Circadian rhythms: Basic neurobiology and clinical applications

Circadian rhythms are major features of adaptation to our environment. Tn mammals, circadian rhythms are generated and regulated by a circadian timing system. This system consists of entrainment pathways, pacemakers, and pacemaker output to effector systems that are under circadian control. The primary entrainment pathway is the retinohypothalamic tract, which terminates in the circadian pacemakers, the suprachiasmatic nuclei of the hypothalamus. The output of the suprachiasmatic nuclei is principally to the hypothalamus, the midline thalamus, and the basal forebrain. This provides a temporal organization to the sleep-wake cycle, to many physiological and endocrine functions, and to psychomotor performance functions. Disorders of circadian timing primarily affect entrainment and pacemaker functions. The pineal hormone, melatonin, appears to be a promising agent for therapy of some circadian timing disorders.