To use non-invasive, naturalistic fNIRS-EEG methods to investigate the coupling of glymphatic clearance with neural rhythms and slow-wave oscillations. 

The brain’s glymphatic system, an interstitial waste and signal transport pathway for cerebrospinal fluid (CSF) and interstitial fluid (ISF) exchange, has pronounced activity during non-rapid eye movement (NREM) sleep and is implicated in neurodegenerative and mood disorders. However, direct measurement in humans is limited because current imaging methods rely on invasive MRI methods, posing a major barrier to advancing glymphatic research in clinical settings.

Water concentrations were highest in slow-wave-sleep (SWS) (d = 1.93, p=0.0002), while HbO concentrations also showed a modest elevation (d = 1.06, ns). After regressing out the HbO contributions, the oscillatory water component correlated with delta power (r=0.63, p=.02). Additionally, low-frequency oscillations (LFOs) of both water and HbO signals exhibited distinct dynamics of suprathreshold envelope peak (SEP) frequencies during NREM as compared to REM and Wake, with effect size (d= 1.31, p = 0.003) for water. These findings indicate that water-sensitive oscillatory processes are selectively amplified during EEG-defined deep sleep, consistent with a role for glymphatic-related fluid transport in human sleep. 

We report novel cortical water shift parameters that are robustly sensitive to sleep stage transitions via a wearable, non-invasive, scalable headband, consistent with predicted glymphatic activity. Future work will focus on MRI cross-validation.