Sleep Duration Associations with CSF-Tissue Coupling Flexibility and Circadian Synchronization: An Observational Study of Glymphatic-Related Dynamics

STUDY OBJECTIVES: The glymphatic system clears brain waste and is most active during sleep. How habitual sleep duration influences its diurnal function in humans remains unclear. We examined whether individuals with longer habitual sleep duration exhibit enhanced diurnal glymphatic fluctuations characterized by flexible cerebrospinal fluid (CSF)-tissue coupling and synchronized temporal dynamics compared with those with shorter sleep duration. METHODS: In this cross-sectional study, 25 healthy adults (mean age 26.5 years; 52% women) underwent multimodal MRI at 8 a.m., 2 p.m., and 8 p.m. on a single day. Participants were stratified into long (≥404 minutes, n = 14) and short (<404 minutes, n = 11) sleep groups. Brain volume, free water, diffusion tensor imaging metrics, and myelin water fraction were assessed. CSF-tissue coupling and temporal synchronization were analyzed. RESULTS: Long sleepers showed diurnal increases in CSF volume and gray matter free water with concurrent white matter reduction, flexible CSF-tissue coupling, and synchronized evening peaks across multiple markers. Short sleepers exhibited rigid CSF-white matter coupling, limited diurnal variation, and desynchronized peaks. CONCLUSIONS: Adequate sleep duration supports flexible fluid-tissue interactions and circadian synchronization, which may reflect more efficient glymphatic-related dynamics relevant to brain health.