Additive framework of hormonal waves accounts for species and age differences in circadian intraocular pressure rhythm

Nocturnal peak in cardiac baroreflex effectiveness aligns with nocturnal trough in systolic blood pressure in young healthy adults: Possible circadian effects.

We tested whether there is a circadian variation in baroreflex function measured as sensitivity and effectiveness, and whether it aligned with circadian variations in systolic blood pressure (BP). Eleven healthy adults (24 ± 2 years; 4 males and 7 females) completed a 30-h circadian protocol incorporating five recurring, identical, 6-h cycles of 4-h standardized wakefulness and 2-h sleep opportunities. Spontaneous beat-to-beat BP (finger photoplethysmography) and R-R intervals (electrocardiogram) were recorded at rest for 10 min during each wakefulness period, to assess cardiac baroreflex sensitivity (BRS) and effectiveness index (BEI). Vascular endothelial function (VEF) was assessed as flow-mediated dilation of the brachial artery. Dim light melatonin onset was used as a circadian phase marker. Systolic BP exhibited a significant trough at ~1:40 am (p = 0.02). This trough closely aligned with the nocturnal peak of cardiac BEI that occurred at ~1 am (p = 0.02). BRS showed no significant variation. VEF exhibited a significant peak at ~10 am In healthy adults, the nocturnal peak in BEI is possibly aligned with the circadian trough in systolic BP. It remains to be determined whether shifting one would shift the other in a similar direction, and whether the alignment holds in people with disease.

The circadian system is affected by Alzheimer’s disease independently from amyloid beta deposits

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.

The dawn phenomenon in diabetes: pathophysiology from periphery to central nervous system and circadian rhythm-based therapeutic strategies.

LightLight and diurnal cycle affect autonomic cardiac balance in human

Circadian Biology and Phase Response: Fundamental Mechanisms and Clinical Applications

The circadian clock, located in the mammalian hypothalamus, regulates biological rhythms with a period of approximately 24 h, influencing nearly all body functions. Its timing is synchronised daily by external cues, primarily light, which align internal rhythms with the environmental cycle. Through this entrainment, the circadian system orchestrates physiological processes such as the sleep–wake cycle, feeding behaviour, gene expression and body temperature regulation. Melatonin, secreted by the pineal gland, also plays a key role as a synchroniser by facilitating sleep onset. Changes in environmental time cues, such as those experienced by shift workers, can disrupt the body’s natural 24-h rhythms. This situation can lead to fatigue, significantly impacting accident rates and productivity, and, in the long term, to an increased risk of various health conditions. A Phase Response Curve (PRC) illustrates how the clock’s phase is affected by stimuli administered at different points in the circadian cycle. In particular, both light and melatonin can induce phase shifts, but the direction and magnitude of this shift depend on the timing of administration. Understanding the PRC enables the design of interventions to realign circadian rhythms and improve adaptation to shift work. This review explores the physiological and clinical effects of circadian disruption in shift workers and discusses strategies to mitigate its impact.