Sleep is important for physical well-being, cognitive functioning, and quality of life. During the last decades evidence accumulates showing that good sleep is also crucial for mental health. Sleeplessness (insomnia) increases the risk for mood and anxiety disorders, prolonged grief disorder and substance use disorders. Psychotherapeutic or pharmacologic treatment of insomnia enhances recovery from depression and reduces the relapse risk. Clinical observations indicate that other sleep disorders also exert negative effects on a wide range of mental diseases. For instance, effective treatment of Obstructive Sleep Apnea or Restless Legs Syndrome may concurrently decrease the symptom severity of a depression and post-traumatic stress disorder. Compared to the general population, insomnia - as well as most other sleep disorders - is more common in individuals with a mental illness. While chronic insomnia occurs in four to ten percent of the general population, the majority of people with a depression, anxiety disorder or schizophrenia spectrum disorder suffer from it. Both in view of the high prevalence of disordered sleep and the negative impact on mental health, early detection and treatment of sleep disorders in psychiatric patients seems of paramount importance. Therefore, this project is designed to implement early – relative to current practice - detection and treatment procedures for sleep disorders in mental health care and study the effects on psychiatric outcome and quality of life. Upon admission to GGZ Drenthe, all patients who approve to participate in this project will be screened for sleep disorders with the 32-item, self-assessment Holland Sleep Disorders Questionnaire (HSDQ). The participants who do not score above the cutoff for a specific sleep disorder (HSDQ-) will receive psychiatric treatment as usual (TAU). The participants with a positive score on one or more sleep disorders (HSDQ+) will additionally receive the standard sleep intervention for the respective sleep disorder(s) (TAU+SI) that has been adapted to vulnerable psychiatric populations. To test the efficacy of sleep interventions we use a delayed-treatment control group design. Participants will be randomly assigned to two groups differing in the timing of the sleep intervention: in TAU+SI-T0 sleep therapy will start immediately and in TAU+SI-T1 the sleep intervention will first begin 6 months after inclusion. All participants will complete the HSDQ, as well as 3 short questionnaires assessing the severity of general and of specific psychopathology and quality of life at admission (T0), 6 months (T1) and 12 months after inclusion (T2). This study will be the first to test the critical prediction that early identification and treatment of sleep disorders would lead to a superior psychiatric outcome. The influence of early sleep disorder treatment will also be investigated in more detail for specific mental diseases (depressive disorder, bipolar disorder, anxiety disorder, post-traumatic stress disorder, Attention Deficit (Hyperactivity) Disorder, autism spectrum disorder, schizophrenia spectrum disorder and personality disorder), taking possible differences related to gender and age group into consideration. Another unique aspect of this project is that, in collaboration with ‘psychiatric empirical experts’ (NL: ervaringsdeskundigen), the standard interventions for the sleep disorders will be adapted to vulnerable psychiatric populations. Examples could be investments in a fruitful therapeutic alliance, motivation for sleep interventions and its components, and adaptations to the intervention structure. The adapted interventions will be further optimized / customized on the basis of evaluations given by participating patients.