Thermoperiod and photoperiod interact to affect the phase of the plasma melatonin rhythm in the lizard, Tiliqua rugosa.

Rhythms of plasma melatonin levels were determined in lizards (Tiliqua rugosa) subjected to 6 h thermocycles (6 h, 33 degrees C thermophase; 18 h, 15 degrees C cryophase) placed at 4 different phases of a 12 h photocycle (12 h light: 12 h dark). The peak of the melatonin rhythm was either shifted at different rates, or inhibited by the light phase of the photocycle, depending upon the phase relationship between the thermocycle and the photocycle. The results indicate that the pineal organ of ectotherms is part of a circadian pacemaker system, transducing photothermal environmental information into a neurochemical signal.