Horses are known as “long-day breeders,” meaning that they will naturally go into heat for breeding when the days get longer in the spring and summer (at least, in the Northern hemisphere). They are also “seasonally polyestrous,” meaning that they will go through multiple estrous cycles during their breeding season. During the winter months or “short days,” the mare will not cycle. This period is known as anestrous. Changes in lighting as the days get longer induce hormonal changes which bring the mare into heat (estrus). As the mare periodically comes out of heat, this bout of reproductive inactivity during the breeding season is known as diestrus.