Abstract
Melatonin suppression by light in humans is largely mediated by the photopigment melanopsin, which is expressed in the intrinsically photosensitive retinal ganglion cells (ipRGCs). Whether the cones and rods also robustly contribute to melatonin suppression is at present not known. In a within-subject 7h in-laboratory protocol individually timed to habitual bedtime, we probed the role of short-wavelength sensitive cones (S cones) in this process. In each of two sessions (order counterbalanced across participants), we exposed participants (n=21 total participated) to light differing in the amount of S cone excitation by more than 1 log unit, with minimal relative change in the L and M cones, melanopsin and rods. Throughout the protocol, at 30 minute intervals, saliva samples (for later melatonin assay) were collected. In addition, participants performed an auditory psychomotor vigilance test (auditory PVT) and provided subjective sleepiness judgments (KSS). During the light exposure phase, pupil size was measured using a head-mounted eye tracker. Salivary melatonin assays were performed using RIA. Forthcoming analyses will allow for inferences of the role of S cones in human melatonin suppression.
Wellcome Trust (204686/Z/16/Z)