This study compared children’s and adolescents’ 24-hour movement behaviour characteristics, examined associations between these characteristics and mental health, and determined the optimal 24-hour time-use behaviour compositions for mental health. Children were more physically active, less sedentary, slept longer, and had lower sleep efficiency than adolescents. Furthermore, based on time-use estimates and directly measured accelerations, boys were more active than girls who in turn accrued more time in sleep. Of the 24-hour rest-activity rhythm metrics, children and boys had the highest M10, mesor and, amplitude values, while timing of acrophase was latest among adolescents. Overall mental health and externalising problems were significantly associated with sleep duration, ST, sleep efficiency, amplitude, and IS. The optimal time-use compositions were specific to the different mental health outcomes and were characterised by more sleep, LPA, and VPA, and less ST and MPA than the sample’s mean time-use composition.
Aim 1
Boys and children accumulated significantly more MPA and VPA, recorded greater activity volume, a higher intensity profile, and had superior M10 values than girls and adolescents, respectively, which concurs with earlier studies (28, 50, 51). Children also had higher mesor values than adolescents which indicate higher activity across the day (20) and are consistent with the age-related differences in M10 values and average acceleration (51, 52). This also reflects the additional 109 min•day− 1 of ST observed in adolescents compared to children, which offset the lower levels of sleep and physical activity in the older group. Such differences in activity have been attributed to various factors including timing and tempo of maturation (53), psychosocial reinforcement (54, 55), as well as changes to the 24-hour rest-activity rhythm (25). The sex differences in physical activity outcomes were also reflected by the higher amplitude values for boys, which represent higher maximum activity and more robust 24-hour rest-activity rhythmicity (20).
Sleep duration was significantly longer in girls and children compared to boys and adolescents, respectively, which aligns with other empirical accelerometry studies (16, 24, 56) and a 2018 meta-analysis of accelerometer-assessed sleep (57). A recent narrative review however, concluded that the direction of sex differences in children’s and adolescents’ sleep duration is equivocal and largely influenced by methodological differences and participant characteristics (58). Age-related differences in sleep are more consistent (57) and reflect the delay in the 24-hour rest-activity rhythm sleep phase during adolescence (i.e., by later sleep and waking times (24, 25)). This was observed in our sample, with adolescents’ sleep onset around 1.4 hours later than the children’s while waking times on most days were similar to accommodate the school day starting around 09:00 for both age groups. Furthermore, adolescents’ acrophase timing was significantly later than children’s, indicating later timing of peak activity, and reflecting a more delayed activity phase following nocturnal sleep (20). Adolescents had 88% sleep efficiency which was slightly, yet significantly higher than the 86% recorded for children. These differences concur with previous research (24), although both values were above the ≥ 85% sleep efficiency threshold for good sleep quality recommended by the US National Sleep Foundation so the difference observed may not have clinical relevance (59). Conversely, sleep durations for both groups were less than the 9 and 8 hours per night minimum ranges recommended for optimal health in children and adolescents, respectively (60). These seemingly contradictory findings highlight the multidimensional nature of sleep and underscore the utility of using complementary indicators of sleep quality.
Aim 2
In our sample the unadjusted mental health outcomes were higher in children than adolescents. This is counter to typically observed age-related differences in mental health problems among youth (61, 62, 63). As reported by others (64, 65), it is possible that mental health issues resulting from the COVID-19 pandemic and lockdown restrictions in the study 2 children may have continued and contributed to the higher than anticipated mental health outcomes in this group. We cannot though speculate beyond this, and importantly the total difficulties SDQ scores (i.e., overall mental health) for both children and adolescents were in the ‘close to average’ range, indicating that the observed differences were not clinically relevant in either group (36).
