The brain has its own cleaning system, working quietly through the night to remove waste that accumulates during waking hours. But new research from the Massachusetts Institute of Technology (MIT) suggests this essential process becomes less efficient with age, potentially raising questions about its role in maintaining brain health and preventing cognitive decline.

Researchers discovered that waves of cerebrospinal fluid (CSF), which help wash away waste products from the brain, are significantly weaker in older adults. The decline appears connected to changes in the slow brain waves associated with deep sleep, rather than simply the reduced amount of sleep commonly experienced with ageing.

The findings, published in Nature Aging, could help scientists better understand why some people maintain sharp cognitive abilities well into old age while others experience memory loss and declining brain function. The research team also hopes to determine whether changes in CSF flow could eventually help identify individuals at greater risk of neurodegenerative diseases.

Led by MIT neuroscientist Laura Lewis, the study builds on earlier research showing that slow brain waves during non-rapid eye movement (non-REM) sleep help generate fluid movements that clear waste from the brain. These waves are also important for memory consolidation, physical recovery and other restorative functions.

To investigate how ageing affects this process, researchers recruited two groups of volunteers aged 18 to 39 and 60 to 84. Participants underwent brain scans and electroencephalogram (EEG) monitoring during late-night sessions, allowing scientists to observe brain activity, blood circulation and CSF movement while participants were awake or asleep.

The results revealed a striking difference. Younger participants experienced stronger slow brain waves and larger CSF movements during sleep, while older adults generally showed considerably weaker fluid waves. Researchers then examined whether age-related changes in the brain’s blood vessels might explain the decline.

Using visual stimulation and breath-holding exercises, the team assessed how effectively blood vessels responded to increased neural activity and changing carbon dioxide levels. Older participants generally demonstrated weaker blood-flow responses, suggesting that declining vascular function contributes to reduced CSF movement.

However, these vascular changes could not fully explain the differences. Researchers believe other factors, including overall vascular health and lifestyle, may also influence the brain’s waste-clearing ability. Interestingly, some older participants maintained CSF waves comparable to those of younger adults, highlighting considerable variation in how the brain ages.

The study also found that some older volunteers showed signs of grey matter loss associated with reduced CSF flow. Although none had been diagnosed with a neurodegenerative disorder, researchers hope further monitoring will reveal whether impaired fluid movement predicts conditions such as Alzheimer’s disease or other forms of dementia.

Scientists are now exploring ways to strengthen the brain’s natural cleaning process. Previous MIT research found that brief bursts of pink noise during sleep could increase CSF flow in younger adults, although the technique remains untested in older people. Understanding why certain individuals preserve healthy fluid circulation could ultimately offer new approaches to protecting brain health as people age.

More information: Sydney Bailes et al, The aging human brain exhibits reduced hemodynamic and cerebrospinal fluid flow waves during sleep, Nature Aging. DOI: 10.1038/s43587-026-01270-4

Journal information: Nature Aging Provided by Massachusetts Institute of Technology