Involvement of a photoreversible process in the circadian clock controlling silkmoth eclosion

1. A preliminary analysis of the silkmoth eclosion clock demonstrated that the endogenous, free-running cycle of the clock could be divided into two parts: an initial synchronization period and a dark decay period. 2. A 1/2 hour light pulse applied during the synchronization period served to restart the free-running cycle. By proper timing of light pulses, a given cycle of the eclosion clock could be restarted ad infinitum. 3. When the length of the interrupting pulse was progressively increased up to 24 hours, the subsequent eclosion rhythms showed a constant phase relationship to the end of the interruption, irrespective of its duration. This phase relationship was the same as observed when the eclosion clock was started by transfer of moths from continuous light to continuous darkness. 4. It is concluded that the synchronization period involves a photoreversible process. During this period light rapidly stops the clock and the cycle can begin only with the onset of darkness.