Sudden Rise in Blood Sugars What Are the Blood Sugar Levels for Diabetes? 4. Feeling Tired After a Meal? 5. What is a High Blood Sugar Level Range? Even under normal conditions, blood sugar levels vary throughout the day, but a sudden rise in blood sugar can be alarming. Stress or illness may cause blood sugar levels to rise suddenly. If you have diabetes, a sudden rise may involve diet or medication. Knowing the cause can help you better manage blood sugar and improve overall health. Normal blood sugar measures 70 to 99 milligrams per deciliter after fasting, or less than 140 milligrams per deciliter two hours after eating. High blood sugar occurs when your body either doesn't make enough insulin or can't properly use the insulin it does make, and is most often associated with diabetes. A fasting blood sugar of 126 milligrams per deciliter or a random blood sugar reading of 200 milligrams per deciliter indicates high blood sugar. Frequent urination, increased thirst, weight loss or fatigue are signs that your blood sugar levels may be high. If left uncontrolled, high blood sugar leads to ketoacidosis, also referred to as a diabetic coma, which is when your body breaks down fat into ketones for fuel and the ketones build up in the blood. If you have diabetes, a sudden rise in blood sugar may be related to your diet. When you eat carbohydrate foods, which include starches, fruit, milk and sweets, your body breaks them down into sugar. If you're eating too many carbs at a meal, your blood sugars may rise rapidly. Or if you're eating carbs that digest quickly, referred to as high-glycemic foods, you may see a sudden rise in blood sugar. High blood sugar levels after eating a meal or snack is called postprandial hyperglycemia. Between the hours of 4 and 5 a.m., most people, whether they have diabetes or not, have a surge in hormones called the "dawn phenomenon.