Original articleDoes insomnia predict sick leave?: The Hordaland Health Study
Introduction
Insomnia has been linked with a range of adverse consequences and conditions, including cognitive and intellectual impairment [1], [2], as well as current and subsequent affective disorders [3], [4], [5]. Persons suffering from insomnia commonly report significant reduction in quality of life [6] and impaired coping abilities [7], and chronic insomniacs have been shown to have reduced immune functioning [8]. In terms of economic costs, the most recent analysis of the economic burden of insomnia in the United States estimates the direct medical costs of insomnia to be US$13.9 billion annually [9], an estimation that increases to $92 to 107 billions when including indirect costs from sleep-related accidents and lost productivity [10].
Still, the relationship between insomnia and work status has received little attention. Although cross-sectional surveys have linked poor sleep with both work dissatisfaction [11], reduced working capacity, employment status [12], as well as self-reported sick leave [13], [14], only a few studies have prospectively examined the effect of insomnia on sick leave, and with mixed results. Vahtera et al. [15] found both moderately and severely disturbed sleep to be associated with higher rate of subsequent sickness absence, whereas Ihlebaek et al. [16] did not find any association between poor sleep and a large increase in sickness absence from 1996 to 2003 in Norway. Similarly, in a cohort study of Norwegian nurses, Eriksen et al. [17] found no effect of sleep complaints on medium-term sick leave 3 months later. In contrast, Akerstedt et al. [18] recently found that working individuals with impaired sleep had 25% increased risk entering a period of long-term sick leave 2 years later compared with good sleepers. However, using only a single item to measure disturbed sleep, this study did not control for important confounders beyond work environment factors, such as mental and physical morbidity. When also adjusting for somatic and mental diseases [19], a Norwegian study found subjective sleep quality (also assessed with a single item) to be a significant predictor for self-reported sick leave 4 years later. The only study so far to examine the effects of a diagnostically based operationalization of insomnia found insomnia (defined as difficulties initiating or maintaining sleep and impaired work performance) to be a strong and independent risk factor for awards of disability pensions during a follow-up period of 4 years [20]. To our knowledge, no studies have examined the independent effects of this definition of insomnia on subsequent sick leave.
Thus, the aim of the present study was to estimate the independent effect of insomnia on sick leave using a historical cohort design linking a large population-based health survey with national registry data on sick leave. We also wanted to adjust the effect for potential confounders, including physical and mental symptoms and conditions, as well as sociodemographic factors.
Section snippets
Population and data material
The Hordaland Health Study (HUSK), 1997 to 1999, was a joint epidemiological research project carried out by the Norwegian Health Screening Service in collaboration with the University of Bergen, Bergen, Norway. The base population for the study reported herein included all 29,400 individuals in Hordaland County, Western Norway, born 1953 to 1957, aged 40 to 45 years at the time of the data collection. Data were collected by questionnaires and clinical examinations. A total of 18,581 (8598 men
Sample characteristics
Baseline characteristics of the 6892 participants completing the insomnia questionnaire are shown in Table 1. The prevalence rate for insomnia was 5.1%, whereas a further 5.9% experienced insomnia without daytime impairment. Insomnia was more prevalent among persons with less education and who exercised less. Insomnia was also significantly associated with symptoms of anxiety, depression, and sleep apnea, as well as somatic diagnosis and symptoms, and pain (Table 1). Insomnia was unrelated to
Discussion
The present study demonstrates that DSM-IV-defined insomnia is an independent risk factor for long-term sick leave up to 4 years later. The effect is especially strong when analyzing sick leave beyond 3 months. Although a substantial part of the effect could be explained by insomnia being associated with a range of confounding factors, including anxiety and depression, somatic symptoms, and pain, insomnia remained an independent risk factor for long-term sick leave also in the fully adjusted
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