8 Facts About Circadian and Circannual Rhythms in Horses

Nearly all organisms, including horses, adapt their lifestyles to the timing of sunrise and sunset. These light and dark cycles result in daily (circadian) and annual (circannual) rhythms of physiology and behavior. And, as a researcher in Ireland recently reported, understanding how horses respond to these cycles can help you manage them accordingly.

Barbara Murphy, PhD, assistant professor in University College Dublin’s School of Agriculture and Food Science, in Ireland, has extensively studied circadian and circannual rhythms and how they affect horses. As one of the keynote speakers at the 2019 Equine Science Society Symposium, held June 3-6 in Asheville, North Carolina, she described them in-depth.

Here are eight important take-homes from her lecture:

  1. Daily external signals, such as the light-dark cycle, meal feeding times, and exercise patterns, can synchronize horses’ circadian rhythms. Horses’ circannual rhythms get synchronized by not only light and dark but also climate changes (especially temperature) and available nutrition throughout the year.
  2. How does light affect horses’ bodies? When it enters the eye, the retina sends a signal to the brain, which sends a time-of-day message throughout the body. Not all types of light, however, are created equal, said Murphy. Blue light, which is present in high amounts in natural daylight, best stimulates the photoreceptors in the eye that regulate circadian rhythms.
  3. Circannual rhythms dictate mares’ reproductive cycles. The natural breeding season runs from April to October to coincide with longer periods of daylight. Breeders have been using artificial light in the form of stable lighting and floodlights in paddocks to effectively manipulate the mare’s reproductive cycle for decades. New research shows that light masks—which Murphy invented and produces with her company, Equilume—that deliver low-intensity blue light to one eye are equally effective. As a bonus, she said, these masks allow mares remain outside on pasture.
  4. Changing day length can affect stallions’ libido, hormone concentrations, and testosterone levels, with each peaking during the long days of summer. Murphy said research shows that blue-enriched LED light by day can help improve stallions’ testosterone levels throughout the breeding season.
  5. There’s also a correlation between circadian rhythms and equine performance. Research has shown that if horses follow a daily exercise regimen, their muscle genes will learn to peak at the time associated with that exercise, she said. By training a horse at the same time each day, you’re essentially training his muscles to perform at their best at that time.
  6. Horses—particularly equine athletes that fly to competitions around the world—can suffer jet lag just like humans do. While they’re immediately on a new light schedule, their body rhythms, such as core body temperature, do not align to the new time zone for up to 11 days, said Murphy.
  7. By stabling horses under fluorescent lights for much of the day (which offer very little if any blue wavelength light), we’re failing to stimulate normal circadian rhythms, she said. We need to mimic natural daylight in stables if we want to maintain good hair coats, musculature, immunity, etc. (remember, light affects all body systems). For stalled performance horses to be at their best, Murphy recommends putting them under blue-enriched light during the day and keeping them in darkness or under red light at night, with transition periods at dawn and dusk.
  8. We disrupt horses’ internal rhythms when we interact with them at night—to toss hay, do night check, medicate, or change bandages, for instance. As soon as you flip that light switch, said Murphy, you’ve disturbed their circadian clocks, which can affect their sleep, cortisol (stress) levels, and even performance. She recommends using red lights rather than fluorescent or incandescent when doing nighttime barn chores. Eliminating white light pollution at night is key to allowing the body’s circadian rhythms to remain strong, she said.

Circadian and circannual rhythms can affect nearly every physiologic, behavioral, and metabolic response in horses’ bodies. We must be cognizant of how our horse management practices might be inhibiting everything from their hormone levels to their immune function. Some relatively simple lighting changes can encourage horses to be alert during the day, rest at night, and maintain healthy body functions, said Murphy.