I find lots of articles prior to 1990 reporting experiments turning off lights in animal lab so that the animal is plunged into darkness, then measuring the intrinsic circadian rhythm (ITR) by activity or some other physiologic event. Then those papers all but disappeared. Did the value of the ITR prove to be unstable, or the assumption invalid, or did the conversation simply move on to other subjects? Humans' ITR of about 25 hours certainly seems to be valid and specific. The exceptions are rare enough to prove rather than challenge the rule.