 minutes of MVPA daily) was only 5.9% among a national sample of Korean adolescents aged 12 to 18 years based on selfreported data [7].

The PA of adolescents occurs in school, and schools is a place of PA for adolescents who spend most of their time [8]. Particularly, Korean adolescents depend on schoolrelated PA [9]. The number of steps at school accounts for 40% of the total number of PA per day [10]. PA through physical education (PE) classes provide more than 50% of daily MVPA [11,12]. Moreover, after-school PAs at school are found to provide up to 50% of PAs to adolescents [10]. In addition, participating in PE classes and psychological well-being among Korean adolescents showed positive relationships. The more active students participated in PE classes, the lower their mental health problems were [13]. More school PA activities (e.g., PE classes, organized sports clubs, after-school programs, etc.) are positive for mental health such as stress, depression, and suicidal attempt [14].

However, given that Korean students spend most of their waking hours in schools or private institutions with highly structured daily schedules [15], it is important to examine the potential benefits of the frequency of participating in PE classes, rather than overall PA per se, to the well-being of adolescents. This is also important in that schools can provide all students (either active or not) with an equal opportunity to be physically active [15,16].

With regard to sleep satisfaction, to date, one study reported that participating in MVPA is associated with sleep satisfaction among Korean adolescents [17]. However, no study has examined associations between PE participation and sleep satisfaction in the Korean adolescent population. This inquiry is particularly important in the Korean context given that Korean adolescents studied most of their waking hours and generally lack sleep due to important academic workload [18]. For example, in typical weekdays, Korean adolescents spend an average 49 hours per week (about 40% of their waking hours in a 24-hour period) in school or private institution studying [18]. As for sleep, on average 13-15-year-olds sleep 7.6 hours and 16-18-year-olds sleep only 5.8 hours per day [18], while the recommendation is 8-10 hours per night. When the maximum sleep duration is fixed along with other academic schedules within a 24-hour period for Korean adolescents, it may be more important to target the quality of sleep rather than the quantity.

As the awareness and consensus on the effectiveness of school PE such as improving health and harmonious personality through various sports activities has been established, school PE curriculum were organized into 272 hours per year for middle schools, 10 units or more and 6 semesters for high schools from 2016 [20]. Therefore, the purpose of this study was to examine associations between participation in PE classes and psychological well-being and sleep satisfaction among a national sample of Korean adolescents. We hypothesized that greater participation in PE classes would be associated with positive psychological well-being and sleep satisfaction.

Of the eligible 132,974, a total of 127,804 adolescents (response rate, 96.1%) had complete data and included in the analyses. Descriptive characteristics of the participants were shown in Table 1. The mean age of the participants was 15.18±0.08 years, with 65,427 boys (51.5%) and 55,466 middle school students (43.4%).

Overall, 16.9% of students (13.7% in boys and 20.3% in girls; 14.2% of middle school students and 19.0% of high school students) did not participate in any PE classes. 33.0% of students were participated in PE classes for more than 3 times per week (Fig. 1). Among them, 38.8% of boys and 26.7% of girls participated in PE classes for more than 3 times per week, indicating a significant gender difference (χ² =1,534.857, p <.001). 55.6% of middle school students and 15.6% of high school students were participated in PE classes for more than 3 times per week (χ² =12,690.161, p <.001). Proportions of participants feeling happy and less/no stress, and sleep satisfaction were 66.2%, 20.0%, and 25.1%. The proportion of participants who participated in PA at least 60 minutes for more than five days a week was 13.5%. The mean breakfast frequency was 5.31 days per week, the high caffeine drink consumption was 1.30 times per week, and the mean sleep duration was about 6.27 hours.

Table 2 shows the gender-stratified associations between participation in PE classes, psychological well-being, and sleep satisfaction before and after adjusting for covariates. Overall, we observed dose-dependent pat-terns on the associations between participation in PE classes and happiness. After adjusting for covariates, participating in PE classes for more than three times per week was associated with happiness in both boys (OR: 1.59, 95% CI=1.42-1.77, p <.05) and girls (OR: 1.32, 95% CI=1.19-1.46, p <.05). Moreover, greater participation in PE classes was associated with higher odds of reporting happiness in a linear fashion in both boys and girls (p-for-trend=.000). Participating in PE classes for more than three times per week was associated with less/no stress in boys (OR: 1.16, 95% CI=1.02-1.31, p <.05). The associations between PE classes and sleep satisfaction were primarily null.

