Circadian rhythms are the approximate 24-h biological cycles that function to 
prepare an organism for daily environmental changes. They are driven by the 
molecular clock, a transcriptional:translational feedback mechanism that in 
mammals involves the core clock genes Bmal1, Clock, Per1/2, and Cry1/2. The 
molecular clock is present in virtually all cells of an organism. The central 
clock in the suprachiasmatic nucleus (SCN) has been well studied, but the clocks 
in the peripheral tissues, such as heart and skeletal muscle, have just begun to 
be investigated. Skeletal muscle is one of the largest organs in the body, 
comprising approximately 45% of total body mass. More than 2300 genes in 
skeletal muscle are expressed in a circadian pattern, and these genes 
participate in a wide range of functions, including myogenesis, transcription, 
and metabolism. The circadian rhythms of skeletal muscle can be entrained both 
indirectly through light input to the SCN and directly through time of feeding 
and activity. It is critical for the skeletal muscle molecular clock not only to 
be entrained to the environment but also to be in synchrony with rhythms of 
other tissues. When circadian rhythms are disrupted, the observed effects on 
skeletal muscle include fiber-type shifts, altered sarcomeric structure, reduced 
mitochondrial respiration, and impaired muscle function. Furthermore, there are 
detrimental effects on metabolic health, including impaired glucose tolerance 
and insulin sensitivity, which skeletal muscle likely contributes to considering 
it is a key metabolic tissue. These data indicate a critical role for skeletal 
muscle circadian rhythms for both muscle and systems health. Future research is 
needed to determine the mechanisms of molecular clock function in skeletal 
muscle, identify the means by which skeletal muscle entrainment occurs, and 
provide a stringent comparison of circadian gene expression across the diverse 
tissue system of skeletal muscle.