Only 24.9% of the Belgian adults (25-50 years) with type 1 diabetes mellitus (T1DM) achieve a good glucose control. This can be explained by the challenging day-to-day diabetes management which places a substantial burden on this population. However, a tight glycemic 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, a healthy lifestyle with sufficient physical activity (PA), limited sedentary behavior (SB) and sufficient sleep time and quality is crucial for a good glucose control. A recent shift in health promotion 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 to investigate the association between the day-by-day 24h-MB patterns of adults (25-50 years) with T1DM and their intra-day glucose control (i.e. time in range and coefficient of variation) on the one hand. On the other hand, associations between he 24-h MB patterns and explanatory variables and cardiometabolic health markers will be investigated. To gain insight into the 24-hour behavior of adults with type 1 diabetes, 150 adults with type 1 diabetes will wear an Actigraph accelerometer, for 14 consecutive days. Daily glucose control will be measured using the participant's continuous glucose meter. Information about the explanatory variables and cardiometabolic health will be obtained by means of a questionnaire, diary and a few measurements (blood pressure, weight, length, Advanced Glycation Endproducts, hip-and waist circumference) during a one-off visit to one of the recruitment- and testing centers namely University hospital of Ghent or University hospital of Antwerp. The results of this cross-sectional study will inform future interventions focusing on the 24-hour movement behaviors in adults with T1DM.