The Role of Circadian Clocks in Metabolism

Circadian rhythm refers to endogenous biological rhythm, which completes cyclic repetitions every 24 hours; the term originated from the Latin words circa, which means “around,” and dies, which means “day” [1,2]. Most organisms on the planet including human beings have evolved their own rhythmic programming of biological and behavioral outputs in response to these stimuli to maintain metabolic balance and promote survival [3,4]. Circadian rhythm helps our physiology to adapt to the environmental or behavioral changes in a time-of-day-dependent manner [5]. Beyond simply adapting to stimulus-response interaction, circadian rhythms are governed by an intrinsic molecular oscillator, the circadian clock [6]. Zeitgeber, literally meaning “time giver,” refers to any stimulus related to time cue from the external environment [1,6]. Our circadian clock constantly communicates with these zeitgebers to maintain circadian rhythm originally settled [1-6]. Without external stimulation, circadian clocks quite predictably control the daily timing of synthesis and release of most hormones [1,7]. In mammals, the circadian clock affects many aspects of the physiology including sleep-wake cycles, feeding-fasting cycles, and metabolic processes of the gastrointestinal tract, kidney, heart, and liver [8,9]. Previous epidemiological studies demonThe Role of Circadian Clocks in Metabolism