Nonrestorative sleep

Summary

The current review presents the empirical findings on varying definitions of nonrestorative sleep (NRS). Despite lacking a standard, operational definition, NRS is investigated in research studies and included in diagnostic manuals. However, because of the absence of standardization, the conclusions that can be drawn about NRS based on the current body of empirical literature are limited. A feeling of being unrefreshed upon awakening that is not accounted for by lack of sleep may occur among a substantial percentage of the population. This experience is correlated with daytime impairment, pain, fatigue, and electroencephalogram (EEG) arousals in non-REM sleep but causal links are unsubstantiated. An immediate converging of researchers toward NRS standardization is needed. We conclude that conceptualizing NRS as a primary symptom of insomnia on par with difficulty initiating sleep and difficulty maintaining sleep is empirically unsubstantiated. We recommend defining NRS as a report of persistently feeling unrefreshed upon awakening in the presence of a normal sleep duration, occurring in the absence of a sleep disorder.

Introduction

Nonrestorative sleep (NRS), a subjective experience of unrefreshing sleep, began to interest practitioners and researchers in the 1970s as a possible lead to the etiologies of chronic fatigue syndrome (CFS) and fibromyalgia syndrome (FMS) as NRS is particularly common in individuals with these conditions.¹ NRS is also common in patients with other organic sleep disorders (e.g., apnea)² and is a primary symptom of insomnia, although there is little empirical evidence supporting this diagnostic status. The wealth of research on insomnia symptoms focuses mainly on difficulties initiating or maintaining sleep, with relatively few studies examining NRS. Currently, the conceptualizations of NRS vary across both sleep research and practice. Further, research suggests that in many cases the report of NRS is not a primary symptom of insomnia, but a symptom of another sleep, health, or psychiatric problem.1, 3, 4 NRS lacks a uniform definition, known causal agents, and empirically validated assessment and treatment. Thus, a review of the small NRS literature is necessary to consolidate our understanding of this unique symptom—a potential clue to the etiologies of chronic fatigue and generalized pain and to the function of sleep.

The concept of NRS borrows from restorative theory, in which brain activity occurring during sleep is thought to replenish the body and mind for daily functioning.⁵ The idea naturally flowing from this theory is that a byproduct of such replenishment is a refreshed feeling upon awakening, the absence of which characterizes NRS. Also congruent with this refreshed sensation are theories positing that sleep serves to restore cerebral functioning, including maintenance of innate brain mechanisms needed for survival and processing or fine-tuning of brain operations and information.⁵ These restorative theories of sleep have empirical support.⁶ Studies have demonstrated that skin renewal and bone growth occur more rapidly during sleep than when awake in rats and in humans, that species with higher daily metabolic rates have longer sleep durations, and that, in humans, higher metabolic demand leads to longer sleep time as well as more slow-wave sleep and more growth hormone.7, 8, 9, 10, 11, 12 Feelings of grogginess, sluggishness, or fatigue that are not attributable to lack of sleep are theoretically and empirically linked with sleep restoration, making NRS a viable label for this subjective phenomenon. However, more research is needed to fully understand sleep restoration and to substantiate it as a causal agent of the otherwise unexplainable unrefreshing feeling upon awakening after an adequately lengthy sleep.