To evaluate hypersomnolence in patients affected by fibromyalgia syndrome.Thirty consecutive patients affected by fibromyalgia syndrome (FMS) (28 F) completed a sleep questionnaire and underwent the following evaluations: lung function tests; polysomnography; the Epworth sleepiness scale (ESS), which measures sleep complaints and daytime hypersomnolence; and the visual analogical scale (VAS) to detect subjective pain, fatigue, anxiety and depression.The FMS patients were divided into two groups based on their ESS score. Patients complaining of daytime hypersomnolence had a higher number of tender points (15 +/- 2 vs. 12 +/- 1, p < 0.01), a higher subjective pain score (72 +/- 15 vs. 52 +/- 13, p < 0.05), and more fatigue (p < 0.05). The diffusing capacity of the lung (Tlco) was more impaired and the occurrence of periodic breathing was higher. FMS patients complaining of daytime somnolence had significantly less efficient sleep than the FMS patients with no daytime somnolence (p < 0.05), i.e. a lower proportion of stage 3 sleep (5 +/- 2% vs. 12 +/- 3%; p < 0.001), stage 4 sleep (1 +/- 0.5% vs. 4 +/- 1%; p < 0.001), and twice as many arousals per hour of sleep (p < 0.01). The respiratory pattern of FMS patients with hypersomnolence showed a higher occurrence of periodic breathing (p < 0.05). The short length of apneas and hypopnoeas did not affect the apnea/hypopnea index (5.1 +/- 3 vs. 7 +/- 4; ns), but FMS patients with daytime hypersomnolence had a greater number of desaturations per hour of sleep (11 +/- 6 vs. 6 +/- 5; p < 0.05). Pulmonary volumes did not differ between the two groups. The EES score was significantly correlated in FMS patients, and even more markedly in the FMS patients with hypersomnolence, TLco, A/I, and disease duration. The ESS score was correlated significantly with the number of tender points only in FMS patients with daytime hypersomnolence.The occurrence of daytime hypersomnolence in FMS patients is linked to a greater severity of fibromyalgia symptoms and to more severe polysomnographic alterations.