Science proves that people burn less fat when they sleep during the day and are active at night. A group of researchers from the University of Colorado studied 14 healthy people for 6 days.
During the first 2 days, subjects slept during the night and didn't have daytime naps. Then they changed their sleeping patterns to imitate owls' sleeping schedules. It turned out that when people took a nap, their metabolism worsened since their biological clocks didn't completely flip to fit their schedules.