Abstract
The present paper reflects a work in progress toward a definition of circadian light, one that should be informed by the thoughtful, century-old evolution of our present definition of light as a stimulus for the human visual system. This work in progress is based upon the functional relationship between optical radiation and its effects on nocturnal melatonin suppression, in large part because the basic data are available in the literature. Discussed here are the fundamental differences between responses by the visual and circadian systems to optical radiation. Brief reviews of photometry, colorimetry, and brightness perception are presented as a foundation for the discussion of circadian light. Finally, circadian light (CLA) and circadian stimulus (CS) calculation procedures based on a published mathematical model of human circadian phototransduction are presented with an example.
Highlights
The suprachiasmatic nuclei (SCN) in the hypothalamus host the master circadian clock that organizes and orchestrates the timing of all daily biological functions, from complicated physiological systems to single cells
Light is formally defined as optical radiation capable of providing visual sensation in humans
Since temporal patterns of retinal light exposures regulate the human circadian system and since disruption of the circadian system has broad implications for health and well-being [2,3,4,5,6,7,8,9,52,53], it is becoming increasingly important to develop a new definition of circadian light
Summary
The suprachiasmatic nuclei (SCN) in the hypothalamus host the master circadian clock that organizes and orchestrates the timing of all daily biological functions, from complicated physiological systems to single cells. The mathematical model of human circadian phototransduction developed by Rea et al [36] is based on the neuroanatomy and neurophysiology of the retina and on published psychophysical studies of nocturnal melatonin suppression using lights of different spectral power distributions. Even the model of human circadian phototransduction based upon nocturnal melatonin suppression and used to calculate CLA and CS is probably incomplete It does not take into account possible participation of different types of ipRGCs [50] and recent evidence that the melanopsin photopigment in the ipRGCs follows a very different regenerative process than that employed by the classical photoreceptors [51]. The information presented here is an important step toward the precise application of light stimuli for the human circadian system
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