Diurnal changes of cerebrospinal fluid and global signal coupling

Cerebrospinal fluid (CSF) bulk movement around the brain is mediated by brain hemodynamics and has been linked to the brain´s waste clearance process and the so-called glymphatic system. Factors such as sleep have been demonstrated to influence global brain hemodynamics, including the coupling between global grey matter (gGM) BOLD and CSF signals, a measure to assess macroscopic CSF flow in relation to global oxygen fluctuations. Although diurnal changes in the amplitude of gGM also occur, whether macroscopic CSF flow couples to gGM varies across the day remains unclear. Using publicly available resting-state fMRI data from healthy adults, we examined the coupling of these signals at varying time points throughout the day. We included data from 875 healthy young adults from the Human Connectome Project, together with Pittsburgh Sleep Quality Index (PSQI) scores. Our results show that coupling strength was highest in the morning and decreased significantly across the day (r = -0.18, p < 0.001). While gGM BOLD amplitude also decreased over the day (r = -0.14, p < 0.001), CSF amplitude did not (r = 0.01, p = 0.74). Among PSQI subcomponents, only sleep duration was significantly associated with coupling (r = 0.10, p < 0.006). Furthermore, self-reported sleep duration was negatively correlated with coupling strength (r = -0.11, p < 0.01), indicating stronger coupling in those who reported shorter sleep. These findings highlight the importance of accounting for both the time of scan and individual sleep characteristics when interpreting fMRI-based gGM-CSF coupling measures.