Hypersomnia is a group of diseases that share the main symptom — excessive daytime sleepiness, not caused by disturbances in nocturnal sleep or circadian rhythms. Excessive daytime sleepiness is present in 15.6% of adults in the world, a Russian study showed a prevalence of 39.2%. It is associated with a wide range of comorbidities, including obesity and mental disorders, on the other hand, the presence of hypersomnia increases the likelihood of mental illness. People with hypersomnia are more likely to take medications, have a decreased quality of life, spend more health care resources, and more often receive social benefits. The heritability is estimated to be about 40% for sleep duration and 17% for excessive daytime sleepiness. Hypersomnia in mental disorders is secondary. It most often occurs in patients with depression or bipolar disorder. To assess the severity of daytime sleepiness, self-observation and objective methods, including the multiple sleep latency test, actigraphy, polysomnography, are used. In the differential diagnosis of hypersomnia in psychiatric disorders, it is necessary to make a differential diagnosis with hypersomnia caused by taking medications or other substances and insufficient sleep syndrome. The etiology of prolonged sleep in psychiatric disorders is complex, and includes biological and psychological causes. The relationship between self-reported hypersomnia and sleep actually obtained is still unclear. Results of daily polysomnography show a significant increase in time in bed during the day and night (clinophilia). Therapy of hypersomniac syndromes should be done taking into account the etiology of the disease. In cases of secondary nature, the main efforts should be directed to the treatment of the underlying mental disorder causing somnolence.