Abstract
Pre-sleep exposure to short-wavelength light suppresses melatonin and decreases sleepiness with activating effects extending to sleep. This has mainly been attributed to melanopic effects, but mechanistic insights are missing. Thus, we investigated whether two light conditions only differing in the melanopic effects (123 vs. 59 lux melanopic EDI) differentially affect sleep besides melatonin. Additionally, we studied whether the light differentially modulates sensory processing during wakefulness and sleep.
Twenty-nine healthy volunteers (18-30 years, 15 women) were exposed to two metameric light conditions (high-vs. low-melanopic, ≈60 photopic lux) for 1 hour ending 50 min prior to habitual bed time. This was followed by an 8-h sleep opportunity with polysomnography. Objective sleep measurements were complemented by self-report. Salivary melatonin, subjective sleepiness, and behavioural vigilance were sampled at regular intervals. Sensory processing was evaluated during light exposure and sleep on the basis of neural responses related to violations of expectations in an oddball paradigm.
We observed suppression of melatonin by ≈14 % in the high-compared to the low-melanopic condition. However, conditions did not differentially affect sleep, sleep quality, sleepiness, or vigilance. A neural mismatch response was evident during all sleep stages, but not differentially modulated by light.
Suppression of melatonin by light targeting the melanopic system does not automatically translate to acutely altered levels of vigilance or sleepiness or to changes in sleep, sleep quality, or basic sensory processing. Given contradicting earlier findings and the retinal anatomy, this may suggest that an interaction between melanopsin and cone-rod signals needs to be considered.
Statement of Significance Metameric light allows to mechanistically investigate the contribution of one specific retinal receptor. Using this approach, we here investigated the effects of high-vs. low-melanopic light for 1 hour in the evening at ecologically valid screen illuminance (≈60 photopic lux). Going beyond earlier research, we also investigated effects on sleep. We found that despite significant suppression of melatonin, other endpoints including sleep and sleep quality were not differentially affected. This underlines that melatonin suppression does not automatically translate to alterations of sleep, sleepiness, or vigilance. Further, it suggests that melanopsin effects may need to be studied in the context of cone-rod signals. Future research should thus investigate the relevance of such an interaction, which may vary between endpoints.
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.
Footnotes
Authors’ email addresses: mn473{at}cam.ac.uk, tb419{at}cam.ac.uk, manuel.spitschan{at}tuebingen.mpg.de, helenchristina.slawik{at}upk.ch, martin.meyer{at}upk.ch, christian.cajochen{at}upk.ch
Clinical Trial Registry, German Clinical Trials Register (DRKS00023602)
We have added further relevant methodological information and included additional references. Furthermore, we have included an additional figure (Suppl. Fig. 4) highlighting individual differences in KSS values between the two light conditions as well as an additional table (Table 4) including the medians and 90% confidence intervals for the sleep parameters.
