More to the point, it's a nerve problem, although the mechanism is unclear. Melatonin, a hormone produced by the body, has a role in regulating the body's seasonal changes. An unusual decrease in melatonin production in certain horses during the spring, summer and fall allows some branches of the trigeminal nerves to be over-stimulated by bright sunlight. In some cases, the irritated nerve becomes painful, thus the headshaking. The irritation of this nerve, which exits the skull about four inches behind the nostril, causes the horse to shake his head and rub his nose on the ground to get relief.
Exercise seems to trigger the condition, or almost always aggravates it.
If a horse shows no clinical signs except during exercise, other causes for the headshaking, such as tooth problems or resistance to training, should be ruled out. A simple test can help you decide if this is the solution for a headshaker that only shows signs during exercise. Lunge the horse in a halter on a sunny day, allowing sufficient time for the nerve to become stimulated. If headshaking occurs, a bitless bridle probably won't solve the problem. If using a blindfold, or lunging after dark, eliminates or greatly reduces headshaking, it's possible the problem is caused by direct sunlight.