Only 24.9% of the Belgian adults (25-50 years) with type 1 diabetes mellitus (T1DM) achieve the glycemic target. This can be explained by the challenging day-to-day diabetes management which places a substantial burden on this population. However, a tight glucose control is fundamental in order to prevent the development of acute and chronic complications. Despite the added value of continue glucose monitors to glucose control, optimizing daily glucose levels is still problematic in adults with T1DM. In addition to self-monitoring of blood glucose, behaviors such as physical activity (PA), sedentary behavior (SB) and sleep can have an impact on glycemic management. A recent shift in health promotion research stresses the importance of considering all these behaviors (i.e. PA, SB and sleep) in one 24-hour day instead of focusing on one behavior in isolation. The aim of this study is two-fold. First, the day-by-day 24h-MB patterns of adults (25-50 years) with T1DM and the association with intra-day glucose control (i.e. time in range and coefficient of variation) will be investigated. The 24-h MB patterns and intra-day glucose control will be associated with explanatory variables and cardiometabolic health markers. Then, a theory-driven intervention will be created together with adults with T1DM using a co-creational approach. The co-created intervention will be tested for feasibility in a pilot intervention.