The aim of this study was to determine whether the human anal sphincter responds dynamically to changing physiological states. In 19 healthy human subjects, intraluminal anal canal pressure was measured with a 5-cm perfused sleeve sensor during the day while fasting (3 hours) and after feeding (3 hours) and at night during sleep (8 hours). Daytime mean anal canal pressures (+/- SEM) while fasting (50 +/- 3 mm Hg) were similar to those after feeding (49 +/- 3 mm Hg) and to those at night during sleep (49 +/- 3 mm Hg). Marked minute-to-minute variations in mean pressure occurred in all three periods, however, as did large phasic increases and decreases in pressure (greater than 20 mm Hg) and small phasic changes in pressure less than 20 mm Hg (anal slow waves). The minute-to-minute variations in mean pressure were greater during the awake fed state (4 +/- 1 mm Hg/min) than at night during sleep (2 +/- 1 mm Hg/min; P less than 0.03), as were the number of large phasic waves per minute (increases in pressure: awake, fed = 0.5 +/- 1 waves/min, night = 0.3 +/- 0.1 waves/min, P less than 0.05; decreases in pressure: awake, fed = 0.4 +/- 0.1 waves/min, night = 0.2 +/- 0.1 waves/min, P less than 0.05). Anal small waves had a similar frequency of about 17 waves/min in all three states. In conclusion, the anal sphincter maintains a continuous pressure barrier to rectal outflow both during the day and at night during sleep. However, marked minute-to-minute variations in mean pressure and large phasic increases and decreases in pressure do occur. Both are fewer at night during sleep.