When insulin is either lacking, or the body does not react to the insulin that is there, hyperglycemia occurs, causing blood sugar levels to become too high. It develops if a person with diabetes does not get it treated sufficiently. If you have diabetes and your fasting blood sugar exceeds 125 mg/dL (milligrammes per deciliter), you have hyperglycemia. Fasting is defined as not eating for at least eight hours.