Complaints about insufficient sleep are among the hallmarks of many psychiatric disorders and, in fact, insomnia in some patients may be a psychiatric disorder of its on. Moreover, psychiatric patients are at increased risk to suffer from genuine sleep disorders, such as obstructive sleep apnea or restless legs syndrome. Vice versa, sleep disorders are risk factors to develop psychiatric disorders and depression in particular. Hence, a thorough work up of psychiatric patients always must include sleep and, shedding light at the other side of the coin, daytime sleepiness. In diagnostic terms, ICD and DSM-IV look at disturbed sleep and daytime sleepiness in psychiatric patients mainly as secondary symptoms. Novel DSM-V, in contrast, defines insomnia and hypersomnia disorders as largely independent of other psychiatric cnditions, an interesting but also quite debatable view. The DSM-V position underlines the fact that sleep related complaints deserve specific attention and treatment (e.g. by CMT-I). The insomnia and hypersomnia disorder concept, however, neglects that still, in the majority of patients disturbed sleep has a distinguishable cause. Just as an example, severe insomnia complaints in psychiatric patients might be caused by the restless legs syndrome and daytime sleepiness might due to obstructive sleep apnea. Therefore, a detailed look at sleep in psychiatric patients leads to more precise diagnostic results and finally in many cases to more appropriate and successful therapy.