1. A method for determining a physiological condition of a person, the method operating in a physiological condition determining system, the method comprising: using a sensor of the physiological condition determining system for sampling a plurality of heart beats of the person; extracting in a control unit of the physiological condition determining system a series of cardiac R-R intervals from the heart beat samples; and using the control unit for providing a two-dimensional representation of subsequent R-R intervals, wherein two subsequent R-R intervals form an entry in the two-dimensional representation resulting in a plurality of entries in the two-dimensional representation, determining in the control unit a centroid, for the plurality of entries in the two-dimensional representation; fitting in the control unit a closed curve through the plurality of entries in the two-dimensional representation, wherein a center of said closed curve coincides with said centroid determining in the control unit distances between the closed curve and the plurality of entries in the two-dimensional representation; and determining in the control unit the physiological condition of the person by determining if the determined distances are above or below a predetermined first threshold.