Fasting blood sugar helps doctors understand how body regulates glucose without food triggers. Dr Vijay Negalur, HoD Diabetology, KIMS Hospitals said fasting blood sugar should be measured right after waking up, before eating or drinking anything other than water. "If you check 1 to 2 hours after eating...it becomes a 'postprandial' reading, which gives different information," Dr Vijay added.