Year: 2025
Type: article
Abstract: In this issue of Sleep, Kuldas et al. [1] published results from an impressive Norwegian cohort study that followed 880 Norwegian youth over seven waves from childhood through young adulthood (ages 6–18). The strong study design included longitudinal assessment of accelerometry data (for sleep, physical activity, and sedentary activity) and clinical interviews for insomnia, addressing methodological concerns from prior between-person studies [2, 3]. Using rigorous cross-lagged panel models, they observed no long-term statistically significant within-person associations between changes in physical activity and changes in sleep outcomes, or vice versa. This study reminds us that science is an iterative process and that prior findings can always be re-examined using rigorous (or more rigorous) methodologies [4]. Furthermore, it underscores the necessity for context, nuance, and continued investigation in behavioral science. These null findings might have been disappointing to the authors. We confess that they were disappointing to us. Wouldn’t it have been nice to show that changes in sleep would have longer term positive associations with physical activity? Or perhaps that getting more exercise will improve sleep outcomes? Rather than bemoaning a lack of a longitudinal association, we should ask what the implications of these results are for future research and for clinical practice. Do they change recommendations for daily behaviors? No, not at all, improving sleep health and increasing physical activity across the 24-h day both remain public health priorities, independent of their relationship to each other [5–7]. Furthermore, given that the results are inconsistent with our current assumptions about movement and sleep, we should consider what else might explain the findings before accepting them. First, as the authors themselves note, there’s a question about the temporal distance between the behavior and the outcome. Perhaps the expectation of finding changes in sleep based on differences in physical activity 2 years prior is unreasonable. A prior micro-longitudinal study from the US-based Future of Families Sleep Sub-study (FFSS) found within-person daily benefits of more daily physical activity on that night’s actigraphy-assessed sleep (longer duration, more efficiency) at age 15, although the results in the reverse direction were inverse (longer sleep duration predicts less physical activity the next day) [8]. It is conceivable that physical activity patterns changed over the course of the observation period enough to account for the null finding. Second, the generalizability and hence the external validity of the study sample deserve closer attention. Norway consistently ranks among the healthiest countries in the world [9]. If most youth are already meeting or exceeding recommended levels of physical activity and sleep, it becomes harder to detect meaningful within-person associations. That is, there’s less room for improvement. The absence of a relationship in this study may therefore reflect a ceiling effect rather than an inherent lack of causality. The Kuldas et al. [1] sample had average 16-year olds sleeping 8.8 h per day measured using actigraphy, in contrast to the US-based FFSS described above, 15.5-year olds slept an average of 7.8 h per day using actigraphy, a full hour less per day [8]. Furthermore, cultural factors, school policies, and built environments all contribute to youth health behaviors [10–12]. In more obesogenic or sleep deficient environments, these same behaviors may have more pronounced consequences. Future research on this topic should prioritize broader and more diverse populations, especially ones where both physical inactivity and sleep deficiency are more prevalent. American youth, for example, may display a different pattern of associations, particularly among marginalized communities. Finally, while the study objectively assessed physical activity and sleep, its measure of sedentary time is non-specific regarding the use of smartphones and digital media. An average American teen spends around 5 h on their smartphone during the day, and that has to be displacing something [13]. While Kuldas et al. [1] assess sedentary time, the granularity of this data is not available in this study. Context (e.g. location and timing) and content (e.g. interactivity, features, and images) matter [14–16]. If youth are on their devices, understanding what types of digital media are being used (e.g. social media, video gaming), when (e.g. at night or during the school day), and where (e.g. in the bedroom or at school), will enhance our understanding beyond just knowing “how long” they are on them. Accelerometry alone as a measurement of sedentary activity does not differentiate between smartphone use or book reading. Future research should rigorously test the hypothesis that specific types of digital media use (e.g. social media, interactive features) displaces both sleep and physical activity outcomes, not just on a daily level, but over the long term [17]. Ultimately, this study provides a valuable contribution. The absence of a within-person relationship between physical activity and sleep in this cohort of Norwegian youth invites us to embrace a more complex understanding of youth health. We must celebrate well-conducted research that challenges our assumptions, even if it does not confirm our hopes. These findings may direct us to more precise, context-sensitive, and inclusive science. Financial disclosure: LH has received consulting/speaking fees from Prosek, Hirezon, Idorsia Pharmaceuticals, and various non-profits and academic institutions. DAC is Editor-in-Chief of JAMA Pediatrics, Chief Science Officer of Children and Screens: Institute of Digital Media and Child Development, and a consultant to Kiwi Crate. DAC and LH both serve as expert witnesses in ongoing social media litigation. DAC and LH also received an honorarium from the National Sleep Foundation for participation in a consensus panel. Non-financial disclosure: LH serves on the scientific advisory board for Children and Screens: Institute of Digital Media and Child Development, the Good Night Advisory Council of Beyond Bedtime, and the Population Health and Methodology Committee of the National Sleep Foundation.
Source: SLEEP
Authors: Lauren Hale, Dimitri A Christakis
Language: English
Topic: Sleep and related disorders
Subfield: Experimental and Cognitive Psychology
Field: Psychology
Domain: Social Sciences
Open Access status: hybrid
Estimated APC paid: $4,388
OpenAlex ID: W4414436352