Sleep is a fundamental biological process with wide-ranging effects on cognition, emotional regulation, immune function, and overall brain health. In recent years, growing evidence has revealed a crucial role of sleep in clearing metabolic waste from the brain via the glymphatic system - a brain-wide fluid transport pathway that relies on coordinated neural and vascular activity. This brain-clearance process is closely linked to slow-wave activity during non-rapid eye movement sleep (SWA during NREM; 0.5–2 Hz) observed in electroencephalographic (EEG) recordings. Increased SWA has been associated with increased cerebrospinal fluid (CSF) flow, enabling glymphatic system function. Disruption of the glymphatic clearance process is increasingly associated with ageing and neurodegenerative diseases such as Alzheimer’s.
Despite growing interest in this field, due to the lack of non-invasive easy-to-use technological solutions, studying glymphatic clearance in natural at-home sleep conditions has remained a challenge. The goal of the action “Exploring neuro-glymphatic coupling during sleep using wearable technology” (GlymphoSleep project) is to overcome these limitations by validating the usage of the first wearable solution capable of assessing changes in CSF dynamics. This technology relies on the multi-wavelength functional near-infrared spectroscopy (fNIRS) which is sensitive to changes in hemodynamics (changes in blood oxygenation), and, importantly, CSF flow. The device “G-meter” was developed by a team of engineers at the University of Oulu, Finland, the main collaborators of this project. By using a combination of CSF-sensitive fNIRS and EEG to monitor electrophysiological sleep markers, this portable setup allows assessing neuro-glymphatic coupling during natural sleep in an at-home environment. Additionally, the project employed non-invasive closed-loop auditory stimulation (CLAS) to enhance SWA, to causally test the link between sleep electrophysiology and brain clearance. CLAS is a validated method that reliably enhances slow waves without inducing awakenings or altering the natural sleep structure.
Thus, the main scientific objectives of the GlymphoSleep project were to: (1) characterize the relationship between sleep neurophysiology and glymphatic clearance; (2) test whether enhancing SWA would change CSF flow; and (3) explore how variability in this coupling depends on the individual genetic profile. While the third objective was not pursued due to logistical reasons, the first two have been fully addressed through successful data collection and publications in preparation.
By bridging neuroscience, biomedical engineering, and sleep research, the GlymphoSleep project contributes to the EU’s strategic goals in health innovation, ageing research, and digital transformation. The technological outputs have implications for early detection of neurodegenerative risk and personalised sleep medicine. Ultimately, the project lays a foundation for wearable tools to monitor brain clearance in ageing populations, supporting preventive health strategies across Europe and beyond.
