The photoperiodic effect of ahemeral light-dark cycles which entrain circadian rhythms.

1. A 27-h cycle of light and dark provides a greater gonadotrophic stimulus to the laying fowl, as judged by sexual maturity and rate of lay, than a 24-h cycle incorporating the same photoperiod. 2. An hypothesis put forward to account for these effects states that Effective Photoperiod equal p + c - b, where p = actual photoperiod, c = cycle length and b = the period of the endogenous biological clock. 3. Two experiments designed to test this hypothesis have yielded results which are consistent with it. 4. A poultryman who use an ahemeral cycle to alter egg weight or shell thickness and then whishes to transfer his flock back to a 24-h cycle should calculate the difference between the two cycle lengths and then add this quantity to the prevailing photoperiod to find the appropriate amount of light to be used in the 24-h cycle.