INTRODUCTION

Circadian rhythm refers to a daily cycle that regulates physiological processes in most living organisms over a 24-hour period [1, 2]. This rhythm is controlled by the suprachiasmatic nucleus, located in the hypothalamus, which responds to both internal and external factors, particularly light intensity, to maintain physiological homeostasis [3–5]. Among cardiovascular parameters, heart rate variability (HRV) is a well-established non-invasive marker of autonomic nervous system activity, reflecting the dynamic balance between sympathetic and parasympathetic modulation [3, 6]. HRV exhibits circadian variation, providing insight into autonomic adaptability under physiological and environmental influences [7–9].

In horses, circadian fluctuations of heart rate (HR) and HRV have been reported primarily under temperate conditions characterized by pronounced day–night temperature gradients and controlled housing systems [10–13]. However, information regarding horses maintained under tropical environments remains limited. Tropical conditions present distinct challenges, including persistently high ambient temperature, high humidity, limited nocturnal cooling, and environmental stressors such as air pollution, particularly in urban settings [14, 15]. In addition, biological disturbances, including insect activity during nighttime, may interfere with rest and recovery, potentially affecting autonomic regulation [16]. These factors may alter circadian autonomic patterns and attenuate the nocturnal parasympathetic predominance typically observed in temperate climates [10, 17]. Aging is also recognized as an important factor influencing HRV [10, 18], with advancing age generally associated with reduced autonomic flexibility and diminished parasympathetic activity.

Although previous studies have characterized circadian changes in HRV and age-associated autonomic alterations in horses, most investigations have been conducted under temperate climatic conditions and have focused on either circadian variation or aging independently [10–13, 17, 18]. Consequently, the combined influence of tropical environmental conditions, age, and circadian rhythm on equine autonomic regulation remains insufficiently understood. Furthermore, previous studies have generally relied on controlled experimental settings and have rarely incorporated continuous monitoring of environmental variables under real-world field conditions. In particular, the potential impact of urban air pollution on resting autonomic function in horses has received little attention, despite growing concerns about environmental quality and equine welfare. Therefore, important knowledge gaps remain regarding how tropical environmental stressors interact with age-related changes and circadian regulation to influence cardiac autonomic activity in horses.

Therefore, this study aimed to comprehensively investigate the circadian characteristics of HR and HRV in horses maintained under tropical environmental conditions. Specifically, the study sought to characterize 24-hour circadian patterns of HR and HRV, compare autonomic regulation between daytime and nighttime, evaluate age-associated alterations in cardiac autonomic function, and examine relationships between environmental factors and HRV parameters. We hypothesized that tropical environmental conditions would attenuate nocturnal parasympathetic predominance, aging would reduce overall HRV, and environmental stressors, particularly heat and air pollution, would shift autonomic balance toward sympathetic predominance.

To the best of our knowledge, this is the first study to simultaneously investigate 24-hour circadian HRV, age-related autonomic alterations, and real-time associations with tropical environmental factors, including air pollution parameters, in a standardized field setting. The findings provide novel insights into how urban tropical environments influence autonomic aging and equine welfare, factors that have not previously been quantified in equine physiology.