Techniques are described for use by a pacemaker or implantable cardioverter/defibrillator (ICD) or other implantable medical device. The techniques are provided for evaluating the likelihood that the patient, in which the device is implanted, has impaired glucose tolerance (IGT) or diabetes mellitus. Briefly, a value representative of sleep quality of the patient is detected and then the likelihood that the patient has IGT or diabetes mellitus is determined based on the sleep quality value. In this regard, it has been found that a decrease in overall sleep quality is associated with an increased likelihood of IGT or diabetes mellitus, which is in turn associated with an increased risk of mortality. Hence, sleep quality may be used as a proxy for evaluating the likelihood that the patient has IGT or diabetes mellitus and for assessing associated mortality risk.