Light colour and our internal clock

There’s certainly no shortage of publications that indicate light intensity and wavelength affect circadian rhythms. But maybe the issue isn’t as clear-cut as it seems. Interestingly, this 2018 systematic review suggests that only 15 out of 128 papers they reviewed on the subject fulfilled their quality criteria; most were excluded due to small sample sizes, narrow cohort diversity, and other factors.

Leena Tähkämö, Timo Partonen & Anu-Katriina Pesonen (2019) Systematic review of light exposure impact on human circadian rhythm, Chronobiology International, 36:2, 151-170, DOI: 10.1080/07420528.2018.1527773

Their review findings were:

“A 2-h exposure to light (460 nm) in the evening suppresses melatonin, the maximum melatonin-suppressing effect of light exposure being achieved at the shortest wavelengths (424 nm), but the melatonin concentration recovers rather rapidly, within 15 min from cessation of the exposure. This suggests a short-term or simultaneous impact of the light exposure. In addition, melatonin secretion as well as melatonin suppression were found to reduce with age, but the light-induced phase advance of melatonin circadian rhythm is not impaired with age. The circadian rhythm was found to respond to both morning and evening light exposure (blue-enriched or blue-depleted) without significant differences between morning-types and evening-types of persons (chronotypes).”​