Insomnia is a common disturbance of sleep which can be treated effectively with cognitive behavioural therapy; a multicomponent ‘package’ of cognitive and behavioural strategies. Sleep restriction therapy is thought to be one of the most potent behavioural components of cognitive behavioural therapy. Subjective measures of sleep and daytime functioning improve not only with cognitive behavioural therapy, but also during and following sleep restriction therapy. However, it is unknown when these changes occur or if there are associated objective changes. This thesis addresses these issues, and presents: 1. a review of the literature of therapy and original research; 2. evaluates the nature and timing of changes in self-reported daytime functioning during therapy; 3. profiles potential objective changes (in measurements of sleep, plasma and salivary cortisol concentrations and temperature); and 4. compares patients with different subtypes of insomnia and healthy good sleeping controls for possible differences within the brain that might serve as future targets for treatment. The final general discussion ties together the results of these data-based chapters. The following section aims to provide a brief summary of the overall thesis. This Ph.D. was undertaken as Cotutelle Agreement between the Universities of Glasgow, United Kingdom and Sydney, Australia. Specific chapters relate to the data collection performed at these two sites. Consequently this thesis has been split into specific chapters from where the data were obtained. Chapters four and five consist of data acquired from Glasgow, United Kingdom whilst in chapters six and seven the data were acquired in Sydney, Australia.