Light targeting the melanopic system suppresses melatonin, but does not alter sleepiness, vigilance, sensory processing, or sleep

Objectives: Pre-sleep exposure to short-wavelength light acutely suppresses melatonin, increases vigilance, and decrease sleepiness with the activating effects extending to sleep. The effects have mainly been attributed to melanopic effects of light. Thus, we investigated whether two metameric light conditions designed to only differ in their melanopic effects (123 vs. 59 lux melanopic EDI) also differentially affect sleep besides melatonin effects. Beyond this, we also studied whether the light conditions modulate sensory processing during wakefulness and sleep. Methods: In a preregistered study, twenty-nine healthy participants aged 18-30 y (15 women) were exposed to two metameric light conditions (high- vs. low-melanopic, differing by a factor of 2x, 7-day washout period) for 1 hour prior to their habitual bed time. This was followed by an 8-h sleep opportunity with polysomnography. Objective sleep measurements were complemented by self-reported sleep evaluation after wake-up. Salivary melatonin levels, subjective sleepiness, and behavioural vigilance were sampled in regular intervals. Sensory processing was evaluated using an oddball paradigm participants completed during the light exposure in the evening and the following sleep episode. Specifically, we were interested in event-related potentials (ERPs) and time-frequency responses related to violations of expectations, that is, the mismatch response. Results: Despite ≈14% melatonin suppression in the high- compared to the low-melanopic condition, light conditions did not differentially affect objectively assessed sleep, self-perceived sleep quality, sleepiness, or vigilance. A neural mismatch response was evident during all sleep stages but not differentially modulated by pre-sleep light. Conclusions: Suppression of melatonin by light targeting the melanopic system does not necessarily translate to acutely altered levels of vigilance or sleepiness and neither to changes in sleep, sleep quality, or basic sensory processing. This may suggest that an interaction between melanopsin and cone-rod signals could be at work for such effects to occur. ### Competing Interest Statement CC and MS declare the following interests related to lighting. MS is currently an unpaid member of CIE Technical Committee TC 1-98 ("A Roadmap Toward Basing CIE Colorimetry on Cone Fundamentals"). MS was an unpaid advisor to the Division Reportership DR 6-45 of Division 3 ("Publication and maintenance of the CIE S026 Toolbox") and a member of the CIE Joint Technical Committee 9 on the definition of CIE S 026:2018. Since 2020, MS is an elected member of the Daylight Academy, an unpaid member of the Board of Advisors of the Center for Environmental Therapeutics. MS is named inventor on a patent application entitled on optimising non-linear multi-primary LED system filed by Oxford University Innovation Ltd. (US Patent Application no. 17/428,073, European Patent Application No 20705492.5). CC has had the following commercial interests related to lighting: honoraria, travel, accommodation and/or meals for invited keynote lectures, conference presentations or teaching from Toshiba Materials, Velux, Firalux, Lighting Europe, Electrosuisse, Novartis, Roche, Elite, Servier, and WIR Bank. CC is a member of the Daylight Academy.