Levels of cortisol, a stress hormone released by the adrenal glands, are meant to peak and dip at different times throughout the day: They're highest in the morning and decline throughout the day, hitting their lowest point around 3 a.m., Moskovitz says. “However, for the chronically stressed , or those in constant over-drive, stress levels stay elevated in the [evening], and well into the night," she says. When cortisol levels are high when they should be low, fat cell production speeds up significantly, which can lead to weight gain, a study from the Stanford University School of medicine found.