Sleep is a sensitive indicator of well-being and a helpful early-warning symptom in many psychiatric disorders. Healthy sleep and a balanced sleep-wake rhythm are desirable goals of a salutary conduct of life. Preventive and psychoeducational measures should take up this point. Non-organic sleep disorders are commonly associated with psychiatric diseases such as major depressive disorder, hypomania, mania, schizophrenia, anxiety disorders, alcoholism and other substance related disorders as well as dementias. Sleep disturbances are often the first symptoms of a psychiatric disorder and thus important early warning signs. Polysomnography is a useful tool to show the characteristic patterns of sleep disturbances in different psychiatric disorders.