1. A method comprising: monitoring at least one physiological parameter of a patient via a medical device that delivers a chronic pain therapy to the patient; determining a value of a metric that is indicative of sleep quality based on the at least one physiological parameter; and controlling delivery of the chronic pain therapy by the medical device based on the sleep quality metric value, wherein monitoring at least one physiological parameter comprises monitoring at least one of electrocardiogram morphology, subcutaneous temperature, muscular tone, electrical activity of a brain of the patient or eye motion.