The ontogeny of daily rhythms in body temperature (T(b)) oxygen intake (VO(2)) and urinary excretion of the major melatonin metabolite, 6-sulfatoxymelatonin (6SMT) was studied in the day-active rodent, Psammomys obesus. Generally, T(b) and VO2 were high during the light phase in this diurnal species. However, after weaning, and only under the short photoperiod, P. obesus individuals display elevated T(b) and VO2 levels during the dark phase, as in nocturnally active species. In parallel, 6SMT and nocturnal activity of pineal arylalkylamine N-acetyltransferase (AANAT) were greatly enhanced. The cDNA encoding P. obesus pineal AANAT was cloned and found to share 90.2% homology with rat and 83.8% with human AANAT, and based on homology modeling, to structurally resemble the ovine enzyme. A robust diurnal rhythm in P. obesus pineal AANAT-mRNA was found, with maximal levels at night. AANAT-mRNA levels were not enhanced in the post-weaning phase, suggesting post-transcriptional up-regulation of pineal AANAT activity. The photoperiod-dependent post-weaning change into nocturnal behavior and up-regulation melatonin production (as evidenced from the increase in both 6SMT and AANAT activity) represent a hitherto unobserved pattern of transition of a diurnal mammal into independent life. Possibly, this pattern may be physiologically important to facilitate T(b) maintenance in the cold nights of winter in the desert.