Most people know this feeling. After sleeping poorly, it becomes harder to stay focused. Thinking seems slower, your mind drifts and easy tasks require more effort than normal. MIT researchers have discovered what happens in the brain during these attention lapses. Their research reveals that when focus drops, cerebrospinal fluid briefly leaves the brain. This usually happens during sleep & helps remove waste that accumulates throughout the day. Scientists think this cleaning process is vital for maintaining brain health and proper function. When someone lacks sufficient sleep, the body seems to activate this fluid movement during waking hours. This might help make up for missed rest but it has a significant drawback. Attention decreases noticeably during these episodes. Laura Lewis explains that without adequate sleep, CSF waves begin appearing during wakefulness when they normally would not occur. These waves bring an attention cost, causing focus to fail when the fluid flows through the brain. Lewis holds the position of Athinoula A. Martinos Associate Professor of Electrical Engineering & Computer Science. She belongs to MIT’s Institute for Medical Engineering and Science and the Research Laboratory of Electronics. She also serves as an associate member of the Picower Institute for Learning and Memory. Lewis leads the study as senior author. The research appeared recently in Nature Neuroscience. MIT postdoctoral associate Zinong Yang wrote the paper as lead author.
The Role of Sleep in Clearing Waste From the Brain
The Impact of Sleep Deprivation on Brain Function Sleep is a basic biological need, yet scientists are still working to fully understand why it is so critical. What is well established is that sleep supports alertness and mental performance. When sleep is cut short attention and other cognitive abilities suffer. One key role of sleep involves cerebrospinal fluid, which surrounds and protects the brain. During sleep, CSF helps remove waste that accumulates during waking hours. In a 2019 study Lewis & her colleagues showed that this fluid moves in a regular rhythm during sleep and that the movement is closely tied to changes in brain wave activity. That earlier research raised an important question about what happens to this system when sleep is disrupted. To investigate, the team recruited 26 volunteers who were tested twice. Each participant underwent testing once following a night of sleep deprivation in the lab and once when they were well-rested. The researchers wanted to see how lack of sleep affected the brain’s natural cleaning process.
How Researchers Measure Brain Activity and Focus
The morning after each session participants completed a standard task commonly used to measure the effects of sleep loss. While they worked through the task researchers monitored multiple signals from both the brain and the body. Each participant wore an electroencephalogram cap to track brain waves while lying inside a functional magnetic resonance imaging scanner. The researchers used a specialized form of fMRI that could measure not only blood oxygen levels in the brain but also the movement of CSF in and out of it. Heart rate and breathing rate and pupil size were also recorded. Participants completed two attention tests during the scan with one based on vision and the other on sound. In the visual test they watched a fixed cross on a screen that occasionally changed into a square and pressed a button when they noticed the change. In the auditory test the visual cue was replaced with a sound. As expected participants who were sleep-deprived performed significantly worse than when they were well-rested. Their responses were slower and in some cases they failed to detect the signal altogether.
Why Brain Fluid Spikes When Attention Drops
When attention briefly failed the researchers observed several physical changes happening at the same time. Most notably CSF flowed out of the brain during these lapses and returned once attention recovered. The results suggest that at the moment attention fails this fluid is actually being expelled outward away from the brain. When attention recovers it is drawn back in according to Lewis. The researchers believe this pattern reflects the brain attempting to make up for missed sleep. By activating a process normally reserved for rest the brain may be trying to restore itself even though this comes at the expense of focus. One way to think about those events is because your brain is so in need of sleep it tries its best to enter into a sleep-like state to restore some cognitive functions according to Yang. Your brain’s fluid system is trying to restore function by pushing the brain to iterate between high-attention and high-flow states.
The Connected Network Between the Brain and the Body
The study found that attention lapses came with changes throughout the body. During these moments breathing and heart rate slowed & pupils became smaller. Pupil constriction started roughly 12 seconds before CSF moved out of the brain and reversed after attention returned. Lewis says what’s interesting is it seems like this isn’t just a phenomenon in the brain but also a body-wide event. It suggests there’s a tight coordination of these systems where when your attention fails you might feel it perceptually and psychologically but it’s also reflecting an event happening throughout the brain & body. This close timing suggests that a single control system may regulate both attention and basic bodily processes such as fluid flow, heart rate and arousal. Lewis says these results suggest there’s a unified circuit governing both what we think of as very high-level functions of the brain like our attention & our ability to perceive and respond to the world and then also really basic fundamental physiological processes like fluid dynamics of the brain brain-wide blood flow and blood vessel constriction. Although the study did not identify the exact circuit responsible the researchers point to the noradrenergic system as a likely candidate. This system relies on the neurotransmitter norepinephrine to regulate both mental & physical functions and is known to fluctuate during normal sleep.
