Chronotypes and disabling musculoskeletal pain: A Finnish birth cohort study.

Background: It has been suggested that chronotype, the individual preference for 24-h circadian rhythms, influences health. Sleep problems and mental distress are amongst the greatest risk factors for musculoskeletal (MS) pain. The aims of this study were first, to explore the associations between chronotypes and MS pain, with special reference to disabling MS pain and second, to test whether mental distress and insomnia have a modifying role in the associations between chronotypes and MS pain.

Methods: The dataset of 4961 individuals was composed of Northern Finns surveyed on MS pain, chronotypes and confounding factors (sex, insomnia, sleep duration, smoking, mental distress, occupational status, education level and number of coexisting diseases) at 46 years. The relationships between chronotypes (evening [E], intermediate [I] and morning [M]) and MS pain were evaluated using multinomial logistic regression. To address the second aim, we included an interaction term (chronotype*mental distress, chronotype*insomnia) in the logistic model.

Results: Compared to the M-types, both the E- and I-types had increased odds of suffering 'disabling pain' in the unadjusted model (odds ratio [OR] 1.79, 95% confidence interval [CI] 1.37-2.33; OR 1.54, 95% CI 1.29-1.84, respectively). However, the association remained statistically significant only after adjusting for all covariates amongst the I-types (OR 1.39, 95% CI 1.15-1.67). Neither mental distress nor insomnia was found to modify the chronotype-MS pain association.

Conclusions: The results highlight the importance of chronotypes for individuals' MS health but suggest the presence of confounding factors in the interplay between these factors.

Significance: This study shows that evening and intermediate chronotypes are associated with disabling MS pain, but that mental distress, insomnia and coexisting diseases also play a role in these associations.