Effects of maternal isolation on the development of activity rhythms in infant rats.

Motor activity of infant rats was measured continuously from day 3 to day 18 of post-natal age. Mother reared pups maintained on a 12 hour light/12 hour dark cycle (MRLD) show a clear predominance of nocturnal activity on day 31 post conception (post-natal day 9 or 10), 4 days before eye opening. Nocturnal partitioning increases progressively to the time of weaning. Rats reared artificially and in isolation from maternal and sibling contact but in the same light/dark regimen (ARLD) fail to develop clear nocturnal partitioning of motor activity.