Hair cortisol and cortisone are decreased by natural sunlight

Highlights • Hair glucocorticoids (cortisol and cortisone) are increasingly used to evaluate long-term HPA axis activity in relation to somatic and psychological health, but are subject to ‘wash-out’: the gradual decrease of glucocorticoid content along the hair shaft. • We hypothesized that wash-out of hair glucocorticoids may in part be caused by ultraviolet (UV) radiation present in natural sunlight. • In our experiments, hair cortisol and cortisone were decreased after exposure to a high amount of artificial UV radiation, but also after 40 h of natural sunlight exposure. • Sunlight exposure should be considered as a potential confounder in studies investigating hair glucocorticoids.