Hold your horses: External factors that influence circadian rhythms in horses

Rita Kochmarjova / stock.adobe.com

Light is the primary regulator of circadian rhythms and it governs physical, mental, and behavioral changes in most living things. A circadian rhythm is a natural, internal process that lasts approximately 24 hours and exists under constant conditions.¹ Barbara Murphy, PhD, BSc, the equine programme director and head of subject for equine science at the University College Dublin, presented how light, exercise, and temperature affect circadian rhythm and fertility in horses during a recent webinar for the University of Connecticut.

To understand the regulation of equine activity rhythms, Murphy cited researchers who recorded 6 mares’ activity patterns for 2 weeks at a time. Horses spent 2 weeks in the pasture, in the stables with light and dark cycles, then in the stables in complete darkness. This study revealed that circadian rhythms with strong ultradian components regulate equine activity patterns.¹ Ultradian rhythms are biological cycles that occur within 24 hours, and they include heart rate, pulse, and blinking. This study also showed that equines contain endogenous circadian rhythms in the absence of environmental stimuli.¹ These findings suggest that humans who implement management regimens that capitalize on these findings can strengthen equine circadian behavioral outputs.¹

Another study Murphy noted tried to identify the time of day when horses would experience optimal exercise performance. Handlers exercised 6 sedentary young thoroughbred mares for 8 weeks using an automated high speed horse exerciser. Researchers then collected muscle biopsies over a 24-hour period. Researchers analyzed equine skeletal muscle and discovered that exercise synchronizes gene expression. This study implies that optimal performance is found at the time of day that coincides with training.¹

Circannual rhythms (processes that occur over approximately 1 year) regulate equine reproduction. Circannual rhythms can be manipulated by external signals such as light, food, and temperature. Extending the daylight allows for a longer breeding season whereas the cold weather and waiting for the grass to grow in the spring slows breeding.¹ Increasing the light stimulates reproductive activity, growth, and lactation.¹ Researchers placed blue light masks on pregnant mares to see the effects on gestation. The study discovered that delivering blue light to a single eye can reduce gestation length, increase foal birth weights, and reduce hair coat length of the post-foaling mare.¹

Current research describes how light, temperature, and exercise influence the circadian and circannual rhythms that horses exhibit. Future research should evaluate what stable lighting is optimal for supporting equine biological and physiological rhythms.

Reference

1. Murphy B. Biological rhythms in horses: Implications for breeding and performance? Webinar; September 24, 2021.

Bean is a 2022 PharmD Candidate at the University of Connecticut.