Chronic fatigue syndrome (CFS) is a disabling condition characterized by severe fatigue lasting for more than six months and the presence of at least four out of eight minor criteria. Sleep disturbance presenting as unrefreshing or nonrestorative sleep is one of these criteria and is very common in CFS patients. Biologically disturbed sleep is a known cause of fatigue and could play a role in the pathogenesis of CFS. However, the nature of presumed sleep impairment in CFS remains unclear. Whilst complaints of NRS persist over time, there is no demonstrable neurophysiological correlate to substantiate a basic deficit in sleep function in CFS. Polysomnographic findings have not shown to be significantly different between subjects with CFS and normal controls. Discrepancies between subjectively poor and objectively normal sleep suggest a role for psychosocial factors negatively affecting perception of sleep quality. Primary sleep disorders are often detected in patients who otherwise qualify for a CFS diagnosis. These disorders could contribute to the presence of daytime dysfunctioning. There is currently insufficient evidence to indicate that treatment of primary sleep disorders sufficiently improves the fatigue associated with CFS. Therefore, primary sleep disorders may be a comorbid rather than an exclusionary condition with respect to CFS.