Light in the built environment: potential role of circadian disruption in endocrine disruption and breast cancer

Life in industrialized societies is primarily life inside buildings. Illumi nation from electric lighting in the built environment is quite different f rom solar radiation in intensity, spectral content, and timing during the 2 4-hour daily period. Humans evolved over millions of years with the day-nig ht pattern of solar radiation as the primary circadian cue. This pattern ma intained a 24-hour rhythm of melatonin release, as well as a host of other physiological rhythms including the sleep-wake cycle. Electric lighting in the built environment is generally more than sufficient for visual performa nce, but may be inappropriate for the maintenance of normal neuroendocrine rhythms in humans; e.g., insufficient during the day and too much at night. Lighting standards and engineering stress visual performance, whereas circ adian function is not currently emphasized. The molecular biological resear ch on the circadian clock and on mechanisms of phototransduction makes it c lear that light for vision and light for circadian function are not identic al systems. In particular, if electric lighting as currently employed contr ibutes to 'circadian disruption' it may be an important cause of 'endocrine disruption' and thereby contribute to a high risk of breast cancer in indu strialized societies.