The Influence of Light Wavelength on Human HPA Axis Rhythms: A Systematic Review.
Level 1 - systematic review of randomized trials
Systematic review of experimental studies
PubMed 37895351 · doi:10.3390/life13101968
What was done
A systematic review was conducted across PubMed, Web of Science, EMBASE, CINAHL, Medline, PsycINFO, and Cochrane Library databases through June 19, 2023. The authors evaluated the effects of light exposure of different spectral wavelengths and luminance intensities on cortisol levels in healthy humans. Twelve studies covering 337 participants met the inclusion and risk-of-bias criteria.
What was found
Exposure to bright light of any color during late night or early morning significantly increased cortisol secretion relative to time-matched dim light control conditions. Furthermore, early morning exposure to bright lights with stronger short-wavelength (blue/green) components elicited greater increases in cortisol secretion compared to lights with stronger long-wavelength (red) components. The abstract reports qualitative directions and does not provide numerical effect sizes or confidence intervals.
Why it matters
This review establishes that human HPA axis cortisol rhythms are modulated by light wavelength and intensity, suggesting that light spectrum should be accounted for in architectural lighting design and circadian intervention trials.
Limits
The total evidence base is small, comprising only 12 studies and 337 healthy participants. The abstract omits numerical effect sizes, variance estimates, and specific exposure durations, and findings in healthy individuals cannot be directly generalized to populations with clinical HPA axis or endocrine disorders.