ObjectivesThe purpose of this study was to detect treatable sleep disorders among patients complaining of chronic fatigue by using sleep questionnaires and polysomnography. MethodsPatients were referred to hospital for investigations and rehabilitation because of a suspected diagnosis of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). The criteria for further referral to full-night polysomnography (PSG) were symptoms of excessive daytime sleepiness and/or tiredness in the questionnaires. ResultsOf a total of 381 patients, 78 (20.5%) patients underwent PSG: 66 women and 12 men, mean age 48.6years, standard deviation 9.9years. On the basis of the PSG, 31 (40.3%) patients were diagnosed with obstructive sleep apnoea, 7 (8.9%) patients with periodic limb movement disorder, 32 (41.0%) patients with restless legs syndrome and 54 (69.3%) patients had one or more other sleep disorder. All patients were grouped into those who fulfilled the diagnostic criteria for ME/CFS (n=55, 70.5%) and those who did not (n=23, 29.5%). The latter group had significantly higher respiratory (P=.01) and total arousal (P=.009) indexes and a higher oxygen desaturation index (P=.009). ConclusionsMore than half of these chronic fatigue patients, who also have excessive daytime sleepiness and/or tiredness, were diagnosed with sleep disorders such as obstructive sleep apnoea, periodic limb movement disorder and/or restless legs syndrome. Patients with such complaints should undergo polysomnography, fill in questionnaires and be offered treatment for sleep disorders before the diagnose ME/CFS is set.