Biophotons are the sunshine your mitochondria make inside your body.
You already know that during cold thermogenesis mitochondria in brown fat cells produce far infrared (FIR) light called heat. Well, they also produce other wavelengths -- including ultraviolet (UV).
It's the UV light of cold thermogenesis that boosted the blood serum levels of Vitamin D in a cohort of Polish women with multiple sclerosis undergoing a whole-body cryotherapy regimen under the supervision of researchers at the University of Krakow.
Your body is such a beautiful, complex system. It knows how to make its own sunshine during the winter, when it's dark and cold outside... as long as you're willing to give it the cold it needs to do it.