Differential routing of spectral light inputs separates circadian timing from energetic responsiveness

Light simultaneously provides phototrophic organisms with energy and with information about environmental time. These two functions need not impose the same response to fluctuations in irradiance: photosynthetic outputs should remain amplitude-sensitive, whereas circadian phase should reject changes that do not alter dawn, dusk, or photoperiod. We formulate this problem for two spectral inputs by decomposing their logarithmic intensities into a common-irradiance coordinate a and a spectral-contrast coordinate r. The contribution of channel i to phase is (Qi=ZiGi, where the non-negative gate Gi determines when the pathway is active and the signed phase-response projection Zi determines whether this activity advances or delays the oscillator. For a locked oscillator, robustness to common irradiance together with retained contrast sensitivity requires two non-zero cycle-averaged contributions of opposite sign, A1simeq-A2=0. Energetic responsiveness is preserved only when the physiological projection of the same inputs is not proportional to their phase projection. A canonical repressilator provides an explicit nonlinear realization of these conditions. Positive gates placed on opposite lobes of its infinitesimal phase-response curve strongly attenuate common-mode phase shifts while preserving contrast sensitivity. A minimal photosynthetic-capacity model then shows how this organization protects temporal alignment under day-to-day irradiance fluctuations. At the largest variability tested, differential routing reduced the mean phase displacement by more than one half and the associated alignment loss by approximately 82%, whereas the resulting production advantage remained small, approximately 0.1%. Thus, multichannel light sensing can stabilize circadian timing without suppressing the energetic response to irradiance.