This narrative review evaluates how full-spectrum sunlight regulates the gut–brain–skin axis, focusing on circadian entrainment, melanin as a holobiont photoreceptor, microbial photoreceptors (melanin, flavins, cryptochromes), and the downstream effects of circadian disruption (shift work, blue light exposure, sun avoidance) on the microbiome, immunity, and metabolism.
The review synthesizes evidence positioning host and microbial melanin as the central signaling molecule of the melanocortin pathway — which is circadian-regulated — governing metabolic and immune homeostasis across the gut–brain–skin axis; no primary outcome data or sample sizes are reported, as this is a mechanistic review.
- Full text was inaccessible; summary is based on the abstract only, limiting depth of appraisal. - As a narrative review, it is subject to selection bias in evidence synthesis with no systematic search or meta-analytic methods described. - Key claims (e.g., circadian regulation of vitamin D, microbial melanin as photoreceptor) are framed as mechanistic models and research gaps rather than established findings.
Clinicians managing conditions linked to circadian disruption — including dysbiosis, skin barrier dysfunction, and metabolic disorders — should consider modifiable light-exposure behaviors (e.g., reducing nocturnal blue light, encouraging morning sunlight) as part of a broader lifestyle approach. The proposed melanin–melanocortin–microbiome framework is mechanistic at this stage and not yet ready to guide specific therapeutic protocols.
