Insomnia, ihn SOM nee uh, is the inability to sleep naturally. Nonmedical people often speak of it as though it were an illness in itself. Doctors, however, know that insomnia is a symptom (sign of an illness) that may result from any one of a number of causes. Pain or discomfort from a physical illness will interfere with sleep. Coffee or other stimulants to the nervous system can cause insomnia. Certain drugs affect the brain in such a way as to produce wakefulness. Some serious sleep diseases, such as sleep apnea, can also produce insomnia.