Almost everyone knows this feeling. After a night of poor sleep, it is more difficult to concentrate. Your thoughts wander, your reaction times slow down, and your mental clarity diminishes, especially when you need to be most alert. New research from MIT sheds light on what happens in the brain during these brief lapses in concentration. The study shows that during a temporary loss of concentration, cerebrospinal fluid (CSF) drains from the brain, a process that normally occurs during sleep and helps remove waste products that have accumulated during the day. This cleansing activity is considered important for brain health and function.

The Body Tries to Compensate for Lost Sleep

Sleep deprivation refers to a condition in which the body and brain do not get enough sleep over a longer or shorter period of time to fully recover and function normally. It can occur when the amount of sleep is regularly below the individual’s needs or when the quality of sleep is reduced, for example due to frequent waking or non-restorative sleep. A distinction is made between acute sleep deprivation, which occurs after individual nights with too little sleep, and chronic sleep deprivation, which extends over weeks or months.

When someone suffers from sleep deprivation, the body appears to try to compensate for the lost sleep by triggering these fluid movements during the waking phase. However, this compensation has a significant disadvantage: greatly reduced attention. “When you’re not sleeping, the CSF waves invade the waking phase, where you wouldn’t normally see them. However, they are accompanied by a loss of attention, as attention wanes during the moments when this fluid wave occurs,” says Laura Lