1. A method for characterizing autonomic nervous system activity of a patient comprising: (a) acquiring electrocardiographic data from the patient with an electrocardiographic device; (b) determining from the electrocardiographic data obtained in step (a) a plurality of time intervals between successive R-waves; (c) analyzing the plurality of time intervals by frequency domain analysis so as to quantitate the following parameters: i) power of high frequency (P HF ); ii) power of low frequency 1 (P LF1 ) (0.07 to 0.15 Hz); iii) power of low frequency 2 (P LF2 ) (0.033 to 0.07 Hz); iv) difference between power of low frequency 1 and high frequency (P LF1 - P HF ); v) maximum amplitude of spectral function in a low frequency band (S LFmax ); vi) frequency at a point of maximum amplitude of spectral function in the low frequency band (F LFmax ); vii) maximum amplitude of spectral function in a high frequency band (S HFmax ); viii) frequency at a point of maximum amplitude of spectral function in the high frequency band (F HFmax ); ix) ratio of power of high frequency to power of low frequency 1 , (P HF /P LF1 ) ; x) ratio of power of high frequency to power of low frequency (sum of powers of low frequency 1 and low frequency 2 ), (P HF /(P LF1 + P LF2 )); xi) heart rate (HR); and, if a spectral peak falls in the range of a middle frequency band, xii) (S MF ) maximum amplitude of spectral function in a middle frequency band; and (d) applying a clusterization algorithm to the parameter values quantitated in step (c) so as to determine two further parameters, one of which correlates with sympathetic nervous system activity and the other of which correlates with parasympathetic nervous system activity, wherein parasympathetic nervous system activity is determined by applying the clusterization algorithm to parameters (c)(i) to (c)(iv) and wherein sympathetic nervous system activity is determined by applying the clusterization algorithm to parameters (c)(ii), (c)(iii), and (c)(v) to (c)(xii), so as to thereby characterize the autonomic nervous system activity of the patient.