When blood sugar levels drop your level of "willpower" may plummet as well, researchers say, especially for people who are already overweight. A study that used brain-imaging scans found that the prefrontal cortex--the area of the brain that regulates impulses and emotions--showed a reduced ability to control desire for sugary foods when blood glucose was low. The study also showed that the stress hormone cortisol played a greater part in triggering the reward center of the brain. Researchers...