To determine whether sleep deprivation (SD) affects the response of circadian rhythms to light, hamsters were forced to walk on a slowly rotating treadmill for 6 or 24 h, ending early in the night, with or without a light pulse during the last 30 min. SD alone did not produce a significant phase shift. Light pulses (300 and 50 lx) alone induced significant delay shifts (55 and 35 min, respectively). Twenty-four hours SD significantly attenuated the delay to brighter light and 6 h SD significantly attenuated the delay to moderate light. Sleep loss or attendant low-intensity continuous activity appear to modulate the response of the hamster circadian system to light.