Systems and methods for evaluating a patient condition using autonomic balance information involve providing an implantable cardiac device that acquires a cardiac waveform from a patient. One or more characteristics associated with autonomic balance of the patient are detected and used to evaluate a patient condition, such as sleep onset, sleep stage, cardiac vulnerability over a predetermined duration, and sleep disordered breathing. Patient activity levels may be sensed and used to evaluate the patient's condition, such as for determining a level of systemic stress. Characteristics associated with the autonomic balance include calculating an LF/HF ratio waveform and/or determining one or more morphological features of the LF/HF ratio waveform. Coordination with a patient-external device may facilitate transmission of information about one or more of the cardiac waveform, the one or more characteristics associated with the autonomic balance, and a marked cardiac waveform.