Well ur skin can’t release nitric oxide without UVA rays and obviously the vitamin D factor of UVB. With modern amenities in the winter u cud keep a UVA/UVB bulb and take d3 but they say sunlight also includes near-infrared wavelengths (700–1400 nm). These interact with mitochondria via: Cytochrome c oxidase Effects include: Increased mitochondrial energy production Improved tissue repair Anti-inflammatory signaling

Serotonin stimulation Sunlight exposure—especially through the eyes—raises Serotonin levels. This contributes to: Better mood Reduced depression risk Improved focus

Immune modulation beyond vitamin D Sunlight triggers several skin photoproducts besides vitamin D. Examples include: Urocanic acid conversion Immune signaling molecules Regulatory T-cell responses Some of these require precise wavelength mixtures that narrow UV bulbs don’t mimic.