The physiological condition of a person is determined in physiological condition determining system by using a sensor for sampling a plurality of heart beats of the person, and using a control unit to perform the acts of: extracting a series of cardiac R-R intervals from the heart beat samples, providing a two-dimensional representation of subsequent R-R intervals, wherein two subsequent R-R interval forms an entry in the two-dimensional representation resulting in a plurality of entries, determining a centroid, an average radius and an average rotation frequency for the plurality of entries in the two-dimensional representation, determining a plurality of distances between the radius and each of the entries in the two-dimensional representation, and determining the physiological condition of the person using the radius in combination with the plurality of distances.