Glucose is the basic fuel used by the cells of the body to produce energy. Although the body stores some glucose as glycogen and can use fat and even protein for energy production, we largely depend on a constant glucose supply from food. As the blood glucose levels drop to lower levels, a person feels tired, irritable, sleepy and has difficulty concentrating.
Normally the body keeps the glucose levels in the blood within a narrow range. This ensures that blood glucose levels does not drop to dangerously low levels in a healthy person. But even at lower levels within the normal range, a person may experience symptoms that we tend to attribute to hunger and low blood glucose levels.