On weekdays, the average American wakes up at 6:10 a.m. But on weekends, that rise-and-shine time shifts to 7:33 a.m., according to the National Sleep Foundation. That hour-and-a-half difference may not seem like a big deal, but it could impact the size of your PJ pants: Women who don't wake up at the same time every day tend to have more body fat, new Brigham Young University research reveals.