The properties of the inner clock—its free-run under constant lighting or darkness, its entrained response to light-dark cycles—were initially understood at the level of oscillating “output” measures, without direct knowledge of the brain mechanisms that produced the oscillation. For example, behavior showed circadian cycles in running-wheel activity, feeding, and drinking, while similar cycles could be measured in body temperature and production of the hormone melatonin by the pineal gland.
In 1972, however, the anatomical location of the brain’s clock was finally identified by a set of experiments that homed in on a tiny cluster of neurons at the base of the brain–the suprachiasmatic nuclei (SCN)—just above the well-known pathways that carry light and dark signals from the eye. The pathway for vision separated from the pathway to the inner clock. Light signals to the SCN are imperceptible!