The compositional analysis of movement behaviour time-use estimates revealed that overall mental health and externalising problems were negatively associated with sleep and positively associated with ST, relative to the remaining behaviours. These findings are broadly consistent with previous literature although the relationships between mental health and sleep or ST are quite nuanced. For example, how ST is defined influences the magnitude of associations, with relatively strong and consistent evidence observed for screen-based ST (11, 66, 67). In our study total ST was estimated from low levels of wrist acceleration (i.e., < 48 mg) (40). Such cut-point approaches are subject to misclassification of ST as stationary standing or LPA and vice-versa and provide no information about type of sedentary behaviours performed – passive versus active screen time for instance. However, our accelerometer data were reflective of weekday and weekend activities and so it is likely that a representative range of sedentary behaviours, such as TV viewing, gaming, computer and mobile phone use, and studying were captured. Similarly, associations between sleep duration and mental health outcomes can be somewhat dependent on exposure and outcome measurement methods and amount of sleep recorded. For example, in Dutch children and adolescents, significant associations were recently observed between self-reported sleep and externalising problems, internalising problems, and dysregulation profile, but these associations were non-significant and much smaller based on wrist-accelerometer derived sleep (24). Moreover, the beneficial influence of sleep on mental health may be strongest among youth who have insufficient sleep, with diminishing returns for those who have more sleep than is necessary for health (66). Nonetheless, recent reviews have concluded that children and adolescents who meet sleep guidelines are more likely to have better mental health outcomes that peers who sleep less (7, 11). Interestingly, achieving recommended levels of sedentary screen time and sleep were also reported to be more strongly associated with mental health than meeting physical activity guidelines (11). This observation is reflected in our findings whereby LPA, MPA, VPA, average acceleration, intensity gradient, and M10 metrics were not significantly associated with any mental health outcomes, but sleep, ST, sleep quality, and 24-hour rest-activity rhythm metrics were.
Sleep efficiency was inversely associated with overall mental health and externalising problems, which is consistent with the associations observed when sleep duration was the exposure. However, the associations between sleep efficiency and externalising problems were only evident among boys. This may be partly explained by the 18% difference in boys’ and girls’ externalising problems scores (boys > girls) which mirror SDQ norms for UK youth (68). Furthermore, sleep efficiency is influenced by number of night awakenings (in our analysis, p = 0.06 and p = 0.05 for associations with overall mental health and externalising problems, respectively) which was recently shown to be more strongly associated with externalising problems in English boys compared to girls in the nationally-representative Millennium Cohort Study (69). These results emphasise the potential importance of sleep quality for boys reducing the risk of hyperactivity and conduct problems. Conversely, non-significant associations between accelerometer-assessed sleep efficiency and youth externalising problems were recently reported in children and adolescents from the Netherlands (24). The study authors suggest that assessing sleep outcomes using self-report methods better captures neuronal domains of sleep that relate more to mental health problems, and which may not be captured by accelerometers (24). On the other hand, accelerometers reduce much of the measurement error associated with self-reported behaviours and allow relatively accurate estimations of important sleep quality metrics. Clearly, there is a need for consistent and valid methodologies to be used to allow meaningful comparisons of sleep metrics between studies.
24-hour rest-activity rhythm amplitude was positively associated with overall mental health and externalising problems. The interpretation is that participants with greater maximum activity and more robust rest-activity patterns (i.e., higher activity in the day and lower activity during sleep) were likely to report higher scores for the hyperactivity and conduct items on the SDQ. This relationship seems counterintuitive because higher activity and more stable rest-activity rhythmicity are desirable for health and wellbeing (70). As the SDQ scores were not clinically relevant (36) the potential implications of this result are unclear and warrant further investigation with a sample providing more heterogenous SDQ scores. IS is indicative of between-day consistency of the 24-hour rest-activity cycle and was negatively associated with overall mental health and externalising problems. This indicates that poorer synchronisation of rest-activity rhythms to external zeitgebers (i.e., environmental cues for the 24-hour clock) was related to lower overall and externalising mental health. For example, participants’ low IS values may have reflected inconsistent bedtimes and wake-up times and daytime napping. Irregular bedtimes have been associated with overall mental problems (71) and poorer cognitive performance among English children (72). Moreover, consistent bedtime routines in general are related to an array of developmental benefits, including emotional and behavioural regulation (73).
