Acceptance and Commitment therapy is a well-established treatment for other disorders including depression, anxiety and chronic pain, but has not been thoroughly investigated for insomnia. The therapy consists of mindfulness, acceptance, identification of personal life values and committed action. In addition, patients in this group will receive sleep restriction, a behavioral therapy component of cognitive behavioral therapy for insomnia. The treatment will consist of six weekly sessions of group psychotherapy and will be conducted in an outpatient setting.

The control intervention is Cognitive Behavioral Therapy for insomnia (CBT-I). This is the first line treatment for adults with chronic insomnia. The therapy consists of education, relaxation, and behavioral therapy, including sleep restriction. The treatment will consist of six weekly sessions of group psychotherapy and will be conducted in an outpatient setting.

Acceptance and Commitment Therapy (ACT) is a newer form of behavioral therapy working with acceptance, and the clarification of personal values. Acceptance, in this case, means the willingness to (temporarily) experience unpleasant sensations such as sleeplessness or tiredness without attempting to control or change them. Aims are, first, to reduce suffering due to unsuccessful control attempts (such as frustration, anger, anxiety) and second, to improve processes that are typically worsened by increased control efforts (such as sleep). Clarification of values means identifying aspects of life that are perceived as rewarding and encouraging approach-behavior in valued aspects of life (instead of avoidance-behavior that is often associated with insomnia and mental disorders).

The ISI is a valid, reliable and change-sensitive self-rating questionnaire that is widely used as an outcome measure in clinical trials in insomnia research. A self-rating questionnaire in contrast to objective sleep measures such as polysomnography or actigraphy is reasonable and widely accepted for measuring insomnia severity because insomnia is defined on the basis of subjective sleep perception and objective measures often deviate from this perception. Advantages of the ISI, compared to a sleep diary, are that it combines different aspects of insomnia such as sleep onset difficulties, sleep maintenance difficulties, and daytime impairment in one summary score, and that it is easier to use for patients. Following Morin et al., response is defined as an improvement of > seven points on the ISI. Remission is defined as a post-treatment ISI score of < 8. The minimum score is 0 points, the maximum score is 28 points. Higher score indicate more severe insomnia.

In the GSII, patients are asked to specify the most important aspect of daytime functioning which they perceive to be negatively affected by their insomnia. The degree of impairment is rated on visual analogue scale. The minimum value of the scale is 0 points, the maximum value is 100 points. Higher values indicate better sleep-related quality of life.

The ISI is a valid, reliable and change-sensitive self-rating questionnaire that is widely used as an outcome measure in clinical trials in insomnia research. A self-rating questionnaire in contrast to objective sleep measures such as polysomnography or actigraphy is reasonable and widely accepted for measuring insomnia severity because insomnia is defined on the basis of subjective sleep perception and objective measures often deviate from this perception. Advantages of the ISI, compared to a sleep diary, are that it combines different aspects of insomnia such as sleep onset difficulties, sleep maintenance difficulties, and daytime impairment in one summary score, and that it is easier to use for patients. Following Morin et al., response is defined as an improvement of > seven points on the ISI. Remission is defined as a post-treatment ISI score of < 8. The minimum score is 0 points, the maximum score is 28 points. Higher score indicate more severe insomnia.