Time spent in outdoor light is associated with the risk of dementia: a prospective cohort study of 362094 participants

The paper outlines how UVB light affects the body not only through vitamin D, but also by sending signals from the skin to the brain. UVB is especially effective at triggering the production of hormone-like molecules, including stress-related hormones and natural opioids like beta-endorphin, which can enter the bloodstream and influence mood, stress, and behavior. These signals also activate the body’s stress-response system (the HPA axis) and support immune regulation. In addition, UVB-driven signals can reach the brain both through circulation and direct nerve pathways, helping coordinate communication between the brain, immune system, and skin.

Vitamin D is involved in various brain processes, and vitamin D receptors are present on neurons and glia in areas of the brain thought to be involved in the pathophysiology of depression. A systematic review and meta-analysis of 14 observational studies, that included a total of 31,424 adults, found that there were lower vitamin D levels in people with depression compared with controls, and there was an increased odds ratio of depression for the lowest v. highest vitamin D categories.

Sub-erythema narrowband UVB exposure can cause a meaningful increase in Beta endorphins in human skin 24 hours after exposure. Beta-endorphin is primarily utilized in the body to reduce stress and pain and has also been shown to positively impact depression and mental health.