Insomnia, the most commonly reported sleep problem, affects about one third of the adult American population. Insomnia impairs the individual's functioning, diminishes his or her quality of life, and contributes to accidents, health care costs, and problems at work. Insomnia is frequently comorbid with physical and mental illness, as either a consequence or a contributing factor. In particular, considerable research supports a strong association between insomnia and depression. Because the majority of individuals who have trouble sleeping attempt to treat the problem independently using over-the-counter medications or alcohol, insomnia is undertreated by health care providers, despite its serious consequences.