Blue light is the part of the spectrum filtered by the eye’s aging lens. In a study published in The British Journal of Ophthalmology, Dr. Mainster and Dr. Turner estimated that by age 45, the photoreceptors of the average adult receive just 50 percent of the light needed to fully stimulate the circadian system. By age 55, it dips to 37 percent, and by age 75, to a mere 17 percent.
via Aging of Eyes Is Blamed in Circadian Rhythm Disturbances – NYTimes.com.