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The gradual yellowing of the lens and the narrowing of the pupil that occur with age disturb the body’s circadian rhythm, contributing to a range of health problems, these studies suggest. As the eyes age, less and less sunlight gets through the lens to reach key cells in the retina that regulate the body’s circadian rhythm, its internal clock.

Photoreceptive cells in the retina absorb sunlight and transmit messages to a part of the brain called the suprachiasmatic nucleus (SCN), which governs the internal clock. The SCN 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 SCN, have a higher incidence of many illnesses, including cancer, diabetes and heart disease.

Scientists believe that with age, people should make an effort to expose themselves to bright sunlight or bright indoor lighting when they cannot get outdoors. Older adults are at particular risk, because they spend more time indoors.

[ New York Times ]