Complaints about disturbed sleep or increased daytime sleepiness are among the most frequent symptoms reported to psychiatrists by patients. Such complaints can be symptoms of an underlying psychiatric disorder or indicative of a separate or comorbid sleep disorder. Hence, basic knowledge in the differential diagnosis of sleep medicine pathologies is pivotal for psychiatrists and psychotherapists. In the present overview following a description of the diagnostic methods, the diagnostic work-up according to the major symptomatic clusters, namely disturbances in initiating and maintaining sleep, abnormal nocturnal movements and excessive daytime sleepiness will be presented.