Abstract
BACKGROUND: A strong relationship between insomnia and painful disorders has been found, but it is still unclear whether chronic pain leads to insomnia. There is a need of large-scale prospective studies to evaluate if there is a causal relationship between painful disorders and insomnia. METHODS: All inhabitants aged ≥ 20 years in Nord-Trøndelag County of Norway were invited to participate in two surveys (n = 92,566 and 93,860, respectively). 27,185 subjects participated in both surveys, and 19,271 of these were insomnia-free at baseline (population at risk). Using logistic regression, we evaluated the influence of headache, CMSCs and coexisting headache and CMSCs on the subsequent risk of insomnia. RESULTS: Compared to subjects without headache and CMSCs, there was an increased risk of insomnia among those with headache, most pronounced among those with headache ≥ 7 days / month (OR = 2.2, 95% CI = 1.9 - 2.6). Similarly, an increased risk among those with CMSCs was found, most evident for those with widespread CMSCs (OR = 2.0, 95% CI = 1.8 - 2.2). Having coexistent CMSCs and headache (OR = 2.0, 95% CI = 1.8 - 2.2) predisposed more strongly to insomnia than having headache (OR = 1.5, 95% CI = 1.3 - 1.6) and CMSCs (OR = 1.6, 95% CI = 1.4 - 1.7) alone. CONCLUSION: In this prospective study headache and CMSCs were risk factors for insomnia 11 years later.