The Journal of School Nursing2020, Vol. 36(6) 423-429© The Author(s) 2019Article reuse guidelines:sagepub.com/journals-permissionsDOI: 10.1177/1059840519842230journals.sagepub.com/home/jsn
Heeseung Choi, PhD, MPH, RN1,2 , Chanhee Kim, PhD, RN2 ,
Heesung Ko, PhD, RN3, and Chang Gi Park, PhD4
Longer sedentary time and insufficient sleep are common and potentially serious problems among adolescents and have substantially adverse effects on their physical and mental health. In this school-based study, we conducted an ecological momentary assessment using actigraphy to examine the within-subject association between sedentary time and sleep duration among 80 Korean adolescents aged 12–17. Objectively measured sedentary time and sleep duration were recorded over 5 days; participants also completed a self-reported questionnaire and sleep logs. Using a generalized estimating equation, analysis revealed that daily sedentary time was significantly and negatively associated with sleep duration (β = −.36, p = .028). Additionally, the interaction between age and sedentary time significantly affected sleep duration (β = .03, p = .012). These findings suggest the need for school-based interventions that aim to reduce sedentary time and thereby improve sleep duration, helping adolescents, especially younger ones, to achieve a healthier lifestyle.
adolescents, sedentary time, sleep duration, ecological momentary assessment, within-subject association
Extended sedentary time among adolescents is becoming a growing concern (Carson et al., 2016; Lee, Carson, Jeon, & Spence, 2016; Tremblay, LeBlanc, Janssen, et al., 2011; Tremblay, LeBlanc, Kho, et al., 2011). As children grow and start spending more time on academic activities, computers, and smartphones, the amount of time that they remain sedentary greatly increases (Lee, Carson, & Spence, 2017; Ortega et al., 2013). In a 2017 survey of 60,250 Korean adolescents aged 12–18 years, the mean sedentary time was 9.18 hr per day, including weekdays and weekends (Korea Centers for Disease Control and Prevention, 2017). This is problematic because prolonged sedentary time, regardless of whether the person engages in regular physical activity, has been associated with unfavorable health outcomes such as obesity, metabolic disease, and mortality (Bae, Kim, Rhie, Lee, & Nam, 2018; Biswas et al., 2015; Koster et al., 2012). Moreover, sedentary behaviors are associated with decreased levels of self-esteem, prosocial behaviors, and academic achievement among adolescents (Tremblay, LeBlanc, Kho et al., 2011).
Similarly, shortened sleep duration has also been a serious concern among adolescents. As children transition into adolescence, they experience significant changes in their lifestyles, including their sleep patterns. Adequate sleep is known to be an essential factor affecting adolescent development, which is necessary for maintaining their physical and mental health (Chaput et al., 2016; Hirshkowitz et al., 2015; Owens & Adolescent Sleep Working Group, 2014). According to the National Sleep Foundation, the proper amount of sleep for adolescents’ healthy development and desirable health outcomes is 8–10 hr per night (Hirshkowitz et al., 2015). However, the average amount of sleep reported was 7.1 hr per night on weekdays among Korean adolescents (Ministry of Gender Equality and Family, 2017).
Inadequate sleep and chronic sleep deprivation are associated with adverse health outcomes, including obesity, high risks of accidents, alcohol abuse, depression, and even suicidal ideation (Chaput et al., 2016; Owens & Adolescent Sleep Working Group, 2014; Shochat, Cohen-Zion, & Tzischinsky, 2014). Additionally, insufficient sleep leads to increased daytime sleepiness and impaired cognitive function, which affects adolescents’ school performance (Owens & Adolescent Sleep Working Group, 2014; Shochat et al., 2014). Both physical factors, including age, gender, and puberty status (Laberge et al., 2001), and mental health problems, such as depressive symptoms, contribute to changes in sleep patterns among adolescents (Owens and Adolescent Sleep Working Group, 2014).
