1. An apparatus useful in diagnosing sleep apnea in a human patient using only an electrocardiogram signal relating to the patient, the apparatus comprising: a computer circuit configured to: analyze the electrocardiogram signal; classify each time period in a set of time periods of the electrocardiogram signal as either apneic or normal; and provide a diagnostic measure of sleep apnea for the human patient based on classification results obtained by combining a plurality of results from the set of time periods.