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M.J. Pauers, M. Neitz, J. Neitz; Diel Changes In The Color Of Sunlight And Its Potential Role In Entrainment Of Circadian Rhythms . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2668.
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To entrain circadian rhythms, organisms have to be able to extract time–of–day information from dawn and dusk during which both the amount of light and its spectral composition change in a systematic way. It is unclear which factors are used by the circadian system to detect this information. The change in luminance between daytime and nighttime is an obvious candidate zeitgeber but it is also possible that the changing color of the sky at dawn and dusk might act as a cue for entrainment (e.g., Foster and Kreitzman, RHYTHMS OF LIFE ISBN 1 86197 235 0). This possibility has become more attractive with the recent discovery that the anatomically distinct 'giant', melanopsin–expressing ganglion cells responsible for setting the circadian pacemaker in the primate retina display a rare, S–Off, (L + M)–On type of spectrally–opponent receptive field (Dacey et al. 2005; Nature 433: 749–54). This characteristic may make these cells particularly responsive to natural changes in the color of the sky at dusk and dawn.
Changes in the spectral composition of light throughout the day in southeast Wisconsin were measured. A series of calculations were performed convolving the solar chromatic stimuli with the photopigment sensitivities of both the human and Mongolian gerbil (Meriones unguiculatus). These incorporated characteristics of ‘brisk–transience’ and spectral–opponence of the ‘giant’ ganglion cells to give an indication of the possible relative strength and importance of color signals in setting circadian rhythms.
The spectral composition data indicate that for the majority of the day, short wavelength light dominates the sky, with two, brief but strong periods of long wavelength light occurring at sunrise and sunset. The photopigment sensitivity calculations indicate that both humans and gerbils would be expected to detect these diel changes in sky color even though the gerbils have UV sensitive cones in place of the S cones of many mammals. Most interestingly, our calculations suggest that the ‘brisk–transient’ nature of the melanopsin–expressing ganglion cells causes them to respond most strongly to the long–wavelength light present at sunrise, with a decrease in the activity of these cells throughout the day.
Consideration of the spectrally opponent nature of the responses of melanopsin–containing ganglion cells suggests that the sudden, transient shift to long wavelength light at sunrise may be an important cue for phase setting of circadian rhythms.
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