Although only half of these subjects consult a doctor, insomnia and sleep disorders represent a real problem. If we want to improve the way sleep disorders are treated medically, we have first of all to be able to recognize them, which means knowing more about them. To do this, a dual assessment is required:

• what are the significance and consequences of these disorders in terms of health, and
• what is the impact of the different therapeutic strategies?

The Sleep-EVAL system can be used in various contexts. During the past years, it was widely used in:

• general population surveys,
• the offices of physicians,
• the sleep disorders clinics and
• in the hospital yards.

The Sleep-EVAL system is useful to test classifications by assessing the symptomatic constellation underlying a diagnosis. The use of Sleep-EVAL ensures that the full spectrum of the classification is covered, including rare diagnoses, which do not necessarily receive the physician's immediate attention. Sleep-EVAL also ensures that at least the minimal criteria for a diagnosis are present and makes it possible to explore the symptomatic constellations of specific diagnoses.

Countries are surveyed using the same methodology, almost the same questionnaires, and the same diagnosis instrument, the Sleep-EVAL system. This is allowing comparisons between countries.