American black bears survive winter months without food and water while in a mildly hypothermic, hypometabolic, and inactive state, yet they appear to be able to return to near-normal systemic function within minutes of arousal. This study's goal was to characterize the cardiovascular performance of overwintering black bears and elicit the underlying mechanisms enabling survival. Mid-winter cardiac electrophysiology was assessed in four wild black bears using implanted data recorders. Paired data from early and late winter were collected from 37 wild bears, which were anesthetized and temporarily removed from their dens to record cardiac electrophysiological parameters (12-lead electrocardiograms) and cardiac dimensional changes (echocardiography). Left ventricular thickness, primary cardiac electrophysiological parameters, and cardiovascular response to threats ("fight or flight" response) were preserved throughout winter. Dramatic respiratory sinus arrhythmias were recorded (cardiac cycle length variations up to 865%) with long sinus pauses between breaths (up to 13 s). The accelerated heart rate during breathing efficiently transports oxygen, with the heart "resting" between breaths to minimize energy usage. This adaptive cardiac physiology may have broad implications for human medicine.