We examined polysomnography (PSG) and body temperature in a patient with delayed sleep phase syndrome who responded to phototherapy. The patient was a 31-year-old woman whose condition had slightly improved by a vitamin B12 administration. Phototherapy was administered to her in combination with the vitamin B12 medication, and this combined treatment successfully advanced her delayed sleep phase. On PSG, the regimen showed shortened sleep latency, decreased total sleep time and stages 1 and 2 sleep, and increased slow wave sleep. Phototherapy also improved temporal distribution of delta half-waves (0.5-2.0 Hz, > or = 31 microV) as well as phase relationship between sleep and body temperature.