Reference phases and circadian rhythms

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Abstract
The choice of an appropriate phase of reference (the reference phase) is important in determining (i) τ characteristics, (ii) α: p ratios and (iii) ? ϕ (phase shifts). Earlier authors have variously favoured the peaks (or troughs), onset, mid‐point or end of activity, center of gravity of the activity period or median values, as reference phases. We report on our work on the activity rhythms of microchiropteran bats in the tropics under field, seminatural and laboratory conditions. Facts and figures have been specially marshalled to show that the greater precision exhibited by bats at emergence and the lability characterizing their end of activity in the field, are also mirrored in the actograms obtained in the laboratory. We, therefore, favour the “onset” of activity as reference value. The onset of activity (i) is often very stable and may thus reflect the state of the basic oscillator better, (ii)unlike mid point it does not depend on the end of activity, (iii) and may be more directly under the clock control than the end which may be decided de novo day to day depending on hunger, satiation, availability of prey, predator pressure and meteorological conditions.