No significant associations were observed between internalising problems and any of the 24-hour movement behaviour characteristics. We have previously reported positive associations between ST and internalising problems (16) and recent systematic reviews have shown how meeting guidelines for sleep, ST, and physical activity are associated with reduced odds of internalising problems, like depressive symptoms, with sleep and ST potentially more important in children compared to adolescents (7, 11). We speculate that the relatively low SDQ scores for emotional and peer problems may have caused a ceiling effect for the internalising problems construct and therefore limited the strength of associations with some of the 24-hour movement behaviour characteristics. The lack of observed associations between time-use physical activity estimates, directly measured acceleration, and mental health could relate to the nature of device-measured movement behaviours capturing more activity than that from self-report measures, which might disproportionately focus respondents’ perceptions and recall on discrete episodes of physical activity, such as exercise and sport, at the expense of incidental activity. Without domain-specific physical activity information, it is possible that incidental activity across the intensity spectrum could attenuate potential beneficial effects of higher intensity physical activity (i.e., MPA and VPA) accrued during structured and unstructured exercise and sports (74). Moreover, it not uncommon for accelerometers to be removed during exercise and sports for safety reasons (75), which would further mask potential beneficial influences of MPA and/or VPA on mental health.
Aim 3
Extending the analysis of time-use estimates of 24-hour movement behaviours and mental health in Aim 2, we calculated the optimal daily compositions of sleep, ST, LPA, MPA, and VPA for overall mental health and externalising problems. Both optimal compositions had subtle differences from each other and were characterised by longer sleep, more LPA, and VPA, and less ST and MPA, relative to the sample mean composition. The only other published study of optimal daily time-use compositions and mental health in youth reported associations with SDQ emotional problems and depressive symptoms (18), which were not included in our Aim 3 analysis because internalising problems were not significantly associated with the mean sample composition in Aim 2. Notwithstanding this and in the absence of other comparable data, we highlight the consistent important contribution of sleep, irrespective of mental health outcome under consideration. In Dumuid et al.’s Australian sample, 582 min•day− 1 was optimised for emotional problems and depressive symptoms (18), compared to 600 min•day− 1 (overall mental health) and 564 min•day− 1 (externalising problems) in our analyses. Displacing ST with LPA seemed to be important for overall mental health and externalising problems in our analyses, whereas Dumuid and colleagues found that relatively less LPA and more MVPA were optimal for emotional problems and depressive symptoms (18). Hypothetical optimal time-use compositions for mental health outcomes have much merit and through data visualisations such as those presented here and elsewhere (18, 76), can be extremely useful in translating key messages to research users (e.g., policy makers) and others with an influence over children’s movement behaviours, such as parents and teachers (77). The optimal compositions though, are sample-specific and may vary according to methods used to measure time-use exposures and mental health outcomes. Thus, caution is urged when comparing them across between studies. Studies involving larger nationally representative samples are required to further investigate the optimal time-use compositions for indicators of mental health.
This novel study has several strengths. We used unfiltered raw acceleration data from wrist-accelerometers to assess 24-hour movement behaviour time-use estimates, sleep quality, 24-hour rest-activity rhythm, and directly measured acceleration metrics. We employed stringent wear time criteria which ensured all participants in the analytical sample wore the device for 24 hours•day− 1 for a minimum of four days, including at least one weekend day. On average the participants had 6.5 valid days wear, indicating strong compliance to the protocol and a high degree of reliability in the resultant data. Further, mental health outcomes were assessed using a validated and well-established self-report tool and we applied the innovative ‘Goldilocks’ compositional analysis approach to generate optimal time-use compositions that were specific to different mental health outcomes. The study also had limitations which should be considered when interpreting the findings. The sample size was not large or representative beyond the region where the research took place. To a degree the sample size was reflective of the strict accelerometer inclusion criteria which increased data attrition, but arguably enhanced the reliability of the movement behaviour data. There was a risk of sampling bias because of observed differences in included and excluded participants who were older and from lower SES families. For these reasons generalising the findings beyond the analytical sample should be done with caution. Moreover, the cross-sectional design precludes any claims of causal inference and directionality between the 24-hour movement behaviour characteristics and mental health outcomes. Lastly, although the analyses were adjusted for variables that are known to influence the exposures and outcomes, we cannot rule out other sources of unmeasured residual confounding.