This study investigated associations between 24-hour activity behaviours and a range of mental health outcomes in youth, which extend beyond those previously examined in compositional analysis studies (i.e., overall mental wellbeing (4) and health-related quality of life (68)). Overall, we found that activity compositions were significantly associated with internalising problems, while among primary school participants they were associated with prosocial behaviours, and the switching and inhibition aspects of executive function. Relative to other activity behaviours, the time spent in ST was significantly associated with internalising problems, and with prosocial behaviour (primary school participants only). Further, significant associations were observed between LPA (relative to other behaviours) and primary school participants’ performances on the switching and inhibition tasks.
Internalising problems encompass emotional and peer relationship difficulties and there is limited literature reporting associations between ST and these aspects of mental health. There is though, systematic review evidence supporting a positive relationship between sedentary screen time and internalising problems in school-aged youth (27). Further, longitudinal findings from the UK show that children who watched TV and/or played video games for more than 3 hour/day were more likely to report emotional and peer-relationship difficulties 2 years later (72). Similarly, US children and adolescents with more than 1 hour/day of daily screen time had lower emotional stability and increased difficulty making friends, with associations typically stronger among adolescents (73). Our findings of higher internalising problems associated with higher ST (relative to other behaviours) may reflect the unfavourable influence of more sedentary screen time. It is possible that in analyses stratified by primary and secondary school, these relationships were only observed among primary school participants because there was more observed variance in the activity exposure and mental health outcome variables (see variation matrix in Additional file 3), and a lack of statistical power for the smaller secondary school sample.
ST was also inversely associated with prosocial behaviour but only in primary school participants. This is consistent with the associations between TV viewing/gaming and prosocial behaviours reported in Carson et al.’s 2016 systematic review (26) and longitudinal data from the UK Millennium Cohort Study (72). While these studies provide some support for our findings it is important to recognise that total ST as reported in our compositional analysis is a different measure than screen time, screen time can occur in non-sedentary postures (74), and high ST is largely independent of screen time (75). It is unclear whether total ST (25) or specific types of ST are more strongly related to mental health outcomes (76). To address this, future studies should employ methodologies such as ecological momentary assessment or device-based pattern recognition approaches to identify specific types of sedentary behaviours (76), which may provide a more precise picture of the compositional relationships between activity behaviours and mental health.
The daily activity compositions were not associated with overall emotional and behavioural mental health, externalising problems, depression, or self-esteem. Contrastingly, non-compositional studies have reported associations between these outcomes and sleep, ST, and physical activity (21, 24, 26, 29), but none accounted for the mutually exclusive and compensatory nature of these behaviours, which are relative to one another over a 24-hour day. Recently, this has been examined with studies showing that youth who met multiple guidelines for sleep, ST, and MVPA were likely to have better mental health than peers meeting fewer recommendations (20, 32, 64). However, as our sample had low prevalence of meeting physical activity and sleep guidelines, it is unlikely that this explanation would hold true with our data. Another possible explanation for the lack of observed associations is the relatively similar and favourable mental health questionnaire scores, which indicated that our sample was at low risk for poor mental health. Further, their SES was moderate to high, which is consistent with previous research reporting positive SES and youth mental health relationships (64, 77). The relatively high SES of our participants may have predisposed them to have favourable mental health status, which could have limited the precision and ability to detect associations between activity behaviours and the mental health outcomes (i.e., a ceiling effect) (64). We recognise though that the IMD was not sensitive to individual family level SES, hence, we exercise some caution with this supposition.
Among the primary school participants, the switching and inhibition aspects of executive function were positively associated with LPA relative to sleep, ST, and MVPA. Moreover, when time was reallocated between behaviours, the effect sizes were large and more substantial than for internalising problems and prosocial behaviour. For example, a modelled reallocation of 10 minutes of LPA to MVPA estimated a -4.20 difference in switching error score with an ES of 5.24. In contrast, a 10 minutes reallocation of ST to MVPA with prosocial behaviour as the outcome, estimated a 0.08 unit difference, with a substantially smaller ES of 0.05. Hence, in our sample the effect of reallocating more time to MVPA from ST or LPA appears to have been be more beneficial for executive function than for indictors of mental ill health.
Evidence exists for a relationship between physical activity and cognition and brain structures that support complex processes in controlled laboratory environments (78). Translating such findings to practice is important, particularly in school settings where there is great interest in how replacing sedentary class time with physical activity can promote learning through improved cognitive performance. Currently though, there is inconsistent evidence of the beneficial effects of such interventions (79, 80). Similarly, evidence for associations between sleep duration and cognitive function is equivocal (29) with the most recent meta-analysis of objectively assessed sleep and cognition reporting small, non-significant effects (31). These authors posited that as most youth sleep for less than the recommended duration (70) the cognitive performance of those with adequate sleep was underrepresented in their analysis. Therefore reported associations between ‘typical’ sleep and cognition were attenuated because of the restricted range of sleep durations observed (31). As less than 25% of our participants achieved the recommended sleep duration this attenuation of a sleep-cognition relationship may have also occurred in our sample.
In the compositional isotemporal substitution analyses, reallocations between MVPA and ST/LPA had the greatest predicted associations with internalising problems, prosocial behaviours, switching, and inhibition (relative to the other behaviours), and estimated differences increased incrementally with the duration of time reallocated. MVPA is characterised by much higher energy expenditure than sleep, ST, and LPA; it may therefore provide a more powerful physiological stimulus for structural and hormonal changes that are favourable for cognition (81). Moreover, MVPA for youth (e.g., sport, structured activities) often involves participation with peers, parental encouragement, and enhancement of emotional status (20, 82), which are consistent with better internalising mental health and prosocial behaviours. While these are possible explanations for the influence of MVPA on the outcomes in this study, it is acknowledged that the mechanistic evidence for this is currently limited (81). Interestingly, associations with the outcomes were greatest when MVPA was replaced by ST or LPA, rather than when MVPA replaced these behaviours. These asymmetrical associated differences in health outcomes involving MVPA have previously been observed in youth studies considering adiposity, fitness, cardiometabolic risk, mental health, and health-related quality of life (4, 6, 68). This was likely due to MVPA constituting a relatively small proportion (50.9 minutes or 3.5%) of the activity composition. Therefore, a 20 minute reallocation in MVPA represents a change of 39%, whereby the equivalent absolute change in SB reflected only 3% of the geometric mean proportion of ST of 635 min⋅day− 1 (67).
Strengths of this study included device-based assessment of 24-hour activity behaviours and use of compositional data analysis to examine how the full activity composition related to mental health. Moreover, ours is the first compositional analysis study to investigate multiple mental health indicators, measured using validated instruments and including objectively assessed cognitive function. There were also limitations which warrant consideration. The cross-sectional design precludes any claims of causal inferences and directionality between the activity composition and mental health indicators. We had an imbalanced sample of primary and secondary school participants, who were relatively homogenous in respect of area-level SES, which may have been reflected in their relatively high mental health status and the limited associations with the activity compositions. Most of the mental health outcomes were measured using questionnaires which although validated, have potential for social desirability bias. Lastly, the activity behaviours were defined using validated wrist acceleration cutpoints which for LPA and MVPA, reflect absolute intensity rather than relative intensity for each participant. This is likely to result in some misclassification of activity behaviours among the sample. Use of machine learning to label activity behaviours and intensities would contribute to addressing this issue in future studies.