General estimates vary depending on the criteria used to define insomnia, and prevalence rates tend to decrease as the stringency of the criteria increases.5 Thirty percent of adults have insomnia when defined as reporting at least one insomnia symptom.6 When daytime impairment or distress is a required criterion, prevalence drops to 10%.1 When the most stringent diagnostic criteria are applied,7 prevalence remains substantial, but further drops to about 6% of adults.5,8 The few studies examining racial differences in prevalence have reported rates of 16.4% to 28.3% in whites, 15.3% to 23.7% in blacks, and 13.4% to 17.1% in Hispanics.9 When separated into age categories, blacks appear to have a greater prevalence of insomnia in middle age (30-59 years), whereas whites have a greater prevalence of insomnia across the life span (Table 1).10 Overall, sex, age, and health and mental conditions appear to be the most significant risk factors for insomnia.1,15 Older age has been associated with increased risk of insomnia.11,16 However, research suggests that age itself is not the risk; instead, the risk is related to inactivity, sleep changes, decreased social activities, and increases in health conditions associated with aging.17,18 Women are at least two times more likely to have insomnia than age-matched men,10 and an increased prevalence of insomnia has been seen in adolescent girls compared with age-matched boys (Table 1).11 The presence of a health or mental condition increases the risk, with insomnia seen in 37.8% of individuals with a comorbid condition but in only 8.4% of those without a comorbid condition.12 Although there are numerous epidemiologic studies of insomnia, criteria for insomnia classification are highly varied among these studies. Therefore, the summary of prevalence data in Table 1 relies heavily on data collected from a single epidemiologic study by Lichstein10 in 2004. Some of the data generated by that study have appeared in the peer-reviewed literature,12 but the bulk of findings to date have been published in Lichstein’s book. Thus, it is important to note that the methodology used in Lichstein’s study was highly rigorous and provides one of the best sources of information currently available on the prevalence of insomnia. In terms of methodology, Lichstein10 used random-digit dialing to sample 50 men and women in each age decade from 20 to ≥80 years. Additionally, unlike the bulk of studies in this area, Lichstein’s study used prospective data collection methods (2 weeks of sleep diaries) and stringent criteria for diagnosing insomnia.10,12 The other studies in Table 1 were also selected because they used the strongest methodology and most stringent criteria available.