Horses are long-day seasonal breeders. The natural reproductive period coincides with the light-filled days between May and September, when the environment is optimal for the survival of offspring. However, the universal birthday for some horse breeds, like Thoroughbreds, is January 1. This creates a demand for early foals in order to produce mature yearlings and 2-year-old racehorses.
Utilization of artificial light to advance the breeding season is common practice within the Thoroughbred industry. Mares are maintained indoors under barn lighting until 11 p.m. for eight to 10 weeks beginning around December 1. The artificially extended day length acts to inhibit the hormone melatonin and fools the mare’s reproductive system into activating earlier in the year. Historically, light from a 100-watt light bulb in a 12-foot by 12-foot stall has been used. This amount of illumination is loosely defined in the literature as "enough light to read a newspaper."
Recently, light in the blue light spectrum (465- 485 nanometers) has been found to facilitate more accurate and efficient levels of melatonin inhibition in other species. In 2011, we investigated the threshold level of blue light required to inhibit circulating concentrations of melatonin in the horse and found it to be within the range of 10-50 lux (the standard international unit of illuminance). The average light intensity at eye level in a stall illuminated by a 100-watt light bulb is 250 lux. Of greater significance was that the level of melatonin inhibition achieved did not differ when light was administered to one or both eyes.
A second multi-institutional study was conducted t