Systems, methods, apparatus, and computer program products for detecting the existence of a sleep disorder in an individual using heart activity. In one aspect, machine-implemented methods include the actions of analyzing a machine-readable heart rate series of a monitored individual in the time domain using one or more digital data processing devices, detecting a cyclic variation in heart rate in the heart rate series as a result of the analysis in the time domain, and outputting, over an output, a report characterizing a sleep disorder event based on the detection of the cyclic variation in heart rate in the heart rate series. The cyclic variation in heart rate is indicative of a sleep disorder.
