Introduction

Even though sleep disorders are a major public health concern and have been shown to cause and exacerbate various psychiatric and medical problems, their assessment, diagnosis and treatment in research and clinical practice remains largely inconsistent. The aim of this chapter is to introduce the various types of sleep disorder that exist and provide a background to the complex relationship between diagnostic criteria and the conception of sleep disorders by the clinician, the researcher and the individual. Moreover, through an examination of the most commonly reported sleep disorder, insomnia, the effect of differing diagnostic criteria on epidemiological estimates are exemplified.

What is a sleep disorder?

It is common for people to experience the odd sleepless night or feelings of fatigue and sleepiness during the day. However, when these problems persist, cause significant distress, and result in modifications in behaviour, it is broadly termed a sleep disorder. Sleep disorders are generally defined under two main categories, parasomnias and dyssomnias, with parasomnias being an abnormality during sleep and dyssomnias being a disruption in the quality, quantity and timing of sleep episodes.

Parasomnias

Whereas parasomnias were traditionally assumed to be a product of an underlying psychological or psychiatric disorder, or physical manifestation of a dream state, more recently they have been attributed to an intrusion of one sleep stage on another or disruption in the transition between sleep stages or between sleep and wakefulness.