Objective:The differential diagnosis of primary (idiopathic) vs. psychiatric hypersomnia is challenging because of the lack of specific clinical or laboratory criteria differentiating these two disorders and the frequent comorbidity of mental disorders in patients with primary hypersomnia. The aim of this study was to assess whether polysomnography aids in the differential diagnosis of these two disorders.Methods:After excluding patients taking medication and those with an additional diagnosis of sleep-disordered breathing, we compared the nocturnal and daytime sleep of 82 consecutive patients with a diagnosis of either primary hypersomnia (N= 59) or psychiatric hypersomnia (N= 23) and normal control subjects (N= 50).Results:During nocturnal sleep, patients with psychiatric hypersomnia showed significantly higher sleep latency, wake time after sleep onset, and total wake time and a significantly lower percentage of sleep time than patients with primary hypersomnia and control subjects (p< .05). In addition, the daytime sleep of patients with psychiatric hypersomnia was significantly higher in terms of sleep latency, total wake time, and percentage of light (stage 1) sleep and lower in terms of percentage of sleep time and stage 2 sleep than in patients with primary hypersomnia and control subjects (p< .05). The daytime sleep of patients with primary hypersomnia as compared with that of control subjects was characterized by lower sleep latency and total wake time and a higher percentage of sleep time (p< .05). Finally, a sleep latency of less than 10 minutes or a sleep time percentage greater than 70% in either of the two daytime naps was associated with a sensitivity of 78.0% and a specificity of 95.7%.Conclusions:Our findings indicate that psychiatric hypersomnia is a disorder of hyperarousal, whereas primary hypersomnia is a disorder of hypoarousal. Polysomnographic measures may provide useful information in the differential diagnosis and treatment of these two disorders.