doi: 10.1111/j.1365-2869.2008.00697.x Insomnia and long sleep duration are risk factors for later work disability. The Hordaland Health Study BØRGE SIVERTSEN 1 , SIMON ØVERLAND 2 , STA ˚ LE PALLESEN 3,4 , BJØRN BJORVATN 4,5 , INGER HILDE NORDHUS 1,4 , JOHN GUNNAR MÆLAND 5 and ARNSTEIN MYKLETUN 2,6 1 Department of Clinical Psychology, University of Bergen, Bergen, Norway, 2 Faculty of Psychology, Research Centre for Health Promotion, University of Bergen, Bergen, 3 Department of Psychosocial Science, University of Bergen, Bergen, 4 Norwegian Competence Center for Sleep Disorders, Haukeland University Hospital, Bergen, 5 Department of Public Health and Primary Health Care, University of Bergen, Bergen and 6 Department of Mental Health, Division of Epidemiology, Norwegian Institute of Public Health, Oslo, Norway Accepted in revised form 25 July 2008; received 25 April 2008 SUMMARY Both insomnia and sleep duration have previously been linked with a range of adverse outcomes, but no studies have explored their relative effect on subsequent work disability. The aim of the present study was to investigate the contribution of insomnia versus sleep duration to later long-term work disability. Using a historical cohort design with 4-year follow-up, data on insomnia, sleep duration and potential confounders were gathered from 6599 working persons (40–45 years). The outcome was award of disability pension, as registered in the National Insurance Administration. After controlling for baseline exposure to disability and sick leave, insomnia was a strong predictor of permanent work disability [odds ratio (OR) = 4.56], and this effect remained significant after controlling for sleep duration, as well as for other possible confounders (OR = 1.88). Short sleep duration was not significantly associated with subsequent work disability, while long sleep duration (>8.5 h) did predict work disability (OR = 2.96), also in the fully adjusted model (OR = 2.14).The present study demonstrates that both insomnia and long sleep duration are strong and independent risk factors for subsequent work disability. keywords cohort studies, disability, historical, insomnia, logistic regression, risk factors, sleep, work INTRODUCTION People with insomnia (PWI) typically complain about diffi- culty initiating or maintaining sleep or of non-restorative sleep, which result in some form of daytime impairment (Edinger et al., 2004; Lichstein et al., 2003). Compared with polysom- nographic recordings, however, PWI tend to overestimate their time awake and consequently underestimate the amount of time they actually sleep (Carskadon et al., 1976; Coates et al., 1982). Still, studies based upon self-reports do not consistently show a linear relationship between sleep duration and insom- nia. In one study, it was found that symptoms of insomnia were more frequently reported by both short sleepers (4–6 h) and long sleepers (9–10 h), compared with sleepers who reported sleep duration in the intermediate range (7–8 h) (Grandner and Kripke, 2004). Based upon such data, we argue that one should distinguish between insomnia and sleep duration when studying possible outcomes of these sleep variables. The many adverse individual correlates of impaired sleep are well documented. Insomnia is associated with both cognitive and intellectual impairment (Pilcher and Huffcutt, 1996; Simon and Vonkorff, 1997; Szelenberger and Niemcewicz, 2000), as well as current and subsequent affective disorders (Mellinger et al., 1985; Vollrath et al., 1989). Patients suffering from insomnia also commonly report significant reduction in Correspondence: Børge Sivertsen, PhD, Department of Clinical Psychology, University of Bergen, Christiesgt. 12, 5015 Bergen, Norway. Tel.: +47 55 58 88 76; fax: +47 55 58 98 77; e-mail: borge.sivertsen@psykp.uib.no J. Sleep Res. (2008) Original article Ó 2008 European Sleep Research Society 1