the mechanism within an organism that controls the periodicity of biological rhythms, including activity rhythms, even in the absence of any external cues. For example, a bird housed in constant light will continue to show patterns of activity and rest with approximately 24-hour rhythms due to the action of the biological clock. A biological clock in mammals is located in the suprachiasmatic nucleus of the hypothalamus. Molecular mechanisms of the circadian rhythm are the same in insects and mammals. Also called internal clock.