Fundamentals of human chronobiology

Chronobiology is the study of biological temporal rhythms, including the circadian, weekly, seasonal, and annual rhythms. The term “circadian” comes from the Latin words “circa”, meaning “around”, and “dies [-ēi]”, meaning "day". A circadian rhythm can be defined as any biological process that manifests an endogenous entrainable oscillation of about 24 hours. Circadian rhythm affects physiology, behavior, cognition, and the sleep-wake cycle in mammalians and humans. The master clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus has a central role in circadian rhythm preservation. The human circadian time-keeping system is characterized by a composite architecture, with the central brain’s SCN pacemaker and subsidiary clocks in nearly every cell. The sleep-wake cycle is a complex and dynamic phenomenon involving numerous cerebral structures, neuronal network, and neurotransmitters. Sleep is generally divided into non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM and REM phases occur in the course of the sleep with cyclicity. Each phase has typical characteristics, including variations in brain wave patterns, eye movement type, and muscle tone. This chapter provides a general overview of the human circadian-generating systems and sleep physiology.