Ontogeny of ultradian and nocturnal activity rhythms in the isolated albino rat

The activity of isolated rats from 6 to 25 days of age was analyzed at 10-min intervals. Ultradian peak activity rhythms, with a rough 1-2-hr periodicity, developed between 9 and 12 days of age, and reached an extreme at 15 days. Ultradian periods of attenuated activity were however detectable even in 6-day-olds. Isolated rats at 15 days of age displayed greatly accentuated profiles characterized not only by heightened activity levels, but by an extreme cyclicity. These subjects spent most of their time oscillating between periods of marked activity and marked inactivity. Both the average and total duration of these extreme periods were greater in 15-day-olds than any of the other ages. Free-running nocturnal activity rhythms were also observed to develop in isolated rats tested in continuous darkness by the 15th day.