Fig. 3.

Circadian phase shifts in response to retinal exposure to 460 nm versus 555 nm light. (A) Dose-response curves for circadian phase resetting are shown in response to 6.5-h of 460 nm light (left, blue circles) versus 555 nm light (right, green circles) exposure. Closed and open circles show phase shifts of plasma or salivary melatonin, respectively, in individual subjects. Black traces show the best-fit dose-response curve with 95% confidence intervals. Black filled circles at 0 log irradiance show phase shifts in response to darkness. (B) The dose-response curves are overlaid, demonstrating a difference in relative spectral sensitivity across irradiance levels. The horizontal dashed line indicates the half-maximal phase-shift response, and vertical dashed lines show the corresponding log ED₅₀ values, which are indicated on the plot. (C) Phase shifts in response to 555 nm light exposure did not match the best-fit univariant dose-response template (black dashed trace). (D) At low irradiances (<13 log photons cm⁻² s⁻¹; ~24 lux for 555 nm and ~2 lux for 460 nm light), phase-resetting responses to 555 nm light exposure were larger than predicted for a response mediated by melanopsin. Phase-shift residuals are shown relative to the predicted melanopsin-driven response, indicated by the dotted line. The predicted ‘melanopsin-only’ response to 555 nm light exposure was derived by translating the dose-response curve to 460 nm light by the predicted difference in log-relative sensitivity at these wavelengths for a photopigment with peak sensitivity to 480 nm light. For each group, the mean is shown with 95% confidence intervals.