It is well known that artificial light at night has broad impacts on the animal world. Migratory birds, insects, and bats all rely on celestial clues to navigate, feed, and mate. Artificial lights can cause them to become disoriented or lose the protective cover of darkness. DarkSky International says on its website that:
Scientific evidence suggests that artificial light at night has negative and deadly effects on many creatures, including amphibians, birds, mammals, insects, and plants.
And now, excess artificial light is being implicated in the health of horses. In her new book, A Horse’s World: A Neuroscientist’s Journey into the Equine Mind, by Janet Jones, Jones observes:
As with most mammals, light, noise, anxiety, and pain interfere with equine sleep. Light disturbance is just now being addressed in competitive environments, where darkness, even during midnight hours, is rare. In 2024, the United States Equestrian Federation activated a new rule for such environments, stating that horses must have minimal light and noise for at least six consecutive hours each night. Light disturbance is an issue at breeding stables as well. Many mares are housed under artificial lights at night to induce fertility in winter and early spring. This practice affords breeders the benefit of earlier births, but it also places the mares at risk of insufficient REM sleep.
Horses have more rod cells in their eyes, leading to more sensitive night vision than humans. Karen Pickering, of the Oregon Horse Council, has said:
Having better night vision has its benefits. On a dark trail, where human vision can be limited, our horse can still see the trail (or a predator for that matter). However, this quality also makes horses more sensitive to bright lights, like the noonday sun or security lights that come on at night. (link)
DarkSky Wisconsin member (and horse owner) Helen Stassen says:
As a lifelong horse gal, I appreciate that attention is being given to light disturbances in the equine world. Owning a mare, I know that her heat cycle, as well as growing her winter coat, is all based on the amount of natural light she is exposed to; artificial light at night disrupts these patterns and is not healthy for her.
Whether it is a migratory bird crossing the night sky, an insect navigating by moonlight, or a horse resting in a stable, darkness is a natural part of the environment animals depend on. Reducing unnecessary artificial light at night helps protect that darkness and the wildlife and ecosystems that rely on it.
Written by John Rummel and Helen Stassen
