The aims of the present study were to a) describe physical activity on school and free days using long-term daily objective measurements in a sample of adolescents, and b) investigate the relationship between objectively-measured (i.e., actigraphy) daytime physical activity and sleep. Overall, our results demonstrate that physical activity declined during adolescence, that boys were more active than girls, and both boys and girls were more active on school as compared to free days. When looking at the association between actigraphy-assessed physical activity and sleep, we find that adolescents with more activity on school days, as indexed by mean steps per day, had slightly later bed and rise times as well as somewhat lower sleep efficiency.
As hypothesized, and in line with previous studies [43, 55, 56] we find that adolescents were less active on free days compared to school days. It is plausible that adolescents are more active during school days due to greater opportunities to participate in physical education classes and extra-curricular organized physical activity. This may be due to formalized physical activity policies at schools, which have been shown to have an influence on children’s activity levels . A further explanation for the difference between school and free days may be that on school days adolescents have to travel from home to school and have movements between class to class, whereas on free days they are more prone to sedentary behaviors, in particular, screen-related sedentary behavior (i.e., computer use and television viewing) on free days [55, 56]. Thus, it is not only vital to implement physical activity policies in schools, but it is also essential to promote physical activity on free days.
Furthermore, as expected, we found that male adolescents were more physically active than their female counterparts, a finding that is consistent with previous studies examining physical activity of boys and girls with actigraphy (e.g., ). Previous research points to several possible explanations as to why girls are less physically active than boys. For instance, although schools provide opportunities to be physically active during school breaks and physical education class, they might be more readily accessible or desirable to boys. Indeed, in a survey study girls often reported that sports in high school are competitive, which is more likely to be enjoyed by boys .
Finally, as hypothesized and in agreement with previous studies conducted in several other countries , we found a significant decline in physical activity with increasing age. As this pattern is consistent across settings, it suggests that the decline may be normative during adolescence due to competing interests and additional academic pressure, which may reduce the time available for physical activity . Taken together, these findings demonstrate how social and cultural factors can play a crucial role in the distribution of physical activity. Hence, the consistency of findings across studies highlight the need to consider the unique activity patterns of adolescents and factors influencing physical activity when developing physical activity intervention programs for this population.
Our second hypothesis that higher physical activity would be associated with better sleep quantity and quality was not confirmed. Our findings were inconsistent with some previous epidemiological, experimental and observational studies reporting a beneficial effect of physical activity on sleep [25, 30, 31]. Participants with more steps showed slightly lower sleep efficiency. This was an unexpected finding but in line with a cohort study examining the relationship between actigraphy-assessed physical activity and sleep in 275 Finnish pre-adolescents . Their findings indicated that higher physical activity during the day was associated with shorter sleep duration, lower sleep efficiency and higher fragmentation of sleep the following night. Another explanation for the negative association between physical and sleep might be found in a trait-like inter-individual variability in activity level. For example, the inherent activity level of a child, might manifest itself in a higher activity level during both the day and the night . A meta-analysis containing of 16 studies comparing sleep in children with attention-deficit/hyperactivity disorder (ADHD) versus controls, showed that children with ADHD had more bedtime resistance, sleep onset difficulties and night awakenings . Therefore, it is suggested that the “trait” activity level may not only explain physical activity level during the day but may also define the level of activity during the night .
Our within subject finding of more physical activity being associated with later bedtimes may be explained within the context of the current sleep hygiene recommendations that advise against exercise in the evening because of the negative impact on sleep . In a meta-analysis higher exercise intensity was associated with prolonged sleep onset latency, lower total sleep time, lower sleep efficiency and more wake after sleep onset, if exercise ended around one hour prior to bedtime . The somewhat detrimental effect of physical activity on sleep may be interpreted through the impact of physical activity on the circadian system. Physical activity can act as a “Zeitgeber” on the circadian system and has been shown to influences the phase of the circadian clock [62, 63]. Future studies should examine not only the association between physical activity and sleep, but also the timing of physical activity.
The findings in this study need to be interpreted in light of some caveats and limitations. For example, we note that while the impact of physical activity on sleep was statistically significant and robust, the effect sizes are small and the benefits of physical activity likely outweigh the detrimental effects of such activity on sleep. Furthermore, even though the actigraphy is an objective measure, there are still some limitations to it. First, the actigraphy used reported the overall physical activity and was not able to distinguish between different intensity levels. This may be key in explaining the mixed findings. Preliminary evidence suggest that especially vigorous physical activity tends to be a better predictor of favorable sleep patterns [30, 64] in comparison to light and moderate physical activity [32, 34]. Second, the time of day in which physical activity occurred was not captured in the study, . Third, the study does not allow any conclusion as to the causal direction of the observed pattern of associations. Fourth, any conclusions have to be made cautiously because the findings are based on a relatively small sample, of which we do not know whether it is representative for all adolescents of this age range since it is based on a data set derived from a twin study. Lastly, the actigraph does not capture physical activity associated with activities in which the wrist is stable – e.g., a participant riding a bicycle, and participants were instructed to take off the actigraph while swimming, which means that this form of physical activity could not be taken into account.
Despite these limitations, the major strength of this study was the long-term examination of the association between physical activity and sleep using different aspects of sleep (actigraphy, and subjective sleep quality) to obtain a comprehensive view of the topic. To the best of our knowledge, this was the first study utilizing all these methods at the same time in a single sample of adolescents. Furthermore, the objective assessment of sleep and activity over several months with the same device in the home environment enhances the reliability of our findings and allowed us to examine potential correlations between physical activity and sleep for both school and free days.