• suprachiasmatic nucleus;
  • hypothalamus;
  • feeding;
  • reproduction;
  • sleep/wake

In the course of evolution, mechanisms have evolved to anticipate the timing of regularly occurring events. These mechanisms are encompassed in a circadian timing system that include a master clock localized to the suprachiasmatic nucleus of the hypothalamus and “slave” oscillators distributed throughout the body. This system serves multiple functions so as to ensure that various physiological processes occur at optimal and nonoverlapping times, to synchronize our activities to local environmental time, and to permit changes required to respond to new environmental circumstances. We suggest that a generalized concept of arousal (which includes alterations in responsiveness to homeostatic pressures, sensory stimuli and emotional reactivity, and to changes in motor activity) serves as a rubric in which to explore the multiple ways in which the circadian system modulates behavior.