To determine the roles of intensity and duration of nocturnal physical activity in causing rapid phase shifts of human circadian rhythms, eight healthy men were studied three times under constant conditions with no exercise, a 3-h bout of moderate-intensity exercise, or a 1-h bout of high-intensity exercise. Exercise stimulus was centered at 0100. Circadian phase was estimated from the onsets of the nocturnal elevation of plasma thyrotropin (TSH) and melatonin. Mean phase shifts of TSH onsets were -18 +/- 8 (baseline), -78 +/- 10 (low-intensity exercise, P < 0.01), and -95 +/- 19 min (high-intensity exercise, P < 0.01). Mean phase delays of melatonin onsets were -23 +/- 10 (baseline), -63 +/- 8 (low-intensity exercise, P < 0.04), and -55 +/- 15 min (high-intensity exercise, P < 0.12). Taken together with our previous findings, this study indicates that nocturnal physical activity may phase delay human circadian rhythms and demonstrates that phase-shifting effects may be determined with exercise durations and intensities compatible with the demands of a real-life setting.