To elucidate the changes in the daily voriation seen in the autonomic nervous system during pregnancy brought about by maternity yoga and to evaluate how maternal yoga affects stress and sleep. A total of 38 women were in the yoga group,with 53 in the control group. At 28−31 weeks’ gestation, heart rate vanability during night and late−night periods was significantly higher in the yoga group than in the control group. At36−40 weeks' gestation, variability between the morning, afternoon and late night was significantly higher in the yoga group. Salivary α−amylase levels decreased significantly immediately after practicing yoga during all evaluation periods in the yoga group. Night−time sleep duration was significantly longet in the yoga group. Our results suggest that practicig yoga activates the parasympathetic nervous systm during the third trimester of pregnancy,consolidating sleep during the night and decreasing α−amylase levels, which indicates reduced stress.