1. A method of analyzing a ballistocardiogram, comprising: selecting a time window of the ballistocardiogram, said time window comprising a cardiac component signal incorporating a plurality of heart beats; defining a reference energy content for said time window; calculating an actual energy content for the cardiac component signal in said time window; comparing the actual energy content with the reference energy content; and generating a further signal when a difference between the actual energy content and the reference energy content exceeds a predefined threshold.