Melatonin accelerates reentrainment of circadian locomotor activity rhythms to new light-dark cycles in the rat.

We examined whether continuous melatonin administration through subcutaneously implanted silastic tubing accelerates reentrainment of circadian rhythms of locomotor activity to shifted illumination cycles. We found that rats required less time to reentrain to an advanced or delayed phase shift of a light-dark cycle while carrying a silastic implant filled with melatonin than while carrying an empty implant. These results suggest that continuous administration of melatonin accelerates the reentrainment of the circadian locomotor activity rhythm to a new light-dark cycle.