The joint contribution of insomnia and obstructive sleep apnoea on sickness absence.

Several studies have indicated a high degree of overlap between insomnia and obstructive sleep apnoea, but little is known regarding how the overlap may affect adverse outcomes associated with each of the disorders. The aim of the current study was to examine the separate and combined effects of symptoms of insomnia and obstructive sleep apnoea on long-term sick leave. We used an historical cohort design with 4 years follow-up. Information on sick leave was obtained from Norwegian official registry data, and merged with health information from the Hordaland Health Study in western Norway, 1997-99. A total of 6892 participants aged 40-45 years were assessed for self-reported symptoms of insomnia and obstructive sleep apnoea (snoring and breathing cessations), as well as confounding factors. The level of overlap between insomnia and obstructive sleep apnoea was low (7-12%). Both insomnia and obstructive sleep apnoea alone were significant risk factors for subsequent sick leave after adjusting for confounding factors (odds ratios ranging from 1.4 to 2.3). Having comorbid insomnia and obstructive sleep apnoea increased the risk significantly. There was an additive interaction effect between the two conditions in the unadjusted analyses, but this was reduced to a non-significant level when adjusting for confounders. This study is the first to report the separate and combined effects of insomnia and obstructive sleep apnoea on any adverse outcome. Having both insomnia and obstructive sleep apnoea increased the risk of later sick leave, but there was no evidence of an independent synergy effect of the two conditions.