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

<jats:p>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.</jats:p>