This new article shows that exposure to UV light produces changes the blood flow to areas of the brain related to reward in ways that are similar to addictive substances. I could see why this might be evolutionarily useful for people who evolved in northern latitudes to ensure that they get enough vitamin D during the sunny summer months to last through the dark winter months. However, I would need to know what the biological mechanism is before I believe this completely.
Title: Activation of the mesostriatal reward pathway with exposure to ultraviolet radiation (UVR) vs. sham UVR in frequent tanners: a pilot study
Abstract: Frequent and excessive tanning persists despite a growing understanding of its associated morbidity and mortality, suggesting that ultraviolet radiation may impart rewarding effects beyond the assumed cosmetic benefits. To empirically measure putative centrally rewarding properties of ultraviolet radiation (UVR), we assessed the effects of a commercially available tanning bed upon regional cerebral blood flow (rCBF), a measure of brain activity, using single-photon emission computed tomography (SPECT). Seven frequent salon bed tanners were placed under a UVA/UVB tanning light during two sessions; one session with UVR and the other with filtered UVR (sham UVR). Session order was randomized and subjects were blinded to study order. During the UVR session, relative to sham UVR session, subjects demonstrated a relative increase in rCBF of the dorsal striatum, anterior insula and medial orbitofrontal cortex, brain regions associated with the experience of reward. These changes were accompanied by a decrease in the subjective desire to tan. These findings suggest that UVR may have centrally rewarding properties that encourage excessive tanning.