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Insomnia and depression are closely associated, but exactly how they might be causally linked is uncertain. These investigators used prospective data on 24,715 individuals (mean age at baseline, 45) from two waves of a Norwegian health survey; the waves were conducted 11 years apart.
Analyses were conducted on two groups — individuals who were initially clinically depressed without concurrent insomnia, and individuals initially experiencing insomnia without concurrent clinical depression. In the initial-depression group, those who were also depressed at the next wave were at significantly greater risk for developing insomnia, even after adjustment for demographics, somatic symptoms, lifestyle factors, and anxiety (adjusted odds ratio, 4.34).…