Lack of strong evidence for cone photoreceptors' contribution to human melatonin suppression and alerting response to light.

Light exposure at night can suppress melatonin production and increase alertness, primarily through the action of melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs). This study investigated whether cone photoreceptors also influence melatonin suppression and subjective alertness using non-visual metameric light. Forty-eight participants with normal trichromatic vision were exposed to three lighting conditions: a baseline (BL: 9 lx), constant background (BG: 149 lx), and cone-modulated flickering light targeting different cone combinations and post-receptor channels (S-, M-L, S+M+L: 149 lx) for 2 h after their habitual bedtime. Salivary melatonin levels and subjective alertness were measured throughout a 9-h protocol. Bayesian analysis showed that cone-modulated flickering light did not significantly affect melatonin suppression or alertness, providing evidence against the hypothesis that cone photoreceptors contribute to these non-visual effects of light. In conclusion, our results suggest cone photoreceptors do not play a measurable role in the effects of light on melatonin suppression and subjective alertness at night.