Review Article
Circadian and Circannual Regulation in the Horse: Internal Timing in an Elite Athlete

https://doi.org/10.1016/j.jevs.2019.02.026Get rights and content

Highlights

  • Horses exhibit circadian and circannual rhythms in physiology and behavior

  • Exercise is a potent entrainment factor for circadian rhythms in equine muscle

  • Circannual rhythms influence ovarian activity, gestation length, and foal birth weight

  • Short-wavelength blue light effectively regulates equine biological rhythms

  • Future research should evaluate stable lighting that supports biological rhythms

Abstract

Biological rhythms evolved to provide temporal coordination across all tissues and organs and allow synchronization of physiology with predictable environmental cycles. Most important of these are circadian and circannual rhythms, primarily regulated via photoperiod signals from the retina. Understanding the nature of physiological rhythms in horses is crucially important for equine management. Predominantly, they have been removed from exposure to their natural environmental stimuli; the seasonally changing photoperiod, continuous foraging and feeding activity, social herd interactions, and the continuous low-intensity exercise of a grassland dweller. These have been replaced in many cases with confined indoor housing, regimental feeding and exercise times, social isolation, and exposure to lighting that is often erratic and does not come close to mimicking the spectral composition of sunlight. Man has further altered seasonal timing cues through the use of artificial lighting programs that impact reproductive behavior, breeding efficiency, and the development of youngstock. Understanding how these new environmental cues (some stronger and some weaker) impact the internal physiology of the horse in the context of the natural endogenous rhythms that evolved over millennia is key to helping to improve equine health, welfare, and performance, now and into the future. This review provides an overview of the field, highlights the recent discoveries related to biological timing in horses, and discusses the implications that these findings may have for the production and management of the elite equine athlete.

Section snippets

Introduction to Chronobiology

Every life form that evolved on our rotating planet was subject to the evolutionary pressures provided by rhythmically fluctuating light–dark (LD) cycles. The daily and seasonal changes in photoperiod represented the dominant environmental cues that gave rise to the development of cellular clock mechanisms, allowing the simplest to the most advanced organisms to tell physiological time. The evolution of biological timekeeping mechanisms meant survival to those organisms most capable of

The Circadian System

The circadian system consists of a network of clocks that drive self-sustained circadian rhythms in mammalian rest-activity cycles [20], hormone secretion [21], cardiovascular activity [22], metabolism [23], immune function [24], alertness [25], and musculoskeletal performance [26]. Suffice to say that few physiological processes are outside the control of the circadian clock. By definition, a circadian rhythm is endogenously generated, has a period length close to 24 hours that persists under

The Circannual Clock

Endogenous circannual rhythms allow anticipation of seasons and drive long-term physiological cycles in long-lived mammals including reproduction, pelage growth, metabolism, and migration [107], [108]. Photoreception by the retina records the annual cycle in day length and acts as the primary synchronizer of these rhythms. Unlike temperature, availability of food, rainfall, or other environmental cues, photoperiod provides seasonal information that remains constant from year to year. The

Lighting for Circadian and Circannual Entrainment

Within the equine industry, there is a general understanding of the application of extended daily lighting for manipulation of seasonal reproduction and coat growth. However, what is lacking is an appreciation for the impact of daily lighting regimes on maintenance of a horse's circadian rhythms. All light is not equal, especially when it comes to the capacity to stimulate and entrain the circadian system. The natural daylight spectral composition encompasses the visible, ultraviolet, and

Conclusions

Circadian rhythmicity is a feature of nearly every physiological, metabolic, and behavioral system. As its primary synchronizer, light reception by the retina directly and indirectly regulates a wide array of biological processes. The concept of “circadian lighting”—lighting that promotes alertness by day and rest and recovery by night—is gaining increasing popularity within human society. The health benefits of regularly timed changes in spectral intensity and composition that mimic the

Acknowledgment

The author would like to acknowledge the late Barry P. Fitzgerald, whose work and interest in equine seasonality and melatonin regulation, sparked my interest for the field of biological rhythms and ultimately led to much of the described research on equine circadian regulation.

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    Animal welfare/ethical statement: Where relevant, all studies were conducted in accordance with the “Code of Good Practice in Research” (University College Dublin, Ireland) and “Directive 2010/63/EU of the European Parliament and the Council on the Protection of Animals used for Scientific Purposes” and had approval of the institutional Animal Research Ethics Committee. Additional studies referred to in the review article have ethical statements that appear in the respective publications.

    Conflict of interest statement: B.A.M. is a member of the Board of Directors of Equilume Ltd. Equilume Ltd. has been granted a license for commercial use of information that is contained in the following granted patents: Practices US Pat. 9,839,791; UK Pat. GB2504244B, and GB2549682B. B.A.M. is named on these patents.

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