Efficacy of Melatonin, Low-dose Quetiapine, or Placebo in Patients With Psychiatric Disorders and Comorbid Insomnia

The goal of this investigator-initiated, randomized, blinded, 3-armed placebo-controlled, pragmatic, clinical superiority trial is to examine the efficacy of melatonin or low-dose quetiapine versus placebo for treatment of insomnia in patients with psychiatric disorders. The aims of the study are: * To examine the efficacy of melatonin or low-dose quetiapine versus placebo for treatment of insomnia * To examine how melatonin or low-dose quetiapine affects global symptom severity, sleep quality, psychosocial functioning and subjective well-being. Participants will receive six weeks of treatment with either melatonin, quetiapine or placebo followed by a brief taper off.