Chronobiology refers to the study of periodic, internal time-keeping in animals, and is partially based on the physiological responses to light. Light and dark cycles over a 24-hour period affect the inner clock, and the physiologic responses to these photoperiods can signal the animal to eat when food is available, to rest, or to avoid predation, and thus affect behaviour. Light periods, as day lengths change throughout the year, can affect other physiologic responses, particularly breeding.

A horse in the wild will have daylight signals much different from today’s highly managed horse. Dr. Barbara Murphy of the University College Dublin has conducted much of the work in the area determining how this affects our domesticated horse’s health and performance. She presented a summary of her work, Chronobiology and the Horse: Internal Timing in an Elite Athlete, at the Equine Science Society’s meeting in Asheville, NC, this past June.

Equine Inner Clocks

The key cells that regulate this internal clock are found in the light-sensitive region of the hypothalamus that is known as the suprachiasmatic nucleus (SCN). Light entering the eye hits photoreceptors at the retina, and these are triggered to send a signal via the retino-hypothalamic tract, whereby the SCN transmits this information by both neural and hormonal paths to affect gene expression and tissue function. Of particular interest is the role of melatonin, a hormone that is produced by the pineal gland with the onset of darkness and which can influence the “clocks” of peripheral tissues.

The internal rhythms include:

Circadian rhythms are those that repeat every 24 hours and would occur regardless of day or night impacts. They appear to control activities such as sleep, wakefulness, and digestive activity. However, “entrainment” (modifying) factors can influence circadian cycle timing and include light/dark periods associated with the rotation of the Earth, but also environmental aspects such as feeding and exercise patterns. (It should be noted that circadian rhythms are different from diurnal rhythms, which result from external time cues, for example, the diurnal rhythm of insulin which is released in response to rises in glucose following feeding.)

Circannual rhythms are those that last about 365 days and can be influenced by the Earth’s movement around the sun.

Infradian rhythms have cycles longer than 24 hours, but less than a year, and an example would be the horse’s estrous cycle (which lasts about 21 days).

Ultradian rhythms have a duration of less than 24 hours.

These internal rhythms can affect the health and performance of horses and understanding the cycles can potentially improve our management. For example, testosterone peaks at about 8 a.m. and is lowest in the evening, between 4 p.m. and 12 a.m. Some research has suggested that horses are more likely to ovulate at night, while most horse owners acknowledge that more foals are born between 7 p.m. and 7 a.m. Horses are also “day active” even when experimentally placed in darkness, and may also show some shorter ultradian three-hour cycles when at pasture in a group. In fact, research suggests that there might be an optimum time for giving vaccines or other drugs to heighten the immune response or response to the drug, which is called chronotherapy.

Daily rhythms also affect performance; circadian “unmasking” experiments involving tricking the body with dark, light, sleep, and eat cycles on human athletes showed peak swim times at 11 p.m., and slowest times between 2 a.m. and 8 a.m., while other studies have shown that exercise performance in humans is at its highest in late afternoon and evening.

Timing is Everything for Horses

Research in horses has shown that when young racehorses are subjected to low-to-moderate exercise training on a regular schedule, it results in an upregulation of genes associated with fuel availability to the muscles a few hours before the anticipated exercise bout. Further, genes associated with muscle cell generation and growth, and those associated with antioxidant mechanisms, had altered expression in preparation for the exercise and later recovery. It also appears that mechanisms to clear blood lactate, a key component of fatigue in racehorses, become accustomed to regular training times. These findings suggest that exercise training of equine athletes should ideally be at similar times as the performance tests (races, events, etc.). Of course, the concern of the effect of jet lag and international travel on horses is another area of emerging research.

Horse owners are well aware that mares show seasonal anestrous, a time where they don’t have reproductive estrous cycles, and are long-day breeders. This natural cycle of reproductive activity allows foals to be born in the later spring/early summer when pasture is plentiful and temperatures are warmer. Melatonin production from the pineal gland is decreased as days become longer, which triggers the release of gonadotropin-releasing hormone, which ultimately stimulates the ovarian cycle to restart.

Playing with Light and Horse Management

For racing thoroughbreds with a January 1st birthday, we need to hasten the start of the season by placing mares under about 16 hours of artificial light for 60 days prior to desired breeding. This “trick” was first identified in 1947, but more recent research is finding that keeping pregnant mares under extended lighting results in larger foal weights with shorter gestation lengths, and foals with shorter and finer hair coats. This could be advantageous in terms of prepping weanlings for sale.

All horses – even non-breeding or high performance horses – may also benefit from some changes in their lighting. Horses that are housed in barns are exposed to artificial lights, particularly when it is dark outside, which can cause “chronodisruption,” a significant disturbance of orderly biological rhythms.

It should be noted that all light is not equal, and daylight has ample blue light (with a wavelength of about 465-485 nm) that trigger special photoreceptors of the retina that play a large role in circadian rhythms. Blue lighting also increases production of the hormone prolactin and neurotransmitter serotonin, which can influence mood and behaviour. In contrast, fluorescent or incandescent lighting provide very little of the blue-light wavelength.

Much of Dr. Murphy’s research has focused on the benefits low-intensity blue light stimulation, whereby horses can wear specially-fitted masks with a single eye light diode (Equilume Light Mask). These devices can advance seasonal cycles and influence foal size and hair coat, while allowing horses to remain outside at pasture. Light fixtures for stalls have also since been developed; these blue-lighting systems have user-reported benefits such as decreased stress and behavioral problems, increased milk yield in mares and increased libido in stallions. The lighting may also improve muscle metabolism and feed utilization. Horses that are housed for much of the day without natural light or are disturbed with lights at night may benefit from this blue-light “therapy.”

At night, it is important to avoid disrupting the darkness and melatonin production by using incandescent or fluorescent lights. Of interest is red light (with a wavelength of greater than 620 nm), which can allow visibility, but has only small disruptions to melatonin production. In humans, the use of red lighting at night has been shown to improve athletic performance and brain activity.

Horse owners are already aware of the role of lighting on cyclic activity in broodmares, but emerging research is continuing to show the importance of lighting, darkness, and environmental patterns such as feeding and exercise times on equine health and performance.