ABSTRACT
When animals or people are kept in isolation from the normal cues of the day–night cycle, they wake up and go to sleep a little later each day. This demonstrates that there is an internal mechanism underlying the circadian rhythm – some sort of biological clock with a cycle of about 25 hours. Moore-Gillon and V. B. Eichler and F. K. Stephan and I. Zucker discovered that lesions in a small area at the base of the medial hypothalamus, the suprachiasmatic nuclei, abolish rats' circadian rhythms. The most obvious rhythm affecting us is the daily sleep–waking cycle. This is known as a circadian rhythm. Lesions in other parts of the basal hypothalamus can disrupt the basic rest–activity cycle, although it is not firmly established whether these are the locations of a clock or clocks, or pathways through which a mechanism elsewhere affects behavior.
