Transdiagnostic Internet-delivered Cognitive Behaviour Therapy (ICBT) is an efficacious treatment option for anxiety and depression; however, not all patients benefit equally and some leave treatment before program completion. Comorbid symptoms of insomnia are extremely common among individuals seeking ICBT, yet sleep problems are rarely a primary focus of treatment in transdiagnostic ICBT despite insomnia being a known risk factor for anxiety and depression. This trial is designed to test whether an ICBT program for anxiety and depression can be improved by providing therapy for insomnia alongside the standard transdiagnostic program. For this randomized controlled trial, patients will be randomly assigned to receive either the standard ICBT program for anxiety and depression, which includes only brief supplementary information about sleep (n = 68), or a modified version that includes a lesson specifically on sleep (n = 132). The sleep lesson will introduce patients to two key behavioural strategies: sleep restriction and stimulus control, which previous research has demonstrated are effective at reducing insomnia. Patients will monitor their sleep throughout treatment and will complete measures of insomnia, anxiety and depression before treatment, at the end of the 8-week program, and 3 months after program conclusion to allow for a comparison of patient outcomes and completion rates between conditions. The acceptability of the new intervention will also be assessed by asking participants to provide feedback on the new materials and to complete measures of treatment satisfaction and working alliance.