The NIH's 2003 National Sleep Disorders Research Plan defines insomnia as "difficulty falling asleep, difficulty staying asleep or short sleep duration, despite adequate opportunity for sleep," and estimates that it affects 30% to 40% of adults. The prevalence of chronic insomnia, defined as sleep disturbances for 4 weeks or more, sleep disruption with daytime impairment, or regular, nightly sleep difficulty, is about 10% of the general population, with higher rates among women, older adults and clinical populations. Total direct and indirect costs of insomnia are estimated to be roughly $113 billion annually. While only about 3 million of the 70 million Americans with insomnia take prescription medications, annual prescription drug costs for insomnia exceed $2.1 billion dollars.