Figure 1.

The circadian clock actions on physiology and health of livestock. Circadian rhythm plays a critical role in livestock’s growth performance, reproduction, metabolism, and the quality of their products. The core mechanism, the transcription–translation–feedback-loop (TTFL) drives the diurnal oscillations to maintain circadian activities. In which CLOCK and BMAL1 directly control the transcription of PER and CRY genes. Whilst PER and CRY proteins can repress their modulator, CLOCK and BMAL1-stimulated transcription occurs in a negative feedback loop, allowing the cycle to begin anew when PER and CRY actions are turned-over. Importantly, REV-ERBs and RORs dominate the BMAL1 gene expression, are the primary players of the interlocked loop. In particular, hormones reach a peak level in the morning. Synthesis and release of melatonin is stimulated in the dark at night, while it is suppressed by light during the day. Loss of function of the circadian genes results in behavioral arrhythmicity, the disruption of the autoregulatory loop, and short period-length phenotypes.