Older adults experience weaker cerebrospinal fluid waves during deep sleep due to diminished slow brain waves, according to a study published in Nature Aging.
As people age, they typically sleep for shorter periods and wake up more frequently throughout the night. MIT researchers discovered that a biological process responsible for flushing waste from the central nervous system is commonly impaired in older individuals. This impairment involves cerebrospinal fluid waves—fluid that surrounds the brain and spinal cord to help remove metabolic debris—which are significantly reduced in older adults.
The reduction in fluid movement links directly to a decrease in the slow brain waves that normally occur during deep non-REM sleep. Researchers noted this fluid deficit is not simply a result of getting fewer hours of sleep overall. During the deepest stages of sleep, high-amplitude, low-frequency electrical oscillations contribute to physical recovery and memory consolidation.
In a 2019 study, Laura Lewis found that these slow oscillations are critical for generating the fluid waves that flush waste out of brain tissue. That earlier discovery led her team to investigate whether reductions in slow waves among older people might suppress this cleansing mechanism. Previous scientific investigations into aging yielded conflicting evidence regarding fluid movement, but those past projects did not monitor flow specifically during sleep.
Assessing Brain Waves and Fluid Flow in Sleep
The research team recruited volunteers across two distinct age groups: 18 to 39 years old and 60 to 84 years old. Testing sessions lasted approximately two hours and began late at night to ensure participants felt naturally sleepy.
Investigators utilized traditional functional magnetic resonance imaging to measure blood flow changes in the brain, alongside a modified MRI technique developed by Lewis to measure cerebrospinal fluid movement. While subjects rested inside the scanner, researchers recorded their brain electrical activity using electroencephalography.
Many participants successfully fell asleep inside the scanner, allowing the team to compare neurological activity during wakefulness and sleep. When younger adults entered non-REM sleep, their electrical slow waves became stronger, and their fluid waves expanded.
However, in the older participant group, average fluid waves measured significantly smaller than those observed in younger subjects. Additional experiments conducted while subjects remained fully awake sought to identify the physiological drivers behind these age-related declines.
Researchers explored whether changes in neurovascular coupling—the process where localized brain activity increases local blood vessel dilation to deliver oxygen and nutrients—played a role. Testing each subject’s neurovascular coupling involved measuring blood flow while participants viewed a flickering checkerboard image designed to drive neural activity.
While neural activity increased in many older subjects, blood flow failed to rise comparably, suggesting that blood vessels possess diminished responsiveness to increased oxygen demands. A second breath-holding task tested vascular system integrity by tracking carbon dioxide-triggered blood flow increases, which also showed smaller increases in older participants.
| Metric | Younger Adults (18–39) | Older Adults (60–84) |
|---|---|---|
| CSF Wave Amplitude | Significantly larger during sleep | Significantly reduced on average |
| Neurovascular Coupling | Normal dilation on demand | Diminished blood flow response |
Deep Sleep Fluid Cleansing Weakens in Older Adults
- Cerebrospinal fluid waves wash metabolic waste out of the brain during deep sleep, but these cleansing waves weaken significantly in older adults.
- This reduction stems partly from fewer slow brain waves during deep sleep, combined with stiffening blood vessels that limit healthy blood flow responses.
- Researchers are investigating whether these sleep-related fluid changes can predict neurodegenerative risk or serve as targets for future sleep interventions.
Reduced Blood Flow and Gray Matter Atrophy Correlate with Fluid Decline
“Aging is so complex, and so many different things change, that it's really hard to disentangle the individual factors,” Bailes said, noting that statistical modeling revealed that reduced neurovascular coupling and diminished blood flow accounted for much of the fluid decline, though other unmeasured vascular and lifestyle factors likely contribute. None of the study volunteers had received a diagnosis of a neurodegenerative disorder.
Nonetheless, researchers observed that certain older adults displayed signs of gray matter atrophy, which correlated with more severe decreases in fluid movement. The team aims to continue monitoring participants to determine whether altered fluid dynamics heighten the long-term risk of developing conditions such as Alzheimer’s disease and other forms of dementia.
Investigators also plan to test potential interventions designed to restore or enhance fluid flow in older populations. In separate work, Lewis’s laboratory demonstrated that short acoustic bursts of pink noise during sleep successfully increased fluid movement in younger adults, though researchers have not yet tested this protocol in older cohorts.
The study said that a subset of older adults maintained normal fluid waves that fell directly within the range measured in healthy young adults. Understanding what preserves this function in certain individuals remains a primary goal for upcoming research.
“We know that people age really differently. Some people age very successfully with great cognitive outcomes, and other people develop cognitive decline. If we can figure out who has great trajectories of aging and what are the factors that are associated with that, we might be able to use that to predict future outcomes.”
Laura Lewis, Athinoula A. Martinos Associate Professor of Electrical Engineering and Computer Science, MIT
The research received funding from the National Institutes of Health, the Simons Collaboration on Plasticity in the Aging Brain, a McKnight Scholar Award, a Pew Biomedical Scholar Award, an MIT EECS Transformative Research Award, the Picower Institute Neurological Disorder Discovery Research Fund, a Sloan Fellowship, and the Department of Defense National Defense Science and Engineering Graduate Fellowship Program.
References
- Nature Aging: Age-related impairments in cerebrospinal fluid dynamics during sleep
- National Institutes of Health: Aging and Brain Health Research Grants
