Brain metabolism depends primarily on glucose as energetic material. States that lead to decreased blood glucose levels can cause a syndrome, or group of symptoms, that is a reaction to that phenomenon. This syndrome is called hypoglycemia.
Hypoglycemia is usually divided into "relative hypoglycemia" and "functional hypoglycemia." Relative hypoglycemia refers to hypoglycemia that is caused by external influences, like diet and medication use. This type is more amenable to management or cure. Functional hypoglycemia refers to hypoglycemia that is caused by a malfunction, possibly metabolic, within the sufferer. This type is harder to manage. Hypoglycemia's symptoms can include fatigue, depression, nervousness, anxiety, constant hunger, cold sweats, cold hands or feet, confusion, delirium, dizziness, fainting, trembling, headaches, muscle pain, memory loss, and insomnia. In severe cases the symptoms can resemble various mental illnesses; when left untreated the condition can even induce coma.
Some members of the medical community believe that functional hypoglycemia does not actually exist, as a disease, at all.