Abstract: Many animals undergo changes in their physiology across seasons in response to day length for survival. This seasonal timing system is conserved across many different species, including humans, and is characterized by large interindividual differences. Strong biological responses (e.g. in melatonin duration) to day length have been linked to seasonal affective disorder, which affects about 5 percent of adults in the U.S. Mathematical models provide a powerful tool to understand the underlying mechanisms driving these responses and their variability across individuals. In this study, we analyze circadian rhythms in medical interns across the U.S. using data from wearable devices. Our findings reveal that individuals with stronger seasonal responses experience greater circadian misalignment due to environmental changes, such as shift work. Using physiologically-based mathematical models, we propose a new perspective on human seasonality, emphasizing the challenges posed by modern-day environmental factors. Furthermore, we investigate the dynamical behaviors in the models contributing to the complexity of circadian rhythms and seasonality.

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