The spectrum of visible light spans the range of violet to blue to green to yellow to orange and red—short to long wavelengths.  When combined, we see white.  The eye has different sensitivity to each wavelength region. In general, for vision, we are most sensitive to green.

Circadian light perception, however, peaks in the blue (but extends from violet into the green range).  Bright white light has a large blue component. Indoor incandescent light also contains blue, but less so, and its appearance is “warmer,” or yellowish.  To maximize a circadian rhythm response, we therefore want a whiter light in the morning (to shift our rhythms earlier) or evening (to shift them later). And we want warmer white if our rhythms are stable and normalized.

Story not finished, of course.  When a light first turns on, the cones immediately respond to the green component of white, and activate the ganglion cells containing blue-sensitive melanopsin.  The melanopsin response comes in later, and is longer-lasting.



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