Relationships between symptoms of hypersomnolence, psychiatric disorders, and hypersomnia disorders (i.e., narcolepsy and idiopathic hypersomnia) are complex and multidirectional. Hypersomnolence is a common complaint across mood disorders; however, patients suffering from mood disorders and hypersomnolence rarely have objective daytime sleepiness, as assessed by the current gold standard test, the Multiple Sleep Latency Test. An iatrogenic origin of symptoms of hypersomnolence, and sleep apnea syndrome must be considered in a population of psychiatric patients, often overweight and treated with sedative drugs. On the other hand, psychiatric comorbidities, especially depression symptoms, are often reported in patients with hypersomnia disorders, and an endogenous origin cannot be ruled out. A great challenge for sleep specialists and psychiatrists is to differentiate psychiatric hypersomnolence and a central hypersomnia disorder with comorbid psychiatric symptoms. The current diagnostic tools seem to be limited in that condition, and further research in that field is warranted.