Night and Day: A Study on Triangular Associations between Light, the Circadian Rhythm, and Wellbeing

This dissertation explores the triangular relationship between light exposure, the circadian rhythm, and wellbeing, with an emphasis on human studies using both social and genetic perspectives. While previous research has focused mainly on the adverse mental health outcomes of circadian disruption—such as depression—this work shifts the focus toward understanding how light and circadian rhythms relate to broader aspects of wellbeing and happiness. Part I: Defining Wellbeing Chapter 2 examines the concept of wellbeing through a network analysis of data from the Netherlands Twin Register. The analysis revealed two interconnected clusters—positive traits (e.g., life satisfaction, happiness) and negative traits (e.g., depression, neuroticism). Four central items emerged as key indicators of general wellbeing, suggesting that these variables could be prioritized in future studies with limited resources. The findings underscore the complex interconnections between wellbeing phenotypes, rejecting the idea of strictly separated wellbeing domains. Part II: Light and Wellbeing Chapter 3 presents a systematic review and meta-analysis of studies examining the impact of light on wellbeing. Out of 24,567 identified articles, 74 met the inclusion criteria, and 30 were used in the meta-analysis. The results show a small-to-moderate positive effect of light on wellbeing (effect size = 0.46), which remained robust after sensitivity analyses. However, the review also highlights issues with study design, such as poor reporting and lack of statistical corrections, leading to recommendations for improving future light-based research. Part III: Circadian Rhythm—Social and Genetic Influences Chapters 4 explored genetic influences on circadian rhythm proxies—Morningness and Relative Amplitude—using data from over 20,000 participants. Polygenic scores (PGSs) for these traits were modestly associated with depressive symptoms but not with wellbeing outcomes. Within-family analyses suggested no strong gene-environment correlations. These results imply that genetic predispositions related to circadian rhythms are more relevant to mental health (particularly depression) than to broader wellbeing metrics. Chapter 5 focused on wake-up time and chronotype changes before and after retirement. While employment influenced wake-up times, it did not affect chronotype, which appeared to be genetically stable. The polygenic score for Morningness predicted chronotype equally in both employed and retired participants, indicating that societal schedules influence behavior but not underlying biological rhythms. Chapter 6 investigated "social jetlag"—the discrepancy in sleep schedules between workdays and free days—and its impact on wellbeing. Although statistically significant, the association was minimal and did not vary with age or day of the week, suggesting that social jetlag is not a primary concern for wellbeing at the population level. Part IV: The ENLIGHTENme Project Chapters 7 and 8 describe a European intervention study targeting elderly participants, assessing how indoor lighting affects sleep, activity, and wellbeing. Despite a robust and ecologically valid design, baseline analyses showed limited associations between light exposure and wellbeing or health. The findings highlight the complex, possibly indirect nature of these relationships and the need for better-designed randomized controlled trials. The thesis concludes with recruitment recommendations for elderly and multicultural populations based on practical insights from the ENLIGHTENme project.