Skin surface blood flow at the fingertips was measured in a 21-year-old woman during sleep using laser Doppler flowmetry. Skin blood flow responses (SBFR) are transient reductions in skin blood flow of about 25s in duration. In our subject, SBFR showed a delay time of 8-9s from the onset of skin potential responses (SPR) and appeared more consistently than SPR. The frequency of SBFR decreased during non-rapid eye movement sleep, especially when the sleep stage became deep. These findings indicate the validity of SBFR as a discrete index of autonomic function during sleep.