Objective: To 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.