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The aims of the study were to assess the effects of light on the production of stress hormones, classroom performance, body growth, and sick leave, of school children. About 90 children were investigated in their school environment for a duration of one school year. The children were situated in four classrooms differing in respect to the access to natural daylight and artificial fluorescent light. The results indicated the existence of a systematic seasonal variation with more stress hormones in summer than in winter. The children situated in the one classroom lacking both natural daylight and fluorescent daylight tubes demonstrated a marked deviation from this pattern. High levels of morning cortisol were associated with sociability, while moderate or low levels seemed to promote individual concentration. Annual body growth was smallest for the children with the highest levels of morning cortisol. Possibly, the production of cortisol had some influence on sick leave. It may be concluded, that windowless classrooms should be avoided for permanent use.