Light regularity: illuminating opportunities to enhance adolescent health

The importance of regular sleep patterns to human health has long been appreciated by our field. However, with recent advances in computational strategies for estimating sleep regularity [1, 2], the last 5 years have observed a proliferation of research into the specific health outcomes associated with regular sleep. Sleep regularity, defined as day-to-day consistency in sleep–wake patterns [1, 2], has been linked to cardiometabolic health, psychological well-being, substance use, neurodevelopment, and daily functioning in school and relationships [1, 3–9]. Although this work has spanned the life course, there has been a particular emphasis on adolescence, during which the “perfect storm” of biological and environmental influences increases risk of irregular sleep patterns [10–12]. Due to rapid cognitive development during this window, adolescence is also a critical period in which targeted prevention efforts, such as those focused on instilling healthy sleep habits, may have a long-lasting impact on health [13].

With increased interest in relationships of sleep regularity to multiple facets of adolescent well-being, there has been a pressing need to better understand the mechanisms underlying these associations. One proposed pathway is through light exposure, the primary zeitgeber entraining the circadian rhythm [14]. Modern adolescents live in a world awash with light. Long after sunset, adolescents are exposed to artificial—and frequently bright—light from multiple sources, from artificial lighting in their home to the cell phone in their hand [15]. In addition to increased exposure, early- to mid-puberty may be associated with greater sensitivity to light [16]. Thus, it is hypothesized that those with irregular sleep patterns have greater variability in light exposure timing, which in turn results in adverse health outcomes by disrupting the circadian system [1, 2]. As intuitive as this hypothesis may seem, it is largely untested. A rate limiter for research on this topic has been the lack of established metrics for quantifying light exposure variability.

Thus, it was with interest that I read the work of Hand et al. (Sleep. 2023;46(8);zsad001. doi:10.1093/sleep/zsad001) in this month’s issue of SLEEP. In their manuscript, these researchers evaluated a novel light regularity measure, derived from actigraphy data, which assesses day-to-day variation in light timing between consecutive days (Light Regularity Index; LRI) and compared it to two additional light regularity metrics, including variation in (1) mean light timing (MLiT) [17, 18] and (2) the daily photoperiod [19]. Computation of the LRI was inspired by the Sleep Regularity Index (SRI), which measures sleep/wake regularity across consecutive days [1]. They examined relationships between the three light regularity metrics and two measures of sleep regularity (SRI, social jetlag) across school and vacation in adolescents and found that greater light exposure regularity on the LRI was correlated with higher SRI, earlier sleep timing, and longer sleep duration. Notably, neither MLiT nor daily photoperiod variation were associated with sleep regularity measures, and the trio of light regularity metrics were not correlated with each other, suggesting multiple facets of light regularity.

Given the dearth of previously established light regularity measures, the work of Hand et al. will likely be impactful as a first step toward advancing methods for quantifying variability in light exposure. Like any new metric, associations between the LRI and sleep measures require replication and validation in new populations. Researchers will also need to evaluate whether the proposed metric adequately captures the patterns of light exposure most germane to the health condition of interest. For example, the LRI converts a light time series into a binary signal based on a set threshold for light intensity, and then calculates the probability of being in the same state (above or below the threshold) at any two time points 24 hours apart. In selecting a threshold, the LRI may not capture potentially important nuances regarding variability in exposure to the full range of light intensity in the environment. For example, if the threshold is set at 50 lux, two individuals—one exposed to 50 lux on day 1 and 50 lux on day 2 at a specified time point, and the other exposed to 50 lux on day 1 and 500 lux on day 2 at the same time point—would each be characterized as being in the same state of light exposure at that time point.

Thus, whether variability in light intensity is a vital characteristic of light exposure regularity, and whether such nuances are integral for understanding adolescent health outcomes, are open questions. However, we may imagine that regularity of exposure to specific light intensities as well as wavelengths (e.g., blue light) may be instrumental in the pathophysiology and treatment of certain conditions, such as mood disorders [20]. Additionally, it is further notable that the LRI was developed using data from wrist worn devices, which are less reliable than eye-level sensors in determining light exposure [21]. Future research may compare LRI computations from wrist-worn versus eye-level measures to determine optimal methods for indexing light regularity in adolescents.

This work may also serve as a driver for new research questions about associations between light exposure regularity, sleep, and human health. Imperative next steps are to evaluate whether assessing light regularity (perhaps by integrating light sensors into consumer-based wearables [22]) and targeting it directly in treatment may enhance adolescent sleep and overall well-being. Consistent with recent research [23], Hand et al. acknowledge that the association between light regularity and sleep regularity is likely bidirectional, but focus much of their discussion on sleep as the “gate” to light exposure, which suggests that if sleep regularity can be improved, health concerns may be mitigated by reducing light exposure variability and subsequent disruptions to the circadian system. However, there may be reasons for investigating the opposite direction of effect for specific adolescent health conditions; i.e., a pathway in which reducing light irregularity and subsequent circadian disruptions improves sleep patterns and downstream health impacts.

An exemplar of such a population is adolescents with attention-deficit/hyperactivity disorder (ADHD), who are at risk for sleep irregularity specifically [24] as well as general sleep disturbances [25], delayed circadian phase [26, 27], and negative health outcomes including obesity and earlier mortality [28]. Light exposure may play a key role in ADHD pathology [29]. In the United States and globally, rates of ADHD prevalence are lowest in geographic areas with highest solar intensity [30], and inattentive symptoms lessen during the summer, with seasonal light variability as the proposed mechanism [31]. Individuals with ADHD may also be particularly sensitive to light exposure and intensity [32], and bright light therapy may improve both sleep and ADHD severity in adolescents [33] and adults [34] with ADHD, although results are mixed [35]. Importantly, the potential importance of light to ADHD pathology is believed to be mediated by the influence of light intensity exposure on circadian rhythm regulation [36]. Thus, in designing targeted interventions for adolescent ADHD, it may be essential to understand the proverbial “chicken and egg”—should interventions focus on sleep regularity, in which sleep serves as a gate to irregular light exposure? Should treatments instead target regularly-timed exposure to light, which may enhance sleep health by supporting circadian functioning? Perhaps both?

Although an oversimplification, this example highlights the need to better understand the intricacies and directionality of the relationships between sleep regularity and light exposure variability and downstream consequences for adolescent health. Timing of both light exposure and sleep are implicated in mood, metabolism, cancer risk, and immune system functioning across the lifespan [37], and vulnerability to health consequences may emerge, in part, from adverse impacts of circadian disruption on brain development during adolescence [38]. Social disadvantage is also associated with both irregular sleep [39] and greater nighttime light exposure [40] among adolescents. If adolescence is indeed a key period for prevention, greater clarity regarding the direction of these effects could impact the focus of interventions aimed at improving a range of health outcomes and reducing health disparities. In a time in which health care systems are increasingly focused on reducing costs through shorter, high-value treatments [41], clarifying the specific intervention components that have the largest and swiftest effects on health improvements is of vital importance. Although these questions are beyond the scope of the investigation undertaken by Hand and colleagues, future research may leverage metrics such as the LRI to shed light on critical mechanisms and clinical targets for enhancing sleep in adolescents, and indeed, illuminate pathways for enhanced health across the lifespan.

Funding

JRLA receives research support from the National Institute of Mental Health (R34MH128440; R34MH131994) and Duke University School of Medicine through the Children’s Health & Discovery Initiative.

Disclosure Statement

JRLA receives research support from Lumos Labs.