“Early studies show that green light is increasing the levels of circulating endogenous opioids, which may explain the pain-relieving effects. Whether this will be observed in humans is not yet known and needs further work.”Read more here.
Mohab M. Ibrahim et al, Long-lasting antinociceptive effects of green light in acute and chronic pain in rats, PAIN (2017). DOI: 10.1097/j.pain.0000000000000767