The Canadian 24-hour Movement Behaviour Guidelines (22) were developed based on systematic reviews demonstrating the physical, mental, and social benefits of sufficient MVPA (67), adequate sleep (68), and limited ST (69); however, since their release, limited research has examined the combined and independent associations with mental health and illness symptoms, particularly over time. In a large sample of secondary school students from Ontario and BC, the current study examined one-year change in adherence to the 24-hour movement behaviour guidelines and depressive symptoms, adjusting for sociodemographic covariates and prior year depression scores. Adherence to the sleep guidelines emerged as the most consistent predictor of depression symptoms. Youth who consistently met sleep recommendations over one year reported lower depressive symptoms in comparison to youth reporting short sleep across both years and those who transitioned from guideline adherence to nonadherence. This study also provides prospective evidence of a link between ST and depressive symptoms; where females who transitioned to meet ST guidelines had lower scores than those who continued to exceed 2 hours/day of total ST. However, no significant effect was found for one-year change in MVPA guideline adherence, when adjusting for the other guidelines, covariates, and prior year depression scores.
In the total adherence models, meeting more guidelines than the year prior predicted lower depressive symptoms in females only. Research has generally supported a cumulative effect, where the more guidelines adhered to, the larger the mental health benefit (50-52). When sex differences have been found in previous studies, significant effects have tended to occur among females only.44,45 Results are unsurprising given known variations in the manifestation of depressive symptoms and movement behaviour engagement. That is, females report higher levels of internalizing symptoms and depressive disorders (2-5) and are generally less likely to meet sleep and MVPA guidelines than males (48,70,71). Overall, effect sizes for depressive symptoms were modest, similar to previous studies of healthy youth populations (50). However, with few youth consistently meeting recommendations, promoting guideline adherence has potential to make a substantial population-level impact on the prevention and early management of depressive symptoms (50,72).
The independent associations for sleep, MVPA, and ST guideline adherence and depression vary across study. Similar to our results, an Australian cross-sectional study found sleep to be the only movement behaviour associated with depressive symptomatology in all youth, while inverse associations were found with ST guideline adherence in females and PA adherence in males (41). In other cross-sectional research, Janssen et al. found adherence to each guideline had an inverse association with ‘emotional problem’ scores to an equivalent degree (50); whereas, Zhu et al. found youth meeting recommendations for MVPA and sleep were less likely to have received a depression diagnosis, but no evidence of an association with ST (51). Conversely, in a cohort study of children and youth, meeting ST and PA recommendations at age 10/11 predicted fewer mental illness-related physician visits over 8 years but meeting the sleep recommendations had no effect (52). Methodology differences may contribute to inconsistent results, including cross-sectional versus prospective designs, types of ST included, and mental health indicators. Hayward et al. used self-reported depressive symptoms, while Janssen et al. employed a composite score (including depressive and other symptoms), Zhu et al. assessed parental reports of physician-diagnosed depression, and Loewen et al. linked survey data to administrative healthcare records (41,50-52). Further prospective research of large youth populations remains necessary to determine the independent associations of adherence to each movement behaviour recommendation for various mental health and illness outcomes; however, overall, evidence suggests benefits for youth meeting more recommendations.
Results bolster calls to give sleep the same attention traditionally devoted to PA and screen use. Our findings augment evidence of an inverse association between sleep and depressive symptoms (2,37,39-41,43,50,51) and conflict with studies that found no prospective effect (52,73)or an effect in females only (44,45). Once considered a core symptom or comorbidity of depressive disorders, some now identify sleep problems as both a prodromal manifestation and an independent risk factor for subsequent episodes, predicting the occurrence and outcome of depressive disorders (74). In fact, given the inadequacy of treatments for youth depression (75), sleep therapy has been suggested as an intervention for adolescent MDD (76). In addition to symptom management, further consideration from a population prevention standpoint is warranted, particularly given the proportion of youth sleeping less than 8 hours/night.
