The researchers tracked 330 young women's sleep habits and found that those who regularly changed their wake-up time were also carrying the most lard. The sleepers with the greatest variability in their schedule—say, getting up at 6 a.m. one day and 8 a.m. the next—had 30.6 percent body fat, compared to 24.7 percent among those with the most consistent sleep schedules (less than 30 minutes variability).