“We believe the effect is huge and that it’s just beginning to be recognized as a problem,” said Dr. Patricia Turner, an ophthalmologist in Leawood, Kan., who with her husband, Dr. Martin Mainster, a professor of ophthalmology at the University of Kansas Medical School, has written extensively about the effects of the aging eye on health.

Circadian rhythms are the cyclical hormonal and physiological processes that rally the body in the morning to tackle the day’s demands and slow it down at night, allowing the body to rest and repair. This internal clock relies on light to function properly, and studies have found that people whose circadian rhythms are out of sync, like shift workers, are at greater risk for a number of ailments, including insomnia, heart disease and cancer.

“Evolution has built this beautiful timekeeping mechanism, but the clock is not absolutely perfect and needs to be nudged every day,” said Dr. David Berson, whose lab at Brown University studies how the eye communicates with the brain.

So-called photoreceptive cells in the retina absorb sunlight and transmit messages to a part of the brain called the suprachiasmatic nucleus (S.C.N.), which governs the internal clock. The S.C.N. adjusts the body to the environment by initiating the release of the hormone melatonin in the evening and cortisol in the morning.

Melatonin is thought to have many health-promoting functions, and studies have shown that people with low melatonin secretion, a marker for a dysfunctional S.C.N., have a higher incidence of many illnesses, including cancer, diabetes and heart disease.