In this study, we modelled concurrent trajectories of outdoor time, screen time and sleep across the early childhood period (from 4 to 60 months). Consistent with previous evidence, both outdoor time and screen time tended to increase with age [40], while sleep duration decreased [10]. However, our concurrent trajectory analyses identified four discrete patterns of change over time within individuals. Almost half the sample belonged to the ‘high sleep, increasing outdoor time, low screen group’, one-quarter of the sample belonged to either the ‘unstable sleep, increasing outdoor time, low screen’ or ‘high outdoor time, low screen, high sleep’ groups, and just 10% were categorized in the ‘high screen, increasing outdoor time, high sleep’ group. There were a number of maternal factors that appeared to be associated with group membership.
The groups identified in the current study were mainly characterized by differences in just one of the three behaviours, i.e., there were no distinct patterns in behaviours grouping together. For example, the ‘unstable sleep, increasing outdoor time, low screen’ group was characterised by unstable sleep with average trajectories for outdoor time and screen time, while the ‘high outdoor time, low screen, high sleep’ group was characterised by higher-than-average outdoor time and average trajectories for screen time and sleep. This is not dissimilar to previous evidence that found three distinct joint-trajectories of physical activity and screen time from birth to 5 years, similarly characterized by a single stand out behaviour: a ‘low activity-low screen’, ‘increasing activity-low screen’, and ‘low activity-increasing screen’group [15]. Collectively, these findings suggest that there may not be a strong interplay between movement behaviours in early childhood; i.e., children tend to be ‘average’ overall but have a single defining behaviour. When designing interventions, it may be important to identify these defining behaviours (i.e., using trajectories) within a cohort and target changes in that specific behaviour.
The differences in outdoor time and screen time between the groups with the lowest and highest levels of the respective behaviours were magnified over time. Outdoor time differed between groups by 30 min at age 4 months, increasing to almost 2 h at 60 months. Similarly, the difference in screen time between groups at age 4 months was around 1.5 h/day, increasing to more than 2.5 h/day at 60 months. These findings underscore the need for early identification of children at risk of unhealthy trajectories of these behaviours, and subsequent early intervention to promote healthy behaviours across the early childhood period. Conversely, sleep stabilized over time, with the difference between the groups with the lowest and highest levels of sleep decreasing from around 4 h/day at age 4 months to less than 1 h/day at 60 months. This finding likely reflects the greater stability of sleep by school starting age, compared to infants where sleep issues are prevalent [41].
The maternal factors that predicted membership in different concurrent trajectories of movement behaviours may be important to consider in the planning of interventions to promote healthy movement behaviours. Higher maternal self-efficacy for limiting screen time and promoting physical activity was predictive of children being in the healthiest group, characterized by high outdoor time. Existing evidence suggests that parental self-efficacy appears to be generalizable across movement behaviors [21, 42], and has also been shown to track over time [43], suggesting that targeting parents’ self-efficacy early, i.e., prior to their children’s first exposure to screens, may be important.
Higher maternal screen time knowledge and lower screen time use for practical reasons were associated with reduced odds of children being in the groups characterized by unstable sleep and high screen time. Additionally, maternal television viewing was strongly associated with increased odds of children being in the group characterized by high screen time. These findings are consistent with previous evidence showing that parental attitudes regarding their child’s screen time (encompassing knowledge and use) and parent’s own screen time are related to children’s screen time [44]. Other potentially important maternal factors identified in this study were maternal views of physically active children and mothers’ own MVPA. Together, these findings suggest that mothers who have a good understanding and value the importance of moving more and minimizing screen time are likely to be doing this more themselves, and promoting this more in their children. Targeting mothers’ modelling of movement behaviours, in addition to their knowledge and beliefs, may be important to promote healthy behaviours from a young age.
Mothers’ sleep quality was associated with higher odds of children being in the group characterized by high sleep compared to the group characterized by unstable sleep. Previous research has found that a number of factors are associated with infant sleep, including co-sleeping, being nursed to sleep, longer sleep latency (i.e., taking longer to fall asleep), and longer and more frequent night-waking [41], and it is likely that many of these factors also affect maternal sleep quality. In the current study, mothers reported their sleep quality at baseline (child age 4 months), so it is possible that there were already bi-directional associations between sleep quality and their infants’ sleep at this age, where the largest difference (of over 4 h) was observed between these groups characterized by high vs unstable sleep.
There are some limitations of the present study. The key limitation is the use of outdoor time as a proxy for physical activity at each time point. Mothers were not asked to report whether this time outdoors was active or sedentary. Evidence from 2-year-old children attending childcare suggests that almost 70% of their time outdoors is active, with 21% of total time outdoors being MVPA [19]. Preliminary evidence suggests that around 57% of infants’ and toddlers’ (ranging from 6 weeks to 36 months of age) unstructured outdoor time in childcare is spent active [20]. Although this study included a relatively small sample (n = 49) in a childcare setting, results provide initial evidence that outdoor time can be active time in very young children. It is likely that a portion of the reported time outdoors in the current study was spent sedentary for children who were mobile. At the earlier time points (particularly at baseline when children were 4 months old), it is likely that a large portion of time outdoors was spent in a pram/stroller and this time may not accurately represent physical activity per se. However, ‘physical activity’ in the first 6 months of life comprises small movements such as reaching and grasping objects, turning the head toward a stimulus, and movement of the arms and legs [45], all of which are difficult to measure (either objectively or via parent report). In children aged 2 years and over, time outdoors is positively associated with habitual physical activity [34], and is therefore often used as a proxy for physical activity [32, 33]. As physical activity has been shown to track through early childhood [11], time outdoors at a very young age may be predictive of time outdoors in future years, and therefore predictive of future physical activity levels. Given this, and that the operationalization of physical activity guideline compliance differs across the early childhood period (i.e., tummy time for infants and total physical activity with a focus on intensity [i.e., MVPA] for toddlers and preschoolers), we decided to use outdoor time for our trajectory analyses as a common indicator of physical activity across the five time points. Results from our sensitivity analyses provide confidence in the findings.
Secondly, mothers in our sample were highly educated and recruited from Metropolitan areas within 60 km of Deakin University’s Burwood campus, which may preclude generalizability to the wider population. Thirdly, data for the main analyses (i.e., children’s outdoor time, screen time, sleep, and parental predictors) were all parent-reported. As such, results may be subject to reporting biases, whereby mothers may have reported their own and their child’s behaviours to be more socially desirable. Fourthly, data were drawn from a randomised controlled trial. It is important to acknowledge that use of data from a trial may have affected the children’s behaviours, and results should be interpreted with this in mind. However, there was no difference in sleep duration between the intervention and control groups, no intervention effect was observed for physical activity, the intervention effect for screen time was attenuated at follow-up, and intervention allocation was included as a covariate in regression analyses. Finally, data were collected in 2008–2013; newer screen technologies such as smartphones and tablet computers were not as ubiquitous then and hence time spent using these devices was not measured. As such, screen time may have been underestimated, particularly at the later time points, which is especially concerning for the group with the highest screen time. Despite these limitations, this study has several strengths. This is the first study to examine concurrent trajectories of outdoor time, screen time and sleep across the early childhood period. We had longitudinal data on outdoor time, screen time and sleep at five time points across the early childhood period in a relatively large sample. In addition, the use of the rigorous analytical technique, GBTM, to examine concurrent trajectories of outdoor time, screen time and sleep in this population is highly novel. A particular strength of this technique is its ability to allow inclusion of participants without complete data, which often hinders analyses in longitudinal samples with multiple time points.