To the Editor The study by Irwin et al1 found that cognitive behavioral therapy for insomnia (CBT-I) had an overall benefit in the prevention of incident and recurrent major depression compared with sleep education therapy (SET) in older adults with insomnia disorder. The authors used a predefined analysis to compare treatment groups as a function of sustained remission of insomnia disorder to test the superiority of CBT-I over SET in remission of insomnia disorder. We suggested that mediation analysis might be used to clarify the role of insomnia remission in the association between CBT-I and SET and the prevention of depression.