Waves that clear waste from the brain during sleep are impaired in older adults

As people age, they often experience shorter periods of sleep and more frequent awakenings throughout the night. MIT researchers have discovered that a crucial process for eliminating waste from the brain during sleep is commonly impaired in older individuals. They found that waves of cerebrospinal fluid (CSF), which assist in removing brain debris, are significantly diminished in older adults.

This impairment is closely tied to a decrease in slow brain waves typically occurring during deep sleep and is not merely due to reduced overall sleep duration.

The researchers now aim to investigate whether the disruption of CSF flow increases the risk of neurodegenerative disorders and if their observed sleep patterns might serve as a means to predict an individual's risk. Laura Lewis, the lead author and an expert in the field, explains, "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."

The study, published in Nature Aging, involved two age groups: 18 to 39 and 60 to 84. Participants underwent two-hour sessions to measure their brain activity using functional MRI (fMRI) and a modified MRI technique to monitor CSF flow. Electroencephalography (EEG) was employed to record brain waves. The findings revealed that slow waves, which strengthen during non-REM sleep, also correspond to larger CSF waves in younger adults. However, in older adults, CSF waves were notably smaller.

To understand the cause of the age-related decline in CSF flow, the researchers examined neurovascular coupling, the process where brain activity increases lead to blood vessel dilation, delivering more oxygen and nutrients. While neural activity increased in older subjects, blood flow did not significantly rise, indicating a possible decline in blood vessels' response to increased oxygen demand.

Additionally, during a breath-holding task, the blood flow increase in older individuals was smaller compared to younger participants.

The researchers concluded that reduced neurovascular coupling and diminished blood flow contribute significantly to the decline in CSF flow, though they acknowledge that other age-related factors may also be involved.