The D/L ratio, which expresses amount of activity during the dark part of the cycle over amount cf activity during the light part of this cycle can serve as a mesure of rhythm contrast: it indicates whether there is a sharp dichotomy between the activity phase and the resting phase. The drawback to this parameter is that it is highly responsive to phase. The solution is to define the active and resting phases in terms of a synchronizer type function which maximizes its intercorrelation with the target rhythm. This function can then be used to define the Active and Resting phases to compute a new, phase‐independant A/R index. Furtermore, the intercorrelation function value is also a measure the best fit of the rhythm to its synchronizer. An example of application of these methods is presented for inbred strains of mice C57BL/6 and BALB/c and their reciprocal Fls. Obtained heterosis effects indicate that the Fl rhythm fits the synchronizer better than the rhythm of the parental strains.