TONIC AND PHASIC EFFECTS OF LIGHT IN THE ENTRAINMENT OF CIRCADIAN RHYTHMS

There is one specific area where the problems of circadian rhythms intertwine with those of tonic and phasic functions of sensory systems: in the entrainment of these rhythms by the natural cycle of light and darkness. Numerous factors in most animals' environment vary in a 24-hour pattern. In the course of evolution on an ever rotating planet, circadian rhythms in animal behavior and physiology may have developed ultimately as an adaptation to any or many of these variations. Sleep, for instance, is currently viewed as a behavioral strategy, immobilizing the organism at a time of day when being active would be inefficient in terms of energy gain and expenditure, or even hazardous owing to increased risk of predation.:' As an evolutionary response to environmental periodicities, innate temporal programs have developed in a wide variety of organisms. down to protozoans and algae. These rhythms include circadian variations in arousal, motivation, and performance. In constant conditions their frequency typically differs from (24 hours)-' and from any other known environmental periodicity; i t varies among individuals and among different genotypes. These facts sufficiently attest to their endogenous nature. No sensory input, whether tonic or phasic, is required to generate these rhythms. By the same token, circadian rhythms must contain facilities insuring their synchrony with environmental periodicity. Otherwise, they would obviously be useless. The most reliable time cue available in the environment is the daily cycle of light and darkness. Virtually all circadian rhythms studied turn out to be sensitive to the light-dark cycle i n the sense that they can be synchronized or "en/rained" by i t . ' The period of the organism's rhythm is corrected for day after day by smaller or larger shifts induced by the light. The mechanism of entrainment has received considerable interest. Yet, the problems are far from solved. One of the main obstacles has been our ignorance about both the photoreceptors involved and the sensory pathways to the circadian pacemakers. The very existence of such pacemakers has only recently been demonstrated. It is of tell-tale significance that in every case analyzed they are located close to photosensory organs. In cockroaches, circadian pacemakers controlling the activity rhythm were localized in the optic lobes,5 in silkmoths a clock in the protocerebrum times pupal cclosion," in the mollusc A p l ~ ~ s i n there is a pacemaker in the eyes,; in sparrows the light-sensitive pineal has a function in driving the bird's daily rhythm of activity and body temperature.'; In mammals there is increasing cvidcnce that the strprachiasmatic nuclei (SCN) in the hypothalnmus are responsible for a diversity of circadian rhythms.!', 1" A plausible interpretation of the facts available, on the one hand, is that the SCN serve as a master-pacemaker triggcring or synchronizing several physiological and bchavioral circadian rhythms, some of thesc probably via the hypothalamic-pituitary regulation centers. They thereby maintain the internal