Updated project metadata. Mitochondria are major suppliers of cellular energy through nutrients oxidation. Little is known about the mechanisms that enable mitochondria to cope with changes in nutrient supply and energy demand that naturally occur throughout the day. To address this question, we applied mass-spectrometry based quantitative proteomics on isolated mitochondria from mice sacrificed throughout the day and identified extensive oscillations in the mitochondrial proteome. We found that rate-limiting mitochondrial enzymes that process lipids and carbohydrates accumulate in a diurnal manner and dependent on the clock proteins PER1/2. In this conjuncture, we uncovered daily oscillations in mitochondrial respiration that peak during different times of the day in response to different nutrients. Notably, the diurnal regulation of mitochondrial respiration was blunted in mice lacking PER1/2 or upon nutritional challenge such as high fat diet. We propose that the clock proteins PER1/2 optimize mitochondrial metabolism to daily changes in energy supply/demand and thereby serve as a rheostat for mitochondrial nutrient utilization.