1 College of Nursing, Seoul National University, Seoul, South Korea
2 The Research Institute of Nursing Science, Seoul National University, Seoul, South Korea
3 Jesus University, Jeonju, South Korea
4 College of Nursing, University of Illinois at Chicago, Chicago, IL, USA
Corresponding Author:Chanhee Kim, PhD, RN, The Research Institute of Nursing Science, Seoul National University, 103 Daehak-ro, Room 402, Jongno-Gu, Seoul 03080, South Korea.Email: chany131@gmail.com
Despite the importance of sedentary time and sleep duration during adolescence, a clear association between them has not been established as the results of previous studies have been mixed and inconclusive. While some studies have reported that sedentary time is not associated with sleep duration (Kakinami et al., 2017; McClain, Lewin, Laposky, Kahle, & Berrigan, 2014), other studies have identified a significant association between them (Brunetti, O’Loughlin, O’Loughlin, Constantin, & Pigeon, 2016; Ortega et al., 2011). Mixed results in previous studies may have occurred because of the cross-sectional and self-reported nature of the data (Chaput et al., 2016), which cannot accurately capture how daily sedentary time influences sleep duration in a natural environment.
Analysis of within-subject associations is an ideal way to capture the dynamic characteristics of sedentary and sleep behaviors, as it helps in understanding dynamic withinsubject variability. On the other hand, between-subject associations are analyzed to identify important structural, dispositional variables that distinguish individuals from one another (Bolger, Davis, & Rafaeli, 2003). Investigation of the potential within-subject association between sedentary time and sleep duration could reveal useful strategies for enhancing sleep among adolescents. Moreover, understanding how sedentary time affects sleep duration as adolescents grow is critical for developing age-specific sleep interventions.
Until now, the association between adolescents’ sedentary time and sleep duration has not been investigated using objective measures in Korea. In addition, changes in sedentary and sleep behaviors during everyday life are still not fully understood. Therefore, we conducted a descriptive, correlational study to examine within-subject associations between sedentary time and sleep duration using actigraphy data for Korean adolescents aged 12–17 years. To assess the dynamic effects of daily sedentary time on sleep duration among adolescents, we used an ecological momentary assessment (EMA) approach. EMA involves repeated assessment of human behaviors, experiences, or physiological data in real time in a natural environment (Shiffman, Stone, & Hufford, 2008). Conceptually, EMA acknowledges that behavior can be influenced by context, and thus assessment of behavior must be conducted in the context in which it naturally occurs. Thus, EMA allows researchers to capture the contextual factors that might influence variables of interest or the interactions between variables, while also limiting the effects of recall bias (Shiffman et al., 2008). The specific aims of the study were to determine (a) whether daily sedentary time affected same-day sleep duration over 5 consecutive days among middle and high school students and (b) if the magnitude of these associations differed by age.
The present study is the second phase of a 3-year research project investigating adolescents’ mental health and sleep and focuses on a school-based sample of adolescents. This study was performed after obtaining approval from the Institutional Review Board of Seoul National University, Korea. In this study, our sample size was calculated using the G-power program. The calculated sample was 68 participants, based on linear multiple regression analysis, considering the following parameters: two predictors and an effect size of 0.15, a Type I error of 0.05, and power of 0.8. We considered 20% more than the calculated sample size to account for the possibility of noncompliance and incomplete actigraphy data.
Adolescents aged 12–17 years were recruited through convenience sampling, following which an information letter describing the study purposes and procedures as well as consent forms (for both adolescents and their parents) were distributed among students and their parents at one middle school and one high school in the Seoul and Gyeonggi Province, South Korea. Participants were enrolled on a “first come, first served” basis until the target sample size was obtained. Parents and adolescents were asked to return their signed consent forms to the school if they agreed to participate in the study. We provided detailed information about the study procedures to the first 80 participants who agreed to participate in our study.
On the first day of data collection, the research team visited the participating schools and gathered all participating students in a classroom to provide more detailed information about the study procedure. The research team instructed all participants on how to wear and remove an actigraphy unit on their nondominant wrist. Participants were asked to wear the actigraphy unit for 24 hr, for 5 consecutive days (Thursday to Monday), except when they were bathing or swimming. The actigraphy unit has been proven to be capable of validly assessing sedentary time and sleep in adolescents (De Vries et al., 2009). The use of an actigraphy unit for at least 5 days is recommended to effectively assess sleep and activity data among adolescents (Sadeh, 2011). Moreover, placement of an actigraphy unit on the nondominant wrist has been validated for assessing sleep and activity (Martin & Hakim, 2011; Rowlands et al., 2014). Although the use of an actigraphy unit for more than 5 days would have been beneficial to obtain more reliable data, we recorded data for 5 days to minimize participants’ noncompliance and sense of burden.
