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A comparison between the theoretical backgrounds of the Cosinor Test and the periodogram method suggested that in many cases the use of the periodogram method is preferable and safer (Van Cauter and Huyberechts, 1972). This work refers particularly to the analysis of multifrequential phenomena, which are commonly encountered in biology and medicine. Statistical analysis of computer‐generated time‐series and 24‐hr cortisol paterns is performed by both methods; comparison between the obtained results strongly support our previous theoretical conclusions. The periodogram method appears to be particularly well‐suited to period estimation in biological data; in some cases, simple additional compulations may improve the approximation of the period.

Classical spectral analysis is usually badly suited for the detection of periodicities in biological or biomedical time series because of the shortness and low density of the series of data. Mathematical study of these series thus requires particular methods adapted to the nature of the data. The well‐known Cosinor test and the periodogram method are compared from that point of view. An extensive description of both methods is given. Comparisons between assumptions, application fields, estimates provided, reliability of the results are made. The Cosinor test requires more initial assumptions, appears as an extremely particular case of the periodogram method, and may lead to wrong interpretation of the data. The periodogram procedure is more profitable and safer but involves more computations. The Cosinor test may avoid unnecessary calculations when the phenomenon has one single, a priori known, natural frequency.