Exposure to light in the evening or at night can profoundly affect human circadian rhythms and sleep-wake physiology. These effects are mediated by a pathway connecting the retina to the suprachiasmatic nuclei, the circadian pacemaker in the hypothalamus. Signals originating in the intrinsically photosensitive retinal ganglion cells that express the photopigment melanopsin control this pathway to a large extent. However, there is converging evidence that also the cones in the retina may contribute to light-mediated effects on circadian physiology and thus sleep. More specifically, changes along the blue-yellow dimension of colour vision, which correspond to a colour-opponent post-receptoral channel pitting signals from the short wavelength-sensitive S-cones against luminance (reflecting a joint signal from long [L] and medium [M] wavelength-sensitive cones) seem to be involved. In this project, we seek to confirm in a direct test that calibrated silent-substitution changes along this +S–(L+M) dimension affect the human circadian system.

In a repeated 32.5-hour within-subjects laboratory protocol, we will expose participants to calibrated lighting scenarios during the night. During a 1-h light exposure, we will employ the following light scenarios: (1) constant background light (100 lux), holding the excitation of the L, M, S-cones and melanopsin constant, (2) intermittently flickering light (1 Hz; 30 seconds on, 30 seconds off) along the blue-dim (+S–[L+M]) dimension, or (3) intermittently flickering light (1 Hz; 30 seconds on, 30 seconds off) along the yellow-bright (–S+[L+M]) dimension. Importantly, all three conditions will not differ in melanopsin excitation. We hypothesize that (1) flickering changes induce greater circadian phase shifts than constant background light and (2) yellow-bright flickering changes induce stronger circadian phase shifts than blue-dim flickering changes. Additionally, we will also examine a set of ringfenced secondary analyses regarding acute melatonin suppression, sleepiness, visual comfort, vigilance, sleep onset latency, and slow-wave activity during sleep.

Our work will directly inform the fundamental biological question which photoreceptors and mechanisms of colour vision in the retina influence circadian and sleep-wake physiology. Last, this will also enable the prediction of the physiological effects of light in real-life contexts such as the effects of artificial screen light in the evening.