This study was an epidemiological questionnaire survey of a representative sample of the French population that included 12 778 individuals and in which adapted DSM-IV criteria for the definition of insomnia were used. Our goals were not only to assess the prevalence of 'insomnia' using these criteria, but also to compare the results obtained with those of prior studies using different definitions of 'insomnia'. The aim of this study was also to identify where areas of agreement and disagreement existed, as we believe that it is important to emphasize these points because DSM-IV recommendations are supposedly reflected in clinical practice. Seventy-three per cent of the individuals surveyed complained of a nocturnal sleep problem, but only 29% reported at least one sleep problem three times per week for a month, and 19% (2428 subjects) had at least one sleep problem three times per week for a month and complained of daytime consequences (DSM-IV criteria). Only 9% had two or more nocturnal sleep problems with daytime consequences and were classified as 'severe insomniacs'. Our study indicates that if DSM-IV criteria are used, the diagnosis of 'insomnia' is lower than in other epidemiological studies. The DSM criteria have an advantage in that they emphasize the daytime consequences of nocturnal sleep disturbances, which seem to be responsible for the most important socio-economic costs of the problem.