Nonrestorative Sleep: A New Perspective

Widely used diagnostic and research criteria such as DSM-IV,¹ RDC for insomnia,² and the ICSD-2³ encompass nonrestorative sleep (NRS) as a symptom of insomnia. It has been a matter of lively and controversial discussion and debate among clinicians and researchers world-wide whether the occurrence of NRS should be counted among the essential criteria of insomnia. Recently, Roth and colleagues⁴ presented a thorough analysis of a large sample of insomnia patients showing that NRS can occur independently of other components of insomnia. It was also shown that daytime symptoms in individuals displaying only NRS were as pronounced in this group as in other insomnia patients (without NRS).

In this issue of SLEEP, Zhang and colleagues⁵ present a highly interesting large-scale investigation based on the National Health and Nutrition Examination Survey (NHANES). In this sample of almost 11,000 individuals with an age of 20 years or older (based on a survey of the general population of the US from 2005 to 2008), not only insomnia symptoms were sampled but also one specific question asked for the presence/ absence of NRS: “In the past months, how often did you feel unrested during the day, no matter how many hours of sleep you have had?” Three questions asked for troubles falling asleep, waking up during the night, and waking up too early in the morning. For the statistical analyses, participants of the study were divided into four groups according to the presence of nocturnal insomnia symptoms (NIS) and nonrestorative sleep (NRS): (1) no symptoms; (2) NIS-only; (3) NRS-only; (4) NIS + NRS. For some of the analyses the groups NIS-only, NRS-only, and NIS + NRS were combined to create an “any insomnia symptoms” group. What has to be considered unique about the study by Zhang et al. is that besides collecting sleep and other data, C-reactive protein (CRP) levels were measured. This served to test whether any of the symptoms under analysis showed correlations with a marker of inflammation. Considering the large sample size, and the measures utilized (including CRP), this study definitely takes research concerning NRS to a new level.

The data analyses performed by Zhang et al. consist of state of the art statistics, as outcome measures medical conditions, CRP, other sleep problems, and general productivity were analyzed. As shown in the publication, the one-month prevalence estimates were 18.1% for any insomnia symptoms, 7.9% for NIS-only, 5.4% for NRS-only, and 4.8% for NIS + NRS. Accordingly, 26% of those with any insomnia symptom had both NIS and NRS. Data analyses revealed that female gender, non-Hispanic whites, lower income, and low educational levels were more likely to be associated with the report of insomnia symptoms. Higher prevalence rates of both NIS-only and NRS-only were found in females, among subjects with depressive symptoms, lifetime smoking, and short sleep duration when compared with the control group without any insomnia symptoms. Interestingly, only NIS increased with age, whereas the prevalence of NRS decreased with age. For analyses of the relationship with medical symptoms Zhang et al. found that there were different patterns between NIS-only and NRS-only. Whereas NIS-only was significantly associated with cardiovascular diseases, NRS-only was not. NRS-only, however, was significantly associated with COPD and sleep problems such as habitual snoring, sleep apnea, and restless legs syndrome, whereas NIS-only was not. Thus, a clearly different pattern of association of NIS and NRS with medical problems arose. Most interesting, Zhang et al. found that CRP data revealed a difference between NIS and NRS. Having NIS + NRS resulted in the highest CRP levels compared to the control group. Second in line was NRS-only in this type of data analysis. With respect to general productivity, the group with NIS + NRS was the most impaired, followed by the group with NRS-only.

The authors thoroughly discuss the data and point out some avenues for future research. They come to the conclusion that, “Furthermore, there are more differences than similarities in the social demographic features, medical comorbidity, other sleep problems, CRP level and functional impairment of NRS-only versus NIS-only.”⁵ Zhang et al. suggest that further research needs to be done in order to thoroughly delineate the biological basis of NRS. The authors were self-critical of their measure of NRS and stress that the development of more refined psychometric methods is necessary to properly identify NRS. With respect to the intriguing results concerning CRP levels in NRS, Zhang et al. suggest that this might constitute a link to the pathogenesis of NRS. An association may exist between NRS, chronic fatigue syndrome (CFS), and some forms of depression, both of which have been shown to be associated with increased peripheral inflammatory markers. Unfortunately, CFS symptomatology was not measured directly in the study by Zhang et al.

Summarizing, Zhang et al.⁵ conclude that further research on diagnostic measures on NRS, its natural history, and its effective management are necessary to learn more about the condition. The authors propose, according to the close association of NRS with medical conditions and depression, the subtyping of NRS into NRS secondary to medical conditions; NRS secondary to psychiatric conditions; and NRS alone. When considering the results of their study, it seems increasingly possible to hypothesize that NRS has some specific features that distinguish it from insomnia. Interestingly, The DSM-V diagnostic category “Insomnia Disorder” (to be published soon) will drop NRS from its symptom catalogue.

CITATION

Riemann D. Nonrestorative sleep: a new perspective. SLEEP 2013;36(5):633-634.

DISCLOSURE STATEMENT

Dr. Riemann has indicated no financial conflicts of interest.