Reduced Cerebrospinal Fluid Waves During Sleep Linked to Aging, Potentially Impairing Brain Waste Clearance

A team at MIT has identified a marked decrease in waves of cerebrospinal fluid (CSF)—which facilitate the removal of metabolic waste from the brain during deep sleep—in individuals aged 60 to 84 compared to younger adults. These CSF waves are tightly coupled with slow brain waves that characterize non-REM sleep stages, which also demonstrated declines with age.

The study involved 18 to 39-year-old and 60 to 84-year-old participants who underwent simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) with a specialized MRI method developed to measure CSF flow. Many subjects were monitored while naturally falling asleep in the scanner, enabling direct comparison of brain and CSF activity during wakefulness and sleep.

Older subjects exhibited significantly smaller CSF wave amplitudes during deep sleep despite similar durations of sleep, suggesting that impairment in brain waste clearance is not merely a consequence of reduced sleep time with age. This diminished CSF flow was linked to a reduction in neurovascular coupling—a process where increased neural activity prompts blood vessel dilation to deliver oxygen—and attenuated cerebral blood flow responses.

Additional tests showed that older adults had weaker vascular responses to elevated carbon dioxide levels induced by breath-holding tasks, further indicating vascular contributions to impaired CSF flow with aging. However, these factors did not fully account for the reduction, implying other age-related mechanisms are involved.

The researchers noted variability within older adults; some maintained CSF wave patterns comparable to younger individuals, a finding they intend to investigate to understand factors supporting healthier brain aging trajectories. The team plans to explore interventions to enhance CSF flow, including experimental use of auditory 'pink noise' stimuli previously shown to promote CSF dynamics during sleep in younger adults.

This research sheds light on a crucial but understudied biological mechanism that may contribute to age-related cognitive decline and heightened risk for neurodegenerative diseases. While the study did not include individuals diagnosed with such conditions, correlations between reduced CSF flow and gray matter atrophy in older participants suggest a potentially important link.

Overall, the findings underscore the complexity of aging-related brain changes and the need for further research to disentangle contributing vascular, neural, and lifestyle factors, with the goal of developing predictive markers and therapies to preserve brain health.

Funded by NIH and other bodies, the study was published in Nature Aging and led by Laura Lewis and colleagues at MIT's Institute for Medical Engineering and Science.