Circadian rhythm modification in rats by whole-life synchrony to a 27-hour day

Resistance to modification is one of the primary characteristics of an organism's circadian rhythm, that spontaneous near-24-hr. biorhythm observed in the absence of daily environmental rhythms. Earlier studies attempting modification met with no success. Once the rhythms ran free of the adiurnal, or non-24-hr., environmental synchronizers they quickly readjusted to the original steady-state circadian period. The readjustment process which was completed within five to eight days consisted of a series of variable-length cycles, denoted "transients." These and related results have led investigators to attribute the origin of circadian timing to alleged innate, but presently undetermined, mechanisms.