The photoreceptor(s) responsible for photoresetting of the human circadian system have not been identified. The aim of the present study was to assess the ability of short wavelength light to alter the timing of circadian rhythms. Eleven male subjects were studied in 15 4-day trials with a single 4 h light pulse administered on day 3, immediately after habitual wake time. The magnitude of the phase shifts in the melatonin acrophase and offset were similar after white (4300 microW/cm(2)) and short wavelength (28 microW/cm(2)) light exposure even though the white light pulse contained 185-fold more photons than the short wavelength light. This finding suggests short wavelength sensitivity of the photoreceptors mediating synchronization of human circadian rhythms.