Insomnia in depressed patients is usually targeted by medication, while cognitive behavioural treatment for insomnia (CBT-I) is the treatment of first choice. Effective online CBT-I is available but is not frequently offered in regular care.<br/> AIM: To determine the feasibility and indications for effectiveness of I-Sleep, an online CBT-I module, for uni- and bipolar depressed patients in specialised mental health care settings.<br/> METHOD: In a pilot study I-Sleep (5 sessions) was offered to all 31 patients. Patients who did not (yet) want to participate in the online intervention, and patients who were included after the intended sample size was reached, participated in the control-group. Feasibility was assessed by compliance rates and satisfaction of patients and therapists (CSQ). Effectiveness was assessed within and between groups by the Insomnia Severity Index (ISI) and the Patient Health Questionnaire (PHQ-9) at baseline and after 6 and 12 weeks.<br/> RESULTS: In the intervention group 41% completed all treatment sessions and 77% completed 3 or more sessions. Patients rated the intervention positively (CSQ 23.6, range 4-32). Sleep improved in the intervention group (Cohen's d = 1.93), as well as depression (Cohen's d=1.05). In the control group there was no significant improvement. The difference between the two groups was high and significant for sleep problems (Cohen's d = 0.99) but not for depressive symptoms.<br/> CONCLUSION: Online CBT-I is a feasible treatment option for depressed patients in mental health care settings. There are indications that CBT-I is also effective in reducing sleep problems in more severely depressed patients.