Because sleep needs vary from person to person, insomnia is defined as the chronic inability to obtain the amount of sleep needed for optimal functioning and well-being. Insomnia, which is a symptom rather than a disease, can be classified into three main etiologic groups: insomnias related to other mental disorders (for example, depression and anxiety), insomnias related to known organic factors (for example, sleep apnea and "nonrestorative" sleep), and primary insomnia (for example, learned psychophysiologic insomnias and insomnia complaints without objective findings). The treatment for insomnia often involves a combination of pharmacotherapy, behavioral and short-term psychotherapy, and sleep hygiene guidelines. Sleep disorders centers can provide specialized knowledge and techniques for patients with severe chronic insomnia.