Exposure to continuous light (LL) results in elongation of the free-running period of the rat circadian rhythm. However, the LL may not always mean constant intensity of light for the suprachiasmatic nucleus (SCN), since rats may regulate contrast of illumination by their eyelids which are closed during sleep phase. The removal of eyelids caused shortening of the free-running period or arrhythmicity under LL. In addition, implantation of carbachol--acetylcholine agonist--near the SCN also resulted in similar effects.