Hair glucocorticoids (cortisol and cortisone) are used to evaluate long-term HPA axis activity in relation to somatic and psychological health.

Hair glucocorticoids gradually decrease along the hair shaft: ‘wash-out’.

We hypothesized that wash-out of hair glucocorticoids may in part be caused by ultraviolet (UV) radiation present in natural sunlight.

In our study, hair cortisol and cortisone decreased after exposure to a high amount of artificial UV, but also after 40 hours of natural sunlight.

Sunlight exposure should be considered as a potential confounder in studies investigating hair glucocorticoids.