To encourage participants to continuously wear the actigraphy unit and complete a daily sleep log during the study period, researchers sent text message reminders to participants on a daily basis. On Monday (the last day of wearing the actigraphy unit), the participants were gathered in a classroom to hand over their actigraphy units and sleep logs. They were also asked to complete a brief questionnaire regarding their sociodemographic information, sleep indices, and mental health (i.e., depressive symptoms), which lasted for 10–15 min. After they returned the actigraphy unit and the questionnaire, participants were given a small gift (US$20 value) for their time and effort. The study data were collected from October through November 2016. We collected data during this specific time of the school year to avoid any effects of midterm and final exams on ordinary sedentary time and sleep patterns among adolescents. Additionally, the two participating schools had a similar daily class schedule (such as their physical education curriculum) since Korean middle and high schools follow a standard curriculum (Ministry of Education, 2017).
ActiGraph wGT3X-BT accelerometer (actigraphy unit). To objectively assess daily sedentary time and sleep duration, we used the ActiGraph wGT3X-BT accelerometer (Pensacola, FL) . In this study, actigraphy data were collected in 1-min epochs. After participants returned the actigraphy units, the researchers downloaded all the data from these units using ActiLife 6 software. Both sedentary time and sleep duration were obtained from the actigraphy. Sedentary time comprises behaviors involving very low levels of energy expenditure, excluding sleep time, and is defined using a standard cut point of 100 counts per minute (Trost, Loprinzi, Moore, & Pfeiffer, 2011). Sleep duration values were drawn from total sleep time (TST) data obtained from the actigraphy units and TST was defined as the actual time spent asleep in a given sleep log-defined interval. Sleep log data were also used to verify the times that participants went to and rose from bed, which were used by the algorithm to calculate the TST.
Sleep log. Besides using actigraphy, we asked participants to complete sleep logs including their specific bedtime and wake up time, as well as any amount of time they did not wear the actigraphy unit and their reason for not wearing it.
Questionnaire. The questionnaire assessed participants’ demographic information and depressive symptoms. Participants’ depressive symptoms were measured using the Korean version of the pediatric Patient-Reported Outcomes Measurement Information System (PROMIS® ) depressive symptom scale (Choi, Ko, & Kim, 2017). This measure consisted of 13 items scored on a 5-point Likert-type scale, ranging from 1 = never to 5 = always, with higher scores indicating higher levels of depressive symptoms. Participants completed the PROMIS based on their recollection of the past 7 days. The scale was rated using T-scores provided by the PROMIS score center. The standardized mean score of this measure is 50, with a standard deviation (SD) of 10, and the total score ranges from 0 to 100 (Cella, Gershon, Bass, & Rothrock, 2016). In this study, the Cronbach’s α was .93.
Statistical analysis was conducted using SPSS Statistics 22 (IBM Corp., Armonk, NY). Participants’ characteristics were analyzed using descriptive statistics. In all further analyses, we included gender and depression scores as confounding variables, since depressive symptoms had strong associations with both sleep duration and sedentary time among adolescents in previous studies (Lovato & Gradisar, 2014; Vancampfort, Stubbs, Firth, Van Damme, & Koyanagi, 2018; Zhai, Zhang, & Zhang, 2015). Prior to using a generalized estimating equation (GEE) model, we assessed the correlations among variables for exploratory purposes. The GEE model was then employed to examine whether (a) daily sedentary time affected same-day sleep duration after controlling for gender and depression and (b) age and the interaction between age and daily sedentary time were associated with daily sleep duration. A GEE model is designed for analyzing the regression relationship between predictors and repeatedly measured outcome variables. The GEE model, an extension of the quasi-likelihood approach, is used to analyze longitudinal and correlated data (Hanley, Negassa, & Forrester, 2003). Because repeatedly measured daily sedentary time and sleep duration were highly correlated in our study, we employed a GEE analysis, as a GEE can handle correlated and missing data.
