Dr Thomas Wehr is a psychiatrist and Scientist Emeritus in the Intramural Research Program of the National Institute of Mental Health, where he was Chief of the Clinical Psychobiology Branch. He has published extensively on his group’s research on effects on human biology of wakefulness, sleep, darkness and light. This work led to the discovery of seasonal affective disorder and its treatment with light, and to manipulations of sleep that improve depression and prevent mania. Dr Wehr discovered a brain-mechanism that tracks dawn and dusk and creates a biological ‘day’ and ‘night’ within the human body. He showed that the actions of this mechanism, which generates seasonal cycles, have been suppressed in humans by artificial light. Dr Wehr received a degree in English Literature and training in psychiatry at Yale University.