In today’s society, insomnia, depression and anxiety are widely present and digital mental health interventions have become essential tools to meet these needs. Of these interventions, smartphone-based, cost-effective, and scalable Cognitive Behavioural Therapy (CBT) is an accessible alternative to face-to-face therapy. This review aims to highlight the effectiveness of smartphone-based CBT for insomnia and mood disorders, access barriers for equity, and ethical considerations with regard to scaling up large-scale digital mental health programmes. There have been randomized controlled trials and systematic reviews conducted to assess the effectiveness of smartphone delivered CBT, which reveal its effectiveness at improving sleep quality, reducing insomnia severity, and improving depression and anxiety symptoms. However, there are unequalities in digital literacy, access to the internet, socioeconomic status and culture which remain to limit the uptake and impact of interventions, particularly for vulnerable groups. The review also delves into the ethical and regulatory challenges and the need for privacy and data security to make the use of digital mental health services safe and beneficial. The evidence-based case study of insomnia in smartphone users is provided to illustrate the use of smartphone CBT, its benefits and implementation problems. The review finds that smartphone based CBT is an effective intervention for improving mental health outcomes, but long-term success will require access to CBT, robust data protection, supportive health policies and continual technological innovation