General Characteristics
Eighty adolescents recruited from one middle school and one high school participated in this school-based study. All students wore the actigraphy unit for all 5 days, except for one middle school student who did not wear the actigraphy unit on the third and fourth days. Table 1 shows the general characteristics of the participants. The mean (SD) age of the students was 14.48 (1.5) years. Of the 80 adolescents, 41 (51.3%) were girls. The daily sedentary time among all adolescents averaged 567.02 min (SD = 92.12) per day and ranged from 291 to 728.25 min. The sleep duration per night averaged 389 min (SD = 60.59) and ranged from 239.75 to 509.25 min. Furthermore, adolescents had a mean depression score of 47.98 (SD = 10.70), with scores ranging from 31.90 to 70.80.
Table 2 shows the correlations between daily sedentary time, daily sleep duration, age, and depression. Daily sedentary time and sleep duration were significantly correlated (r = .20, p < .001). Furthermore, age was significantly correlated with both daily sleep duration and sedentary time.
The GEE analysis was used to perform an EMA of the associations between daily sedentary time, daily sleep duration, and the interaction between age and sedentary time, after adjusting for gender and depression (Table 3). The results indicated that daily sedentary time was significantly and negatively associated with daily sleep duration (β = −0.36, p = .028). Furthermore, age was significantly and negatively associated with sedentary time (β = −29.53, p < .001). There was also a significant effect of interaction between age and sedentary time on sleep duration (β = .03, p = .012).
In this school-based study, we performed an EMA study to examine within-subject associations between objectively measured sedentary time and sleep duration among Korean adolescents aged 12–17 years. Participants’ average sedentary time was about 9.5 hr per day, which is longer than the 9 hr among European adolescents (Ruiz et al., 2011) and 6.9 hr among U.S. adolescents (Fletcher et al., 2017). Additionally, participants had an average of 6.5 hr of sleep per night, which is substantially lower than the recommended amount of nightly sleep for adolescents (Hirshkowitz et al., 2015). The overlong sedentary time and insufficient sleep duration observed in this study might be largely attributable to the highly competitive educational system of South Korea. Korean high school students spend an average of 12 hr per day in school to help them attain better academic achievement than their peers, leading to greater sedentary time (Kim, 2015). Furthermore, Korean adolescents spend a considerable portion of their leisure time using electronic devices such as a computer, smartphone, or television, leading to problematic levels of sedentary behavior (Lee et al., 2016; Lee et al., 2017; Lee & Spence, 2016). Since adolescence often involves dramatic physiological, psychological, and lifestyle changes, the consequences of prolonged sedentary time on adolescents’ health outcomes, including their sleep duration, require greater attention in a school environment (Carson et al., 2016).
Notably, we found that daily sedentary time had a significant and negative effect on daily sleep duration among adolescents. Only a few studies have investigated the association between sedentary time and sleep duration. Some studies that assessed sedentary time using an actigraphy showed no significant associations between them among adults (McClain et al., 2014; Vallance, Buman, Stevinson, & Lynch, 2015). In an adolescent sample, no significant association was found between self-reported sedentary time and sleep duration (Kakinami et al., 2017). It should be noted that the previous findings were based on betweensubject association analysis. On the other hand, consistent with our findings, a previous study found a significant within-subject association between daily sedentary time and sleep duration in adolescents (Master, Lee, Nahmod, Hale, & Buxton, 2018). Regardless of the inconsistent findings associated with differences in the measurement and data analysis method, results from the present study add evidence to the limited body of knowledge about the association between sedentary time and sleep duration. Understanding the association between daily sedentary time and sleep duration is necessary for designing educational programs that teach healthy sleep habits to adolescents and promote adequate sleep. Based on our findings, interventions aimed at reducing sedentary time should be considered to improve sleep duration among adolescents.
In our study, age was significantly and negatively associated with sleep duration, and the interaction of age and sedentary time was significantly and positively associated with sleep duration. In other words, the effect of sedentary time on sleep duration decreased with age. Based on the current findings, it is not clear why the magnitude of the effect of sedentary time on sleep duration is greater for younger adolescents; thus, futures studies with more diverse and larger samples are needed. Previous studies (Harris, King, & Gordon-Larsen, 2005; Sawyer et al., 2012) have suggested that adolescents’ age plays an important role in their developmental trajectories with regard to a healthy lifestyle (i.e., getting adequate exercise and sleep). Our findings therefore suggest that reducing sedentary time, especially among younger adolescents, may be critical to improving their sleep duration. Early adolescence is a critical period for building the foundation of a healthy lifestyle. The earlier that adolescents are motivated to reduce their sedentary behavior, the sooner they will be able to achieve healthier lifestyles.
