The influence of light wavelength on HPA axis function in non-human animals: A meta-analysis

The suprachiasmatic nucleus regulates circadian rhythms in physiological and behavioural processes partly through modulation of the hypothalamic-pituitary-adrenal (HPA) axis. The suprachiasmatic nucleus is entrained by light via input from intrinsically photosensitive retinal ganglion cells that are particularly sensitive to short wavelengths, theoretically linking light spectra to HPA axis function. This systematic review and meta-analysis evaluated the effects of wavelength of light on biochemical markers of the HPA axis in terrestrial non-human animals. A search of seven databases on 13 January 2025 identified 23 studies meeting inclusion criteria. Study subjects spanned a variety of animal species. Subjects were typically exposed to coloured light for the entire daytime light period for multiple weeks prior to HPA axis assessment. Corticosterone was the most frequently reported HPA axis measure. Melatonin, an additional hormonal index of circadian rhythms, was reported alongside corticosterone when data were available. Meta-analyses showed that in poultry, blue, blue-enriched, and red light rearing induced medium to large statistically non-significant increases in daytime corticosterone levels relative to white light, while green and UV-enriched light elicited weaker or inconsistent effects. In melatonin-proficient laboratory rodents, blue-enriched light induced a large significant decrease in daytime corticosterone and a large significant increase in nighttime melatonin. These findings highlight the importance of considering light wavelength for animal welfare and the optimisation of lighting conditions in research, agricultural, and ecological settings. Registration: osf.io/rvu4w.