Glucose (a type of sugar) is the body's main energy source. Hormones, such as insulin, control the level of glucose in the blood. It's unhealthy if a person's glucose levels get too high, or too low. Hypoglycemia occurs when the levels get too low. This can happen to people who have diabetes. It can occur if the person doesn't eat enough or if the person takes too much insulin, which lowers glucose levels. A person with hypoglycemia may feel hungry, shaky, sweaty, weak, drowsy, or dizzy. If left untreated, hypoglycemia may even make someone faint or pass out.