Polysomnographic studies in hypersomniac patients with mood disorders are rare. Previous studies investigated patients with a severe mood disorder, but our study was done in patients with dysthymia, who complained of sleepiness. Mean sleep latency test (MSLT) and continuous polysomnographic recording (CPR) were done in 12 dysthymic patients, in comparison with 12 idiopathic hypersomnia patients, and 12 normal controls. In dysthymic patients mean sleep latency on the MSLT (13 +/- 1) was normal, and when CPR was done during 24 hours, no hypersomnia was found (553 +/- 24). Dysthymic patients showed an abnormal macrostructure of sleep (characterised by an excess of sleep stage I and a decrease of stages 3 and 4), which could be related to their complaint of hypersomnia.