Insomnia treatment using an internet-based cognitive–behavioural therapy for insomnia (CBT-I) program reduces depression symptoms, anxiety symptoms and suicidal ideation. However, the speed, longevity and consistency of these effects are unknown.

To test the following: whether the efficacy of online CBT-I was sustained over 18 months; how rapidly the effects of CBT-I emerged; evidence for distinct trajectories of change in depressive symptoms; and predictors of these trajectories.

A randomised controlled trial compared the 6-week Sleep Healthy Using the Internet (SHUTi) CBT-I program to an attention control program. Adults (N=1149) with clinical insomnia and subclinical depression symptoms were recruited online from the Australian community.

Depression, anxiety and insomnia decreased significantly by week 4 of the intervention period and remained significantly lower relative to control for >18 months (between-group Cohen's d=0.63, 0.47, 0.55, respectively, at 18 months). Effects on suicidal ideation were only short term. Two depression trajectories were identified using growth mixture models: improving (95%) and stable/deteriorating (5%) symptoms. More severe baseline depression, younger age and limited comfort with the internet were associated with reduced odds of improvement.

Online CBT-I produced rapid and long-term symptom reduction in people with subclinical depressive symptoms, although the initial effect on suicidal ideation was not sustained.