Table 3 shows the associations between participation in PE classes, psychological well-being, and sleep satisfaction by grade. After adjusting for covariates, participating in PE classes for more than three times per week was associated with happiness in middle school students (OR: 1.30, 95% CI=1.16-1.47, p <.05) and high school students (OR: 1.60, 95% CI=1.43-1.80, p <.05). Moreover, greater participation in PE classes was associated with higher odds of reporting happiness in a linear fashion in high school students (p-for-trend=.000). Participating in PE classes for more than three times per week was associated with less/no stress in high school students (OR: 1.22, 95% CI=1.03-1.44, p <.05). Additionally, greater participation in PE classes was associated with lower odds of reporting sleep satisfaction in a linear fashion in high school students (p-for-trend=.000).

This study demonstrated that participation in PE classes is associated with positive psychological well-being among Korean adolescents. compared to students who did not participate in any PE classes, those who participated in PE classes at least one time per week were more likely to be happy and less likely to feel stressed. Further, these associations were slightly stronger in boys than girls, and high school students than middle school students; however, the strength of the associations was small. Overall, our findings suggest that the more PE classes Korean students report participating in the better the associations are with psychological well-being. That being said, no relationships existed between participation in PE classes and sleep satisfaction except high school students.

Positive associations between PE and psychological well-being have previously been reported. For example, participation in PE classes was associated with higher levels of happiness and lower levels of perceived stress and depression in the Korean adolescent population [15,18,30,31]. The present study adds to the available evidence on the topic by providing contextual information on which PA setting may be beneficial to psychological well-being among Korean adolescents. School-based PA programs can provide the equal opportunity for students to engage in structured PA that may supplement to achieve the overall recommended PA for numerous health benefits [32]. Furthermore, beyond educational benefits such as the acquisition and development of movement skills and physical competence, PE can provide a space where students can feel en-joyment, relatedness, and social support while interacting with a sup-portive PE teacher and among students themselves [32,33]. Such social interactions within PE classes may play a role in the psychological well-being of adolescents.

Another explanation for the better psychological well-being among participants reporting greater participation in PE classes could be due to the PA-associated changes in the central catecholamine system, that is, changes in β-endorphins and/or norepinephrine levels that can lead to more positive emotions while reducing pain [34–37]. These neurotrans-mitters, which produce feelings of well-being, may act as a mood en-hancer during and post PA [37]. Given these, we may speculate that participation in PE classes is mediated by a psychological function and in turn acts as a buffer against stressful events [38] such as studying for prolonged hours. Indeed, recent evidence suggests that breaking up prolonged sitting can be beneficial to health and well-being [39]. However, it is important to note that, in our study, only a high dose of PE classes participation was associated with the less/no stress indicator. These results may indicate that whereas a minimum dose of PE can be beneficial to adolescents feeling happy, more doses of the PE exposure would be required for adolescents to attenuate their daily stress level.

No associations were observed in the relationship between participation in PE classes and sleep satisfaction after adjusting for covariates in our study except high school students. The decrease in sleep satisfaction in high school students as the frequency of participants in physical education classes increases could be interpreted as high school students have relatively less sleep time and less time in physical education classes than middle school students. In addition to the covariates selected in this study, more in-depth analysis is needed, such as selecting and modifying additional variables-smoking and drinking that are more suitable for adolescents. It may also be suggested that the physical activity through PE classes strength, type, and participation time presented in the survey in this study fell short of the threshold level for a positive change in subjective sleep satisfaction [40]. This is contradictory to the majority of studies that have suggested that PA is associated with sleep satisfaction among adolescents in different countries [40–42] as well as in Korea [18,43]. The previous studies showed that high-intensity and/or vigorous physical activity (VPA) was associated with favorable sleep patterns and less sleep complaints [41,42]. Furthermore, even if the frequency of participating in school PE classes was high, the degree of fatigue recovery through sleep had been negative [43]. According to the literature, associations between VPA and sleep satisfaction could be explained by two mecha-nisms-physiological and biological changes. First, VPA has been shown to increase cortical blood flow, endorphin release, and epinephrine and norepinephrine synthesis in the short term [44]. Second, VPA may positively contribute to increasing the volume of slow-wave sleep while reducing stage-2 sleep, and decreasing sleep-onset latency, which all may result in better efficiency of sleep [18,41]. No association between PE participation and sleep satisfaction may be due to the fact that in PE classes most PAs occur at moderate-to vigorous intensity rather than vigorous intensity [18,42]. Another possible explanation of our findings between PE participation and sleep satisfaction is that perhaps sleep satisfaction may be more closely related to PA outside of PE and/or screen time behaviors rather than PE [42,45]. Given this, future researches on PE/PA and sleep satisfaction are needed because sleep is affected by various factors such as depression, anxiety, screen time, smoking and drinking [44,46].

