1 Introduction
There has been a long-standing debate in the chronobiology community regarding the relationship between latitude and human circadian phenotypes (chronotypes) (e.g., Bohlen & Simpson (1973); Randler (2008); Leocadio-Miguel et al. (2017); Wang et al. (2023)), with many assuming that this association is well-established. The hypothesis is based on the varying amounts of solar radiation experienced by populations across different latitudes. Since light exposure serves as a primary zeitgeber — a periodic environmental cue that influences or regulates biological rhythms (Aschoff, 1960; Pittendrigh, 1960) — such variations, along with temperature differences, are thought to result in observable differences in chronotype distributions globally. This thesis investigates the so-called latitude or environmental hypothesis in human circadian phenotypes, addressing the question: Is latitude associated with chronotype?
The central hypothesis is that latitude is associated with human chronotype distributions, with populations closer to the equator exhibiting, on average, a shorter or more morning-oriented circadian phenotype compared to those living near the poles (Bohlen & Simpson, 1973; Horzum et al., 2015; Leocadio-Miguel et al., 2014, 2017; Randler, 2008). The primary objective of this study is to model and test this hypothesis by critically examining whether a significant association exists between latitude and circadian phenotypes in the Brazilian population.
This study emerged from an insightful debate with my former supervisor, sparked by results published in 2017 in the journal Scientific Reports (Leocadio-Miguel et al., 2017). In this paper, the authors conclude that there is a significant association between latitude and chronotype in the Brazilian population, consistent with theoretical predictions. However, the results were not as clear-cut as presented, and the methodology used to test the hypothesis was not optimal. This thesis revisits the hypothesis using an improved statistical approach, aiming to provide a more accurate and reliable answer to the research question.
In the following chapters, the latitude hypothesis is tested using Popper’s hypothetical-deductive method (Popper, 1979) and an enhanced approach to Null Hypothesis Significance Testing (NHST), rooted in the original Neyman-Pearson framework for data testing (Neyman & Pearson, 1928a, 1928b; Perezgonzalez, 2015). This involves a series of analyses conducted on a large dataset of \(65,824\) individuals, collected from the Brazilian population in 2017. The dataset is based on the Munich Chronotype Questionnaire (MCTQ) (Roenneberg et al., 2003, 2012), and includes data on sleep habits and geeographical and demographic characteristics from all of Brazil’s states.
It is important to emphasize that this thesis does not aim to propose or discuss the mechanisms underlying the latitude-chronotype relationship. Instead, it focuses solely on the statistical association between them concerning only human populations. An association is a necessary precursor to any causal relationship — and this thesis aims to determine whether such an association exists.
The analyses utilized nested multiple regression models to assess the variance explained by latitude in predicting chronotype. This method of procedure builds on the method used in Leocadio-Miguel et al. (2017). The results will contribute to the ongoing debate on the latitude-chronotype relationship, offering new evidence on the influence of environmental factors on human circadian rhythms.
In accordance with the graduate program regulation, this thesis follows an article-based format, inspired by the structure of Reis (2020)’s PhD thesis. Chapters 2, 3, and 4 consist of essays and literature reviews related to the thesis topic that provide essential background for understanding the research. Chapter 5 presents the core investigation, including an article detailing the hypothesis test and addressing the research question. Finally, Chapter 6 offers conclusions, discusses limitations, and proposes directions for future research. Additionally, supplementary materials are provided to offer a richer, more comprehensive understanding of the research. The reader is encouraged to explore them in detail.
All analyses in this thesis are fully reproducible and were conducted using the R programming language (R Core Team, n.d.) alongside the Quarto publishing system (Allaire et al., n.d.).