The effects of post-awakening light exposure on the cortisol awakening response in healthy male individuals.

Background: It is assumed that the expression of the cortisol awakening response (CAR) is modulated by light exposure during the peri-awakening period. While initial evidence supports this principal effect, the specific role of the spectral composition of light (brightness and wavelength) is still incompletely understood.
Method: Two counterbalanced within-subject experiments were conducted in a standardized sleep laboratory setting to investigate the effect of light intensity (study I; two days: dim vs. bright light) and spectral composition (study II; three days: red vs. blue vs. green light) on the CAR. Across studies, light exposure was conducted for one-hour post-awakening and the accuracy of CAR assessments (based on eight saliva samples) was well-controlled in line with recent guidelines.
Results: The two studies revealed consistent effects of light exposure on the CAR. Specifically, an increased CAR was found after exposure to bright (vs. dim) light (study I; (F _{(3.7, 106.4)}  = 11.93, p < .001, η² _p  = .29) and following blue and green (vs. red) light exposure (study II; F _{(4.9, 194.6)}  = 2.49, p = .037, η² _p  = .10).
Conclusion: Our findings illustrate the crucial role of light intensity and wavelength for expression of the CAR, in line with current theoretical knowledge of underlying neurobiological mechanisms.
(Copyright © 2019. Published by Elsevier Ltd.)