Brain homeostasis depends on delivery of nutrients and oxygen, as well as removal of waste products. In recent years, a spike in interest for brain clearance pathways emerged with a strong focus on sleep, since brain clearance seems enhanced during deep sleep. This has resulted in studies on the role of sleep that gained much attention in the field and from the public. Yet, the interpretation of these studies is heavily debated, since the exact mechanisms remain obscure. Similarly, the role of impaired brain clearance in vascular and neurological pathologies is unclear. Within the current debate on the mechanisms of brain clearance, perivascular spaces and movement of cerebrospinal fluid (CSF) within these, play a central role. However, important factors remain unknown, such as the processes determining the efficiency of brain clearance, including the driving forces. Moreover, a major limitation is that at the moment almost all knowledge arises from rodent models with very limited translation to human studies. This limitation also holds for the role of sleep in brain clearance. Brain clearance involves diffusion (along a concentration gradient), flow (along a pressure gradient), and mixing. Mixing, as a result of vital oscillations, can greatly enhance solute clearance when net flow is absent or limited, such as in perivascular spaces along penetrating arteries. Three important natural forces might generate such oscillations, acting at different frequencies: the cardiac (≈1 Hz), the respiratory (≈0.2 Hz) and the vasomotor forces (≈0.1 Hz). In the current project, we aim to quantify the contribution of these physiological processes to brain clearance with regard to mixing as well as net flow and to assess if and how this changes during sleep. The current team of researchers provides a great opportunity to address the project in a truly translational manner: it will combine the expertise of the UMC (Amsterdam) on mechanistic studies of brain clearance in rodents with the LUMC’s (Leiden) newly-developed non-invasive MRI approach in humans, exploiting the improved resolution capabilities of ultra-high field MRI (7 Tesla). This will enables us to: 1) image the relevant anatomical substructures with high resolution techniques in rodents (perivascular spaces around penetrating arteries and veins); 2) obtain a 3D reconstruction of perivascular spaces in humans in vivo; 3) perform experimental manipulation of cardiac, respiratory, and vasomotor forces in rodents; 4) study the role of sleep in brain clearance in rodents; and 5) apply human MRI to capture whole brain CSF dynamics, including perivascular spaces of penetrating arteries in awake and during sleep. A better understanding of the process of brain clearance is crucial for new treatment strategies in neurodegenerative diseases as well as a clearer insight into the impact of e.g. aging, hypertension and sleep on brain clearance.