Sleep helps the brain to cleanse itself – and now this process can be measured in humans entirely non-invasively. Researchers at the University of Oulu have developed a method that allows the increased movement of brain fluids during sleep to be tracked quickly and safely, without the need for injected contrast agents.
The brain’s cleansing mechanism is driven by pulsations, natural bodily rhythms that move blood and cerebrospinal fluid through the brain. These pulsations fall into three main categories: cardiovascular pulsations generated by the heartbeat in arteries, respiratory pulsations affecting veins and cerebrospinal fluid spaces, and slow vasomotor waves in the walls of blood vessels. Previous research has shown that both these pulsations and the brain’s waste clearance are enhanced during sleep.
These pulsations drive the flow of fluids through brain tissue, helping to remove metabolic waste. When this fluid circulation weakens, waste products may begin to accumulate in the brain. The phenomenon has been linked to memory disorders, among other conditions, but has been difficult to measure directly in humans.
Fluid flow accelerates during sleep
An ultrafast magnetic resonance imaging method developed by the
University of Oulu’s functional neuroimaging research group (OFNI) now makes it possible to measure brain fluid circulation directly by tracking the movement of water molecules in cerebrospinal fluid. The scan takes only about five minutes and does not require contrast agents.
The researchers found that the behaviour of brain pulsations changes markedly during sleep. The propagation of respiratory and vasomotor pulsations—both of which promote brain-cleansing fluid circulation—speeds up, while cardiac pulsations slow down. This shift is thought to reflect more efficient water filtration in brain tissue, alongside a slowing of arterial pulse waves as blood vessels dilate and blood pressure decreases during sleep.
Brain control dynamics partly reverse during sleep
The studies also revealed a shift in the brain’s fundamental operating logic during sleep. When awake, electrical activity in neurons modulates blood flow and fluid movement: neural activation comes first, followed by increased blood flow. During sleep, however, this relationship is no longer strictly one-directional.
“During sleep, vasomotor waves in particular, slow pulsations below 0.1 hertz, begin to locally influence not only fluid movement but also the brain’s electrical activity,” explains Professor Vesa Kiviniemi, who led the research.
This effect is especially pronounced in posterior brain regions, such as the sensory cortex. These same areas also show a marked increase in fluid flow through brain tissue, pointing to enhanced clearance.
New possibilities for monitoring the ageing brain
The findings are based on two recently published studies, one in Advanced Science and the other in The Proceedings of the National Academy of Sciences (PNAS). Both studies involved measurements in healthy volunteers.
According to the researchers, the results provide a more detailed understanding of how and where sleep enhances the brain’s cleaning processes.
It is already known that brain fluid circulation declines with age. “New measurement methods open up possibilities to monitor—and in the future potentially treat—age-related changes in brain fluid dynamics,” says Kiviniemi.
The research group has also developed wearable technology that can track brain electrical activity and blood flow during sleep without the need for MRI. The results correspond well with MRI measurements, suggesting that brain cleansing could in future be monitored more easily in clinical settings.
The team is now working on ways to enhance the fluid circulation and pulsation mechanisms that weaken with age, with the aim of slowing down the effects of ageing on the brain.