Blood sugar, also known as blood glucose, refers to the amount of sugar present in a person's blood. Glucose is the primary source of energy for the body's cells, and it comes from the foods we eat. The normal range for blood sugar varies depending on the time of day, when the person last ate, and their overall health status. Generally, normal blood sugar levels are: Fasting (before meals): 70 to 99 milligrams per deciliter (mg/dL) of blood After meals (1 to 2 hours after eating): less than 140 mg/dL of blood A high blood sugar level, called hyperglycemia, may indicate conditions such as diabetes, pancreatitis, or hormonal disorders. Hyperglycemia can also be caused by stress, infection, or certain medications. Symptoms of hyperglycemia may include increased thirst, frequent urination, blurred vision, and fatigue. A low blood sugar level, called hypoglycemia, may indicate conditions such as insulin overdose, liver or kidney disease, or hormonal disorders. Hypoglycemia can also be caused by not eating enough, exercising too much, or drinking alcohol. Symptoms of hypoglycemia may include shakiness, dizziness, sweating, confusion, and weakness.