Sleep Medicine Reviews (2008) 12, 275–288 THEORETICAL REVIEW Nonrestorative sleep Kristen C. Stone a, , Daniel J. Taylor b,1 , Christina S. McCrae c,2 , Anupama Kalsekar d,3 , Kenneth L. Lichstein e,4 a Clinical Psychology Training Consortium, Brown Medical School, Box G-BH, Providence, RI 02912, USA b Department of Psychology, The University of North Texas, P.O. Box 311280, Denton, TX 76203-1280, USA c Department of Clinical and Health Psychology, University of Florida, P.O. Box 100165, Gainesville, FL 32610-0165, USA d Global Health Outcomes Research, Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46285, USA e Sleep Research Project, Department of Psychology, The University of Alabama, Box 870348, Tuscaloosa, AL 35487-0348, USA KEYWORDS Nonrestorative; Nonrestorative sleep; Unrestful and sleep; Alpha–delta sleep; Unrefreshing 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 conceptualiz- ing 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. & 2007 Elsevier Ltd. All rights reserved. Introduction Nonrestorative sleep (NRS), a subjective experi- ence of unrefreshingsleep, began to interest practitionersand researchersin the 1970s as a possible lead to the etiologies ofchronic fatigue syndrome (CFS) and fibromyalgia syndrome (FMS) as NRS is particularly common in individualswith ARTICLE IN PRESS www.elsevier.com/locate/smrv 1087-0792/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.smrv.2007.12.002 Corresponding author. Tel.: +1 901 486 1087; fax: +1 401 274 0771. E-mail addresses: Kristen_Stone@Brown.edu (K.C. Stone), djtaylor@unt.edu (D.J. Taylor), cmccrae@phhp.ufl.edu (C.S. McCrae), kalsekar_anupama@lilly.com (A. Kalsekar), lichstein@ua.edu (K.L. Lichstein). 1 Tel.: +1 940 565 2655 (office); fax: +1 940 565 4682. 2 Tel.: +1 352 273 6053; fax: +1 352 273 6156. 3 Tel.: +1 317 277 2861; fax: +1 317 433 2997. 4 Tel.: +1 205 348 4962; fax: +1 205 348 4965.