“Melatonin is a hormone which has gotten lots of press lately because of its many claims, one of which is to help people sleep. In horses Melatonin has been used to simulate short day cycles. (Kind of approaching the estrous/anestrus subject from the opposite end.) Lights shorten anestrus and hasten the onset of estrous.
“Melatonin, if given in mid summer, shortens estrous and hastens winter anestrus. Once properly exposed to short days, the mare can then respond to lights or the stimulus of long days. Consider this therapy if you find your mare not pregnant this summer. If such a mare was given Melatonin at that time, followed by lights next November, regular cyclic activity could be achieved early next season.” (Oregon Equine, Ron Friedman, MS, DVM, Lake Oswego, Oregon, USA)
“Melatonin is the primary endocrine signal in the horse that produces the effects of season on other parts of the body e.g. reproductive cycle. The circulating melatonin level is highly dependent on light, it increases in concentration during periods of darkness (night). Seasonal head shakers (spring onset) can be treated with melatonin. Daily administration of melatonin mimics the winter conditions of melatonin kinetics and thereby may decrease the seasonal body changes which lead to the development of headshaking.” (VetPro, Kumeu, New Zealand)
“Headshaking is a mysterious problem in horses. Recent research suggests that it can be caused by a variety of factors: it can be behavioral (a vice or disobedience) or biological (caused by physical discomfort arising from tack, teeth or ear problems, allergies, nerve stimulation, sensitivity to light, and a variety of other possible causes).”
“Some researchers theorize that melatonin, a hormone secreted by the pituitary gland, may play a role in photic headshaking. 12 mg melatonin per 1000 lbs, in concert with antihistamines or alone, may help your horse. Consult with your vet before trying this, as melatonin is a powerful supplement and can act as a sedative.” (Lisa Jadwin, Rochester, New York, USA)