Inclusion Criteria:  students from the faculty of medicine, pharmacy, and law major exam at least 5 weeks ahead possession of smartphone and data plan  Exclusion Criteria:  major depressive disorder (PHQ-9): Score > 4 suicidal ideation (BDI, Item i): Rating > 1 seasonal affective disorder (PIDS-A): Score > 11 taking medication which increases photo-sensitivity taking medication to increase cognitive performance starting or changing pharmacotherapy for affective disorders and anxiety disorders within the last two weeks changing of all kinds of medication within the last two weeks headache (PHQ-D, Item f): Rating "severly impaired" increased light sensitivity: Item "Wearing sunglasses outside on sunny days" is answered with 'often' or 'very often' currently undergoing psychotherapeutic treatment the presence of an eye disease for which light therapy is contraindicated