Children and adolescents with psychiatric or neurologic disorders often suffer from sleep problems. Disrupted sleep might lead to different comorbidities in the child/adolescent. These symptoms often mimic other psychiatric symptoms, which makes the diagnostic process challenging. Sleep problems can lead to aggravation of existing symptoms, exacerbation into psychiatric problems, or arise as a response to pharmacological treatment. In order to provide an efficient and well-qualified treatment, it is important to understand the pathogenesis of sleep problems to be able to distinguish between the cause and consequence, as argued in this review.