The peripheral circadian clock is entrained by factors in the external environment such as scheduled feeding, exercise, and mental and physical stresses. In addition, recent studies in mice demonstrated that some food components have the potential to control the peripheral circadian clock during scheduled feeding, although information about these components remains limited. l-Ornithine is a type of non-protein amino acid that is present in foods and has been reported to have various physiological functions. In human trials, for example, l-ornithine intake improved a subjective index of sleep quality. Here we demonstrate, using an in vivo monitoring system, that repeated oral administration of l-ornithine at an early inactive period in mice induced a phase advance in the rhythm of PER2 expression. By contrast, l-ornithine administration to mouse embryonic fibroblasts did not affect the expression of PER2, indicating that l-ornithine indirectly alters the phase of PER2. l-Ornithine also increased plasma levels of insulin, glucose and glucagon-like peptide-1 alongside mPer2 expression, suggesting that it exerts its effects probably via insulin secretion. Collectively, these findings demonstrate that l-ornithine affects peripheral clock gene expression and may expand the possibilities of L-ornithine as a health food.