Developmental changes in sleep have been suggested to partly account for the emergence of depressive symptoms and MDD over adolescence (37). A natural shift towards later sleep onset contributes to a steady decline in youth sleep durations, coinciding with growing school pressures, increased extra-curricular activities, and less parental monitoring. As a result, youth bedtimes become progressively later, yet early school start times prohibit compensating with delayed wake times. Aligning school schedules to adolescent sleep patterns appears an effective public health strategy for promoting longer sleep. Evidence suggests even modest school start time delays predict increased sleep durations (70,77) as well as improved mental health and fewer emotional problems (77). In addition to developmental trajectories, evidence suggests a population shift towards shorter average sleep durations occurred over the past several years (71,72), parallel with increased reports of tiredness and difficulties sleeping (80,81) These trends coincide with increased reports of psychological distress and internalizing symptoms among youth (82) and further empathize the need to consider sleep in efforts to address youth mental health.
ST is the most debated of the movement behaviours, in terms of prospective links with mental health and illness. Some researchers have dismissed the effects as miniscule (27), while others argue a small risk of depression may result in substantial burden at the population level, considering the majority of youth exceed recommendations (72). In the current study, so few youth reported ST guideline adherence that we are limited in interpreting results. Transitioning to meeting ST recommendations may have a modest effect on depressive symptoms in females. Several reviews have concluded research supports a positive relationship between leisure-time screen use and depressive symptoms, or internalizing symptoms more broadly, among adolescents (33,34,83-85); however, much of this research has been cross-sectional. Proposed mechanisms for depression risk primarily focus on ST displacing more active, productive, or social activities. In this study, an association remained after adjustment for PA and sleep, suggesting depressive effects independent of their potential displacement; however, again, we are limited in interpretation. Other hypothesized mechanisms point to the context and content of ST engagement (72). Sedentary behaviours often take place in solitude, potentially giving rise to rumination and feelings of isolation (72); whereas PA typically occurs in the presence of peers among youth. The content itself could have a more direct effect, through social comparison or exposure to cyberbullying, for example. Further prospective research is needed exploring ways in which screens are used. Mixed findings in the literature may partially reflect inconsistencies in the form of ST assessed and included in determining guideline adherence, with prospective evidence varying by texting, computer, video game, or television time (23,26,72,86).
Our MVPA results coincide with reviews of observational studies among largely healthy populations, in which prospective associations between PA and depression are typically small or null, as opposed to intervention studies among samples with clinically diagnosed depression (20,21,67,87). Only PA volume was examined, but the context likely has unique relationships with mental health (88-90). Some evidence indicates MVPA is no longer associated with depressive symptoms when extracurricular activities such as team sports participation are accounted for (28), suggesting the mental health benefits relate more to positive social interaction and identity development. Many youth also report negative PA experiences, contributing to differences in PA and sports engagement by gender, body size, and age over adolescence (91-94). Motives may also play a role. Appearance or weight loss PA motivations have been associated with negative psychosocial outcomes, as opposed to functional or enjoyment motivations (95). Continued research is needed to explore various moderators in the relationship between PA and depression risk among youth.
Key strengths of this study include the linked data and adjustment for previous depression symptoms, as the reliance on cross-sectional evidence has been the most common criticism of previous research. The primary limitation pertains to self-report measures. As measures do not account for multi-tasking or for intermittent patterns of use, they likely overestimate total ST. The low frequency of students meeting ST guidelines may present power issues. Also, only two waves of linked data were available at time of this analysis. One-year change may not be optimal to understand effects over time. Lastly, COMPASS was not designed to be representative. The current sample was a pilot subsample of the larger study.
To our knowledge, this study is the first to examine change in the adherence to the 24-hour movement guidelines and depression symptoms over time in a population sample of youth. Adherence to the sleep recommendations emerged as the most consistent predictor of depression symptoms. Promoting adherence to the movement guidelines, particularly for sleep, should be considered a priority in the prevention and management of youth depressive symptoms. The decline in sleep duration over adolescence (49,66) has long been recognized yet remains undervalued in efforts addressing youth mental health, despite simultaneous increases in depressive symptoms and MDE onset (37). Depression in adolescence is associated with problematic substance use, suicidality, poor physical health, reduced academic achievement, and elevated risk for future episodes or other mental disorders continuing into adulthood (5,13). Given the large burden of depression (14,15) and low adherence to the movement guidelines, supporting youth in continuing to meet recommendations over adolescence appears a worthwhile public health investment, with clear benefits for multiple aspects of youth physical, mental, and social health (48,49,67).