A system comprising: an electrocardiograph configured to generate an electrocardiogram of a monitored individual, a tangible data storage device encoding one or more modules of computer program instructions which, when executed by a data processing device, cause that the data processing device implements a method, said method comprising: analyzing a heart rate series derived from the electrocardiogram of the monitored individual in the time domain, using the data processing device; establishing cyclic variation in heart rate indices from the heart rate series; establishing tachycardia indices from the heart rate series; and establishing bradycardia indices from the heart rate series; wherein the step of establishing cyclic variation in heart rate indices includes auto-correlating the heart rate series to detect a cyclic variation in the heart rate which is indicative of a sleep disorder; a data processing device configured to implement the one or more modules of computer program instructions encoded on the tangible data storage device, and configured to detect obstructive sleep apnea of the monitored individual based exclusively on the electrocardiogram, by determining that obstructive sleep apnea is indicated based on tachycardia indices indicating that tachycardia is not present, bradycardia indices indicating that bradycardia is present, and cyclic variation in heart rate indices indicating that cyclic variation in heart rate is present; and a transmitter configured to report information characterizing the physiological condition of the monitored individual during the detected obstructive sleep apnea. an electrocardiograph configured to generate an electrocardiogram of a monitored individual, a tangible data storage device encoding one or more modules of computer program instructions which, when executed by a data processing device, cause that the data processing device implements a method, said method comprising: analyzing a heart rate series derived from the electrocardiogram of the monitored individual in the time domain, using the data processing device; establishing cyclic variation in heart rate indices from the heart rate series; establishing tachycardia indices from the heart rate series; and establishing bradycardia indices from the heart rate series; analyzing a heart rate series derived from the electrocardiogram of the monitored individual in the time domain, using the data processing device; establishing cyclic variation in heart rate indices from the heart rate series; establishing tachycardia indices from the heart rate series; and establishing bradycardia indices from the heart rate series; wherein the step of establishing cyclic variation in heart rate indices includes auto-correlating the heart rate series to detect a cyclic variation in the heart rate which is indicative of a sleep disorder; a data processing device configured to implement the one or more modules of computer program instructions encoded on the tangible data storage device, and configured to detect obstructive sleep apnea of the monitored individual based exclusively on the electrocardiogram, by determining that obstructive sleep apnea is indicated based on tachycardia indices indicating that tachycardia is not present, bradycardia indices indicating that bradycardia is present, and cyclic variation in heart rate indices indicating that cyclic variation in heart rate is present; and a transmitter configured to report information characterizing the physiological condition of the monitored individual during the detected obstructive sleep apnea.