ABSTRACT

INTRODUCTION As the earth spins on its axis once a day, and completes an orbit around the sun within a year, most regions experience changes in environmental lighting conditions, and major temporal structures are created. This includes the day-night cycle, the lunar cycle and the seasons. The regular alternation between light and dark, and the seasonal changes in their relative durations, impose major constraints on living organisms. This has led to the key evolutionary feature of predictive adaptation to the cycles. These adaptive mechanisms have evolved to promote survival and reproduction, utilize the solar cycle, and are termed “circadian rhythms” (circa about, diem a day). The most evident manifestation of human adaptation to day/ night alternation is the diurnal temporal organization of behavior into periods of wakefulness during the day and sleep during the night. These rhythms are regulated by the circadian timing system, and depend on genetically controlled rhythmic molecular events that control a wide range of rhythms in cellular, systemic, and behavioral processes. The circadian timing system is a fundamental homeostatic process that powerfully influences human behavior and physiology throughout development. The present chapter reviews information regarding the basic neurobiology of the circadian timing system, its ontogenesis, circadian disorders commonly seen in children and adolescents along with their comorbidities, and treatment of these sleep disorders in youth.