Presence-absence cycles of the mother and not light-darkness are the zeitgeber for the circadian rhythm of newborn mice.

The relative roles of conflicting zeitgebers [presence/absence (PA) cycles versus light/dark (LD) cycles] on entrainment of circadian rhythm of locomotor activity were tested in pups of the night active mouse Mus booduga. During the early days of the pups' life the PA cycles of the mother acted as a zeitgeber and entrained their activity rhythm, even though the LD cycles were available. Entrainment by LD cycles took place only when the pups' eyes opened and probably became functional.