Several epidemiological studies suggest a correlation between eating time and obesity. Night eating syndrome characterized by a time-delayed eating pattern is positively associated with obesity in humans as well as in experimental animals. Here, we show that oil intake at night significantly makes more fat than that at day in wild-type mice, and circadianPeriod 1(Per1) contributes to this day–night difference.Per1-knockout mice are protected from high-fat diet–induced obesity, which is accompanied by a reduction in the size of the bile acid pool, and the oral administration of bile acids restores fat absorption and accumulation. We identify that PER1 directly binds to the major hepatic enzymes involved in bile acid synthesis such as cholesterol 7alpha-hydroxylase and sterol 12alpha-hydroxylase. A biosynthesis rhythm of bile acids is accompanied by the activity and instability of bile acid synthases with PER1/PKA-mediated phosphorylation pathways. Both fasting and high fat stress enhancePer1expression, increasing the fat absorption and accumulation. Our findings reveal thatPer1is an energy regulator and controls daily fat absorption and accumulation. CircadianPer1controls daily fat absorption and accumulation, suggestingPer1is a potential candidate of a key regulator in stress response and the relevant obesity risk.