Before revision:  Our calculation demonstrates that every circadian clock with a dead zone is optimally adapted to the daylight cycle . Our result also explains the lack of a dead zone in oscillators of mammalian somatic cells, and justifies more effective entrainment by dawn and dusk than by on/off lighting protocols in animals including humans .
After revision:  Our calculation demonstrates that every circadian clock with a dead zone is optimally adapted to the daylight cycle .