And one way of classifying animals, based upon when they are most active, is as nocturnal , diurnal or crepuscular . This 18h cycle forces them to phase-advance their cycle by 6h in every 24h cycle. One test is to place animal in a constant environment , one in which presumably all external cues have been eliminated. When this was done the cyclic behavior still persisted. But the cycle drifted iv.
Behavioral biologists refer to the cycle as having become free running .
That is the rhythm, while continuing, has become uncoupled from the rhythmic cues of the environment. On Sept. 14 he was informed that the experiment was over at which time he thought it was August 20 . Isolation experiments lent support to endogenous control of rhythmicity. If a circadian rhythm that is persisting under constant conditions could be timed by some uncontrolled external stimulus related to the earth s cycle of rotation, then transporting the animal longitudinally around the earth should immediately reset its rhythm to that of the new location. Support for endogenicity.