Biological Implications of Artificial Illumination

There can be little doubt that man's visual system provide him with critically important information about his environment. The individual who cannot perceive light and its reflections suffers an immense disadvantage; in the civilized world he is cut off from the written word, unable to use most tools, and poorly oriented; in the world of Nature his very survival is in constant jeopardy. The obvious significance of light in providing the substrate for vision has tended until recently to obscure the fact that light also exerts important biological effects which are not dependent upon vision. Some of these effects of light are, like vision, initiated by responses of specialized photoreceptor cells in the retina. The photic input is transduced to nerve impulses which, instead of traveling to visual centers in the brain, terminate in brain regions that control glandular function. Other biologic effects of light result from direct effects of photic energy on the skin and the subcutaneous tissues. This report will summarize the former, neuroendocrine effects of light, and will comment on the latter. Recent experiments will be described which indicate that highly specialized pathways have evolved in the brain to mediate the extra-visual effects of light. The implications of these biologic consequences of light for the design of artificial light sources will be considered.