Insomnia is generally defined as a difficulty initiating or maintaining sleep or a poor quality of sleep. Insomnia may be primary or secondary to other conditions such as depression, anxiety, mania, or substance abuse. Primary insomnia appears to be uncommon relative to secondary insomnia. Various population surveys have found self-reported rates of symptoms of insomnia in about a third of adults. About 10% of the U.S. population report having experienced symptoms of chronic insomnia that may have lasted for many years. However a smaller percentage of patients have insomnia with significant daytime impairment. According to the "NIH State of the Science Conference Statement on Manifestations and Management of Chronic Insomnia in Adults Statement" (2005), perhaps 10% of the population has clinically significant insomnia if the criterion of daytime impairment is included in the criteria. Women and older adults appear to be the demographic groups at greatest risk for significant insomnia, but of course transient insomnia can affect anyone.