BACKGROUND: Healthy sleep is a fundamental component of physical and brain health. Insomnia, however, is a prevalent sleep disorder that compromises functioning, productivity, and health. Therefore, developing efficient treatment delivery methods for insomnia can have significant societal and personal health impacts. Cognitive behavioral therapy for insomnia (CBTI) is the recommended first-line treatment of insomnia but access is currently limited for patients, since treatment must occur in specialty sleep clinics, which suffer from an insufficient number of trained clinicians. Smartphone-based interventions offer a promising means for improving the delivery of CBTI. Furthermore, novel features such as real-time monitoring and assessment, personalization, dynamic adaptations of the intervention, and context awareness can enhance treatment personalization and effectiveness, and reduce associated costs. Ultimately, this "Just in Time Adaptive Intervention" for insomnia-an intervention approach that is acceptable to patients and clinicians, and is based on mobile health (mHealth) platform and tools-can significantly improve patient access and clinician delivery of evidence-based insomnia treatments. OBJECTIVE: This study aims to develop and assess the usability of a Just in Time Adaptive Intervention application platform called iREST ("interactive Resilience Enhancing Sleep Tactics") for use in behavioral insomnia interventions. iREST can be used by both patients and clinicians. METHODS: The development of iREST was based on the Iterative and Incremental Development software development model. Requirement analysis was based on the case study's description, workflow and needs, clinician inputs, and a previously conducted BBTI military study/implementation of the Just in Time Adaptive Intervention architecture. To evaluate the usability of the iREST mHealth tool, a pilot usability study was conducted. Additionally, this study explores the feasibility of using an off-the-shelf wearable device to supplement the subjective assessment of patient sleep patterns. RESULTS: The iREST app was developed from the mobile logical architecture of Just in Time Adaptive Intervention. It consists of a cross-platform smartphone app, a clinician portal, and secure 2-way communications platform between the app and the portal. The usability study comprised 19 Active Duty Service Members and Veterans between the ages of 18 and 60. Descriptive statistics based on in-app questionnaires indicate that on average, 12 (mean 12.23, SD 8.96) unique devices accessed the clinician portal per day for more than two years, while the app was rated as "highly usable", achieving a mean System Usability Score score of 85.74 (SD 12.37), which translates to an adjective rating of "Excellent". The participants also gave high scores on "ease of use and learnability" with an average score of 4.33 (SD 0.65) on a scale of 1 to 5. CONCLUSIONS: iREST provides a feasible platform for the implementation of Just in Time Adaptive Intervention in mHealth-based and remote intervention settings. The system was rated highly usable and its cross-platformness made it readily implemented within the heavily segregated smartphone market. The use of wearables to track sleep is promising; yet the accuracy of this technology needs further improvement. Ultimately, iREST demonstrates that mHealth-based Just in Time Adaptive Intervention is not only feasible, but also works effectively.