Daylight spectrum and intensity reduction and its impact on melatonin, cortisol, alpha-amylase, sleep parameters and sleepiness in humans

Light serves as the main synchroniser of the circadian system. The amount of light and its spectral distribution throughout the day influence hormonal secretion and sleep-wake regulation. There is a knowledge gap regarding the impact of the spectrum and intensity reduction of short-wavelength light during the day on circadian system outputs. In the present study, 23 participants spent 5 working days in the reference office with full-spectrum lighting. In the experimental week, participants spent five working days in the office with reduced intensity and short-light spectrum up to 500 nm. We measured melatonin, cortisol, and salivary alpha-amylase (sAA) activity in morning and evening saliva under dim light or light exposure (LE) and sleep by wrist actigraphy. Daylight impacted sleep quality only in females. The melatonin did not differ due to a changed daylight, but the LE had a stronger suppressive effect during the experimental week. The cortisol in the morning was higher in females, with no differences between weeks. To conclude, modified daylight has an impact on sleep quality without significant hormonal or sAA changes. LE before sleep can influence melatonin and sleep quality depending on the previous light history with high interindividual differences.