Background: Atopic dermatitis (AD) is a chronic inflammatory skin disease often associated with non-atopic comorbidities. Recently, a severity-dependent relationship between AD with sleep and mental health diseases has been proposed, but few studies investigated this topic through validated questionnaires. Therefore, the present study aimed to: (i) assess the impact of AD on sleep and psychological disorders using validated instruments and (ii) estimate the association of AD severity with sleep disorders and psychological symptoms distinguishing between clinical-oriented and patient-oriented measures. Methods: We conducted a cross-sectional, case-control study, recruiting 57 adult AD patients matched for age and sex with 57 healthy adults. To investigate the differences in sleep quality, insomnia, depression, and anxiety between the two groups, we performed independent samples t-tests. Moreover, we conducted several univariable linear regression analyses to examine the relationship between objective/subjective severity of AD and sleep quality, insomnia, and psychological symptoms. Results: AD patients presented poorer sleep quality and more severe symptoms of insomnia and depression than healthy adults. Objective and subjective AD severity were similarly associated with worse sleep quality, anxiety, and self-perceived stress. However, subjective AD severity was more strongly associated with insomnia and depressive symptoms than clinical-oriented AD severity. Conclusions: The study demonstrated poor sleep quality and high levels of insomnia, depression, and perceived stress in AD patients, with an aggravated psychological status for adults with more severe disease. We suggest implementing a multidisciplinary approach to AD management and treatment that considers objective and subjective measures of disease severity.