Autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) are neurodevelopmental disorders with disturbed melatonin secretion profile and sleep problems. The growing incidence of ASD and ADHD inspires scientists to research the underlying causes of these conditions. The authors focused on two fundamental aspects, the first one being the presentation of the role of melatonin in ASD and ADHD and the second of the influence of melatonin treatment on sleep disorders. The authors present the use of melatonin both in the context of causal and symptomatic treatment and discuss melatonin supplementation: Dosage patterns, effectiveness, and safety. Sleep disorders may have a different clinical picture, so the assessment of exogenous melatonin efficacy should also refer to a specific group of symptoms. The review draws attention to the wide range of doses of melatonin used in supplementation and the need to introduce unified standards especially in the group of pediatric patients.Brain Sci. 2020, 10, 219 2 of 21 requiring flexibility and behavioral problems [9]. In the presented disorders, the clinical picture is heterogeneous [10] and the etiology is still unclear [11].Autism spectrum disorder (ASD) can be defined based on the diagnostic criteria which include communication and social interaction disorders and limited repetitive patterns of behavior and interests. In addition to the fundamental symptoms, many patients also present with anxiety attacks, (self)-aggression, mood disorders, and sleep difficulties [12,13]. The first symptoms of autism are observed already in early childhood and usually persist throughout life. An increase in the incidence of ASD has been reported over recent years. According to the CDC's Autism and Developmental Disabilities Monitoring (ADDM) report in 2000 the incidence of ASD was estimated at 1 in 150 children, in 2014 this incidence was 1 diagnosis in 59 children at the age of 8 [14]. The causes of this tendency may be different, e.g., a higher standard of medical and diagnostic care [15], higher social awareness, and the older age of parents at childbearing [16,17]. The etiology of ASD is most likely multifactorial [11], being the result of genetic and environmental factors [6]. Hallmayer et al. indicated the predominance of environmental factors, estimating the shared environmental component to be 58% and heritability to 38%. This distribution was similar among women and men [18]. So far multiple potential risk factors for ASD have been reported in the literature, including the use of illicit drugs during pregnancy, medications, organophosphate pesticides [19], fatty acid deficiency [20], gestational diabetes, and low birth weight [11]. Despite numerous scientific studies, no genetic abnormality that would account for more than 1% of ASD has been identified yet [11]. One of the hypothesis defines ASD as a collection of many forms of a rare monogenic disorder with a different etiology [21]. Ten percent of ASD can be explained as a component of a specific genetic sy...