Symptoms of high blood sugar (hyperglycemia) include increased thirst, frequent urination, dry skin, hunger, blurry vision, sleepiness, and slow healing of injuries or infections. High blood sugar is caused by too much food, too little insulin or diabetes pills, illness, or stress and can lead to a medical emergency if not treated. If blood sugar levels are higher than the goal for three days and the cause is unknown, one should call their doctor.