Clinical review
Sleep in the chronic fatigue syndrome

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Summary

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.

Introduction

Recurring complaints of disturbed sleep and fatigue are very common among the general population. Patients who present with a combination of these symptoms may perceive malfunction of sleep as the prime cause of tiredness and other impairments in daily life. Because of this attribution, dissatisfaction with daytime functioning may be an incentive to seek medical help for a presumed disturbance of sleep.

Fatigue is a common denominator referring to various aspects of impaired physical, mental, emotional and neurocognitive functioning. Lack of energy, weakness, attention deficits, memory problems and irritability are typically associated with the construct of fatigue. It is a frequent manifestation of a variety of medical, neurological and psychiatric diseases. It may also appear as a side effect of pharmacological treatment.

Presently, there is ample evidence to confirm that sleep curtailment, whether experimentally induced or self-imposed, is causally associated with fatigue. Likewise, primary sleep disorders (PSD) are a known cause of fatigue and excessive daytime sleepiness (EDS). Clinical improvement of these symptoms can be expected from adequate treatment of the underlying sleep disorder.

Finally, fatigue often remains unexplained, leading to the construct of chronic fatigue syndrome (CFS), in which unrefreshing sleep is a prominent (but ill-defined) feature.

The aim of the present review is to:

  • •

    give an overview of definitions, health impact and epidemiology of CFS;

  • •

    explore current insights into restorative and nonrestorative aspects of sleep;

  • •

    assess the relations between sleep and CFS.

Section snippets

Definitions, health impact and epidemiology of CFS

CFS is characterized by long lasting pathologic fatigue with a disabling impact on professional, social and daily functioning. The absence of any obvious underlying disease, and the presence of a number of associated clinical features are fundamental to this disorder. The term CFS was coined in 1988 by Holmes et al. in a publication of the US Centers for Disease Control and Prevention (CDC).1 Since then, several new case definitions have been introduced. In 1994, revised CDC criteria were

The restorative function of sleep

Because unrefreshing or nonrestorative sleep (NRS) is a hallmark of CFS, insights into the restorative function of sleep are mandatory. The present section gives an overview of our current understanding of this feature, while the next section expands on the construct of nonrestorative sleep.

Sleep is a universal phenomenon in living creatures. While sleep is conceived essential for normal life, its functions are as yet incompletely understood. Regarding non-rapid eye movement (NREM) sleep, most

Nonrestorative sleep

While adverse effects of sleep loss on daytime performance were already substantiated more than a century ago,34 medical attention for insomnia-like daytime symptoms in the presence of normal sleep duration is of a more recent date. The clinical phenomenon of interest is a subjective experience of unrefreshing sleep. Typically, patients report awakening unrestored or unrefreshed after a preceding night with sufficient sleep duration. From the 1970s on, unrefreshing sleep was observed as a

Sleep complaints and assessment of sleep in chronic fatigue and CFS

In all available case definition guidelines of CFS, sleep problems are described as a minor criterion (Table 1). The terms used vary substantially from (aspecific) sleep disturbance, to unrefreshing or nonrestorative sleep, to various aspects of sleep quality, sleep duration and elements of insomnia and/or hypersomnia. Evidently, the lack of uniformity in working definition mirrors the gap in our understanding of the pathophysiological role of sleep in CFS.

Sleep disturbance is reported by the

Acknowledgments

This work was supported by the scientific fund of the Department of Internal Diseases, Infectious Diseases and Psychosomatic Medicine.

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