The evidence for significant associations between prolonged sedentary time and poor health outcomes continues to grow. Given this trend, our findings contribute to the literature in several ways. A particular strength of this study is its use of objective measures of sedentary time and sleep duration through actigraphy during an important developmental period. Furthermore, our application of EMA in a natural environment and the use of a GEE analysis benefited our identification of significant within-subject associations between daily sedentary time and sleep duration. Given the amount of time that Koreans spend in school and the observed effects of sedentary time on sleep duration, school-based interventions aimed at reducing sedentary time are needed to improve adolescents’ sleep duration. In a systematic review (Hegarty, Mair, Kirby, Murtagh, & Murphy, 2016), multicomponent interventions including a teacher-led curriculum, homework tasks, and, in particular, standing desk activities were found to be effective in reducing sedentary time in youth. Therefore, school nurses can conduct health promotion programs to change adolescents’ sedentary behaviors and to enable them to be more active as often as possible during the day (e.g., reducing sitting time via standing desks, taking more steps per day). Additionally, the significant interaction effect of age and sedentary time on sleep duration suggests that such interventions are urgently required for younger adolescents.
We obtained data using a convenience sample of highly motivated students. In addition, considering the lack of research on the relationship between sedentary time and sleep time, more studies with diverse sample using objectively measured data based on both between- and withinsubject associations need to be conducted. Since sedentary time measured by actigraphy cannot provide any information on the types of sedentary behavior (i.e., screen time or nonscreen time), it is recommended that researchers consider the associations between specific types of sedentary time and sleep outcomes in future studies. Moreover, further studies should aim to expand on our findings, such as by examining the adverse effect of prolonged sedentary time on sleep duration over longer periods using EMA. In future studies, examining the effects of a specific school and neighborhood on sedentary time and sleep duration in adolescents is also necessary, considering that these contextual factors can greatly influence adolescents’ daily lives (Philbrook & El-Sheikh, 2016). Finally, research findings have shown that breaks in sedentary time are significantly associated with better cardiometabolic outcomes (Belcher et al., 2015; Chastin, Egerton, Leask, & Stamatakis, 2015). As such, examining the associations between breaks in sedentary time and sleep outcomes in further studies is worthwhile; encouraging breaks in sedentary time may be a plausible intervention for adolescents in a school setting.
Insufficient sleep and prolonged sedentary time are growing concerns among Korean adolescents. Our findings provide useful information on the within-subject association between sedentary time and sleep duration based on an EMA over a critical period of life. This understanding would be especially useful for school nurses in Korea, as the country’s adolescents spend most of their time in school. Thus, school nurses can conduct health promotion programs that aim to reduce sedentary time, such as “interrupting sedentary time,” using an ambulatory device to measure and encourage their activities. In addition, policy makers need to modify school environments or classroom settings to reduce sitting time.
We are grateful to the adolescents who participated in this study, as well as the school nurses who supported data collection. We also thank Mr. Jon Mann of the University of Illinois at Chicago for his editorial support.
All authors contributed to the overall data acquisition and analysis, critical revisions, gave final approval and agree to be accountable for all aspects of work ensuring integrity and accuracy. Heeseung Choi and Chanhee Kim drafted the manuscript.
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the Promising-Pioneering Researcher Program of Seoul National University.
Heeseung Choi, PhD, MPH, RN https://orcid.org/0000-0002-2649-4972
Chanhee Kim, PhD, RN https://orcid.org/0000-0001-5829-0889
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Heeseung Choi, PhD, MPH, RN, is a professor at the College of Nursing and the Research Institute of Nursing Science, Seoul National University, Seoul, South Korea.
Chanhee Kim, PhD, RN, is a researcher at the Research Institute of Nursing Science, Seoul National University, Seoul, South Korea.
Heesung Ko, PhD, RN, is an assistant professor at the Jesus University, Jeonju, South Korea.
Chang Gi Park, PhD, is a research assistant professor at College of Nursing, University of Illinois at Chicago, Chicago, IL, USA.
