A case of moderate obstructive sleep apnea in which effective treatment with continuous positive airway pressure did not reduce excessive daytime sleepiness was described. Polysomnography with tibial muscles activity recording revealed frequent periodic limb movements in sleep (PLMS) with sleep fragmentation. PLMS index was 13.6/h of sleep. Clonazepam combined with iron and magnesium supplementation reduced limb movements, excessive daytime sleepiness and improved the patient's mood. Causes of PLMS and treatment options are discussed. PLMS and restless legs syndrome (RLS) should be considered in the differential diagnosis of excessive daytime sleepiness.