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.
- Terman M, Remé CE, Rafferty B, Gallin PF, Terman JS. Bright light therapy for winter depression: Potential ocular effects and theoretical implications. Photochemistry and Photobiology1990:5;781-793.
- Foster RG. Bright blue times. Nature 2005;433:698-699.
- Remé CE, Rol P, Kaase H, Terman M. Bright light therapy in focus: Lamp emission spectra and ocular safety. Technology and Health Care 1996;4:403-413.
- Smith MR, Revell VL, Eastman CI. Phase advancing the human circadian clock with blue-enriched polychromatic light. Sleep Medicine 2009;10:287-94.
- Terman M. Blue in the Face. Sleep Medicine 2009;10:277-278.
- Gooley JJ, Rajaratnam SM, Brainard GC, Kronauer RE, Czeisler CA, Lockley SW. Spectral responses of the human circadian system depend on the irradiance and duration of exposure to light. Science Translational Medicine 2010;2:31ra33