To determine whether a single episode of physical activity is capable of inducing rapid phase shifts in human circadian rhythms, 17 subjects were studied two times under constant routine conditions, once in the absence of stimulus and once with a 3-h nighttime pulse of exercise interrupting the constant routine conditions. The profiles of plasma cortisol, thyrotropin (TSH), and melatonin and of body temperature were monitored continuously to derive estimations of circadian phase position. The phase shifts were measured on the 1st day after exercise exposure. The timing of the exercise period ranged from -5 h to +4 h around the time of the minimum body temperature rhythm. Nighttime exercise was associated with 1- to 2-h phase delays of both the melatonin and TSH rhythms, with the size of the delays tending to be smaller when the exercise was presented in the latter part of the nighttime period and in the early morning. These data demonstrate that nonphotic stimuli may exert phase-shifting effects on the human circadian pacemaker.