Regardless of potential benefits of PE participation on psychological well-being shown in our study, PE classes often get replaced by self-di-rected study time for other important subjects in the competitive education environment of Korea [17]. In addition, Korea adolescents do not exercise hard even during insufficient PE classes [45]. Given that Korean children scored the lowest in the happiness scale among children in 22 countries for the past seven years [47,48], our findings on the potential benefits of participating in PE classes on psychological well-being pro-vides further justification for increasing mandatory PE classes per week. According to the most current regulation associated with PE in Korea, middle schools are mandated to deliver 4 hours of PE per week while compulsory PE for middle school students is 272 hours per year. That being said, it is unknown if these regulations associated with mandatory PE are actually being delivered in schools [49].

In addition to increasing quantity, improving the quality of PE should also be considered simultaneously. In particular, gender is a major corre-late of PA among Korean adolescents [50]. In a systematic review on correlates of PA among girls, gender discrimination and gender stereotypes by PE educators, and gender-biased teaching and evaluation methods have been identified as major barriers to PA [51]. This may explain the stronger associations between PE classes, psychological well-being, and sleep satisfaction in boys than girls shown in our study. It is also noted that the current PE programs in Korea may not appropriately represent the needs of female students [52]. Therefore, the development of future PA promotion strategies within a PE setting should be gender-specific. Furthermore, like gender differences, grade differences were also clearly indicated in this study, with high school students participating in school PE classes having higher subjective happiness and less stress than middle school students. This special nature of Korean education system, which values academic performance, could be explained as trying to get rid of the stress of high school students, especially through PAs in PE classes in school [29]. Securing high school PE classes, which are replaced by self-study hours or other major subjects – Korean, English, and math classes, is especially essential in high schools, where the burden of studying is heavier than in middle schools [29].

The major strength of our study is the use of a large and nationally representative sample. However, this study has limitations that should be acknowledged. We were unable to test causality due to the nature of the cross-sectional design of the KYRBS. Research with stronger study de-signs and/or more robust measures is required to confirm our findings. Another limitation is the use of self-administered questionnaires. The questions are subjective and simple, and the measurement errors that occur in the response process cannot be excluded [25]; however, the KYRBS questionnaire is known to be valid and reliable over time [53]. Nonetheless, the association between PE participation and sleep satisfaction observed in our study may be due to the fact that sleep satisfaction was measured using a single item. Given the importance of sleep that makes up healthy 24-hour movement behavioral patterns [54], and its significant implications to overall health and well-being [55], compre-hensively measuring different aspects of sleep behavior. Furthermore, it is unknown how much PA was performed by students during PE classes; thus, we are not able to confirm whether the observed associations are due to actual PA or other factors. It is also noteworthy that residual con-founding by unmeasured variables (e.g., diet quality, screen time) is al-ways a possibility in observational studies.

Our findings suggest that, primarily, participating in PE classes more frequently is positively associated with psychological well-being among Korean adolescents, regardless of family economic status, academic achievement, the frequency of breakfast and high-caffeine drink consumption, and PA level. School is a key environment in which most Korean adolescents spend a large portion of their waking hours during weekdays [52,56]. Not only PE classes in school plays a key role in improving adolescents’ health, but also PE classes could lead adolescents to be active and establish physically active lifestyle [30].

Given this, emphasizing the importance of PE and increasing mandatory PE classes per week, as well as providing quality PE may be a first step toward promoting psychological well-being among Korean adolescents. In addition, more research is required to elucidate whether participating in PE classes do not have any implications to sleep satisfaction that either confirms or rejects our findings.