A new study suggests there might be something to the notion, first proposed by physicians in ancient Greece, that seasonal mood swings are caused by “humors.” According to Yale investigators, patients with seasonal affective disorder (SAD) have lower nocturnal levels of the bile pigment bilirubin. The study is the first scientific investigation of circadian levels of bilirubin in the blood of people with depression. Psychiatrist Dan A. Oren, M.D. ’84, HS ’88, and colleagues found that nighttime bilirubin levels were lower in nine patients with SAD, compared to a group of seven healthy volunteers. The patient group’s levels of bilirubin increased after exposure to a light source, a standard therapy for the disorder. The article was published in the March 1 issue of the journal Biological Psychiatry.