Coupling strength between global grey matter BOLD and CSF signals was highest in the morning and decreased significantly across the day, highlighting the importance of accounting for both the time of scan and individual sleep characteristics when interpreting fMRI-based gGM-CSF coupling measures.
Key Findings
Results
Global grey matter BOLD and CSF signal coupling strength was highest in the morning and decreased significantly across the day.
The correlation between time of day and coupling strength was r = -0.18, p < 0.001.
Data were drawn from 875 healthy young adults from the Human Connectome Project.
Resting-state fMRI data were used to examine coupling at varying time points throughout the day.
This finding indicates that diurnal variation in macroscopic CSF flow relative to global oxygen fluctuations is time-dependent.
Results
Global grey matter BOLD amplitude also decreased over the course of the day.
The correlation between time of day and gGM BOLD amplitude was r = -0.14, p < 0.001.
This suggests that the diurnal decline in coupling is at least partly driven by changes in global grey matter hemodynamic amplitude.
Data were from the same sample of 875 healthy young adults.
Results
CSF signal amplitude did not change significantly across the day.
The correlation between time of day and CSF amplitude was r = 0.01, p = 0.74.
This finding indicates that CSF amplitude itself is stable across the day, in contrast to gGM BOLD amplitude.
The divergence between gGM and CSF amplitude trends suggests that the two signals are modulated by different mechanisms across the day.
Results
Among Pittsburgh Sleep Quality Index (PSQI) subcomponents, only sleep duration was significantly associated with gGM-CSF coupling strength.
The correlation between PSQI sleep duration subcomponent and coupling was r = 0.10, p < 0.006.
Other PSQI subcomponents were not significantly associated with coupling.
PSQI scores were collected alongside resting-state fMRI data for the 875 participants.
Results
Self-reported sleep duration was negatively correlated with gGM-CSF coupling strength, indicating stronger coupling in individuals who reported shorter sleep.
The correlation between self-reported sleep duration and coupling strength was r = -0.11, p < 0.01.
Shorter reported sleep was associated with stronger gGM-CSF coupling.
This finding may reflect a homeostatic or compensatory relationship between sleep duration and macroscopic CSF dynamics.
Self-reported sleep duration was distinct from the PSQI sleep duration subcomponent analysis.
Methods
The study used publicly available resting-state fMRI data from the Human Connectome Project along with Pittsburgh Sleep Quality Index scores.
The sample consisted of 875 healthy young adults.
Pittsburgh Sleep Quality Index (PSQI) scores were included to assess sleep characteristics.
The study examined gGM BOLD and CSF signal coupling as a measure of macroscopic CSF flow in relation to global oxygen fluctuations.
The dataset allowed examination of scan time as a continuous variable across the day.
What This Means
This research suggests that the movement of cerebrospinal fluid (CSF) — the fluid that surrounds and flows through the brain — is not constant throughout the day but follows a diurnal pattern. Using brain imaging data from 875 healthy young adults, researchers found that the coordinated relationship between global brain activity (measured by blood oxygen levels) and CSF movement was strongest in the morning and declined as the day progressed. Interestingly, while overall brain activity amplitude also dropped across the day, CSF signal strength itself stayed stable, suggesting these two aspects of brain function are regulated by different mechanisms.
The study also found links between sleep and this brain-CSF coupling. Among various sleep quality measures, only sleep duration was meaningfully associated with coupling strength. Somewhat surprisingly, people who reported sleeping fewer hours showed stronger coupling between brain activity and CSF movement. This could reflect the brain's response to insufficient sleep, potentially related to its waste-clearance processes, which have been linked to the glymphatic system — a network thought to help flush waste products from the brain, particularly during sleep.
This research suggests that when scientists interpret brain imaging studies involving CSF movement, they need to carefully account for what time of day scans were collected and participants' individual sleep habits. Failing to do so could introduce confounding variability into findings, particularly in studies investigating brain waste clearance, neurological disease, or the effects of sleep on brain health.
Müller L, Berberich C, Heinrich P, Nuttall R, Pilge S, Zott B, et al.. (2026). Diurnal changes of cerebrospinal fluid and global signal coupling.. NeuroImage. https://doi.org/10.1016/j.neuroimage.2026.121833