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ObjectiveTo assess the effects of a 12-week Tai Chi exercise program on sleep using the sleep spectrogram, a method based on a single channel electrocardiogram (ECG)-derived estimation of cardiopulmonary coupling, previously shown to identify stable and unstable sleep states. Methods We retrospectively analyzed 24-h continuous ECG data obtained in a clinical trial of Tai Chi exercise in patients with heart failure. Eighteen patients with chronic stable heart failure, left ventricular ejection fraction ⩽40% (mean [±standard deviation] age, 59±14 years, mean baseline ejection fraction 24%±8%, mean) were randomly assigned to receive usual care (N=10), which included pharmacological therapy and dietary and exercise counseling, or 12 weeks of Tai Chi training (N=8) in addition to usual care. Using the ECG-based sleep spectrogram, we compared intervention and control groups by evaluating baseline and 12-week high (stable) and low (unstable) frequency coupling (HFC & LFC, respectively) as a percentage of estimated total sleep time (ETST). Results At 12 weeks, those who participated in Tai Chi showed a significant increase in HFC (+0.05±0.10 vs. −0.06±0.09 % ETST, p=0.04) and significant reduction in LFC (−0.09±0.09 vs. +0.13±0.13 % ETST, p<0.01), compared to patients in the control group. Correlations were seen between improved sleep stability and better disease-specific quality of life. Conclusions Tai Chi exercise may enhance sleep stability in patients with chronic heart failure. This sleep effect may have a beneficial impact on blood pressure, arrhythmogenesis and quality of life.