Provided is a clock gene expression level modifier having extensive history of use as a food, high safety, ready availability, and exceptional processability. A clock gene expression level modifier having as an active ingredient one or more compounds selected from the group comprising oleuropein, 3,4-DHPEA-EA, derivatives thereof, and salts thereof. The clock gene expression level modifier of the present invention makes it possible to adjust the expression levels of the Per1 gene and Bmal1 gene to the opposite phase and thereby tune the expression rhythm of the Per genes and Bmal genes to the normal phase.
