Seasonal and daily rhythms in healthy adults and individuals with obesity: associations with selected hormones and lifestyle factors
Biological timing systems, including circadian and seasonal rhythms, play a key role in regulating human physiology. The central circadian “body clock,” located in the suprachiasmatic nucleus, coordinates daily rhythms both centrally and via peripheral clocks in metabolic tissues. Seasonal rhythms, in turn, are shaped by environmental cues such as photoperiod and temperature and may influence mood, appetite, sleep, physical activity, hormonal secretion, and thermoregulation. Although seasonal responses are generally weaker in humans than in many other animals, evidence suggests that some physiological processes remain seasonally regulated. From a physiological anthropology perspective, these rhythms can be viewed as adaptive mechanisms that align human physiology and behavior with predictable environmental changes. These timing systems are increasingly recognized as important regulators of metabolic homeostasis. Disruption of circadian and seasonal rhythms may contribute to insulin resistance, altered energy expenditure, fat accumulation, and obesity-related dysfunction. Human studies suggest that biological timing is reflected in chronotype-related behaviors, endocrine responses, seasonal hormonal variation, and thermoregulatory adaptation. Obesity is a complex, multifactorial disease that cannot be explained solely by excessive energy intake and low physical activity. It develops through interactions between external factors, such as an unhealthy diet and a sedentary lifestyle, and internal factors, including genetic, metabolic, endocrine, and behavioral mechanisms. Therefore, obesity should be understood not only as a disorder of energy balance but also as a condition influenced by disrupted biological timing. This article builds on our 2015 review (Kanikowska et al., Int J Biometeorol 59:377-84, 2015) by synthesizing recent literature and emerging perspectives on the roles of circadian and seasonal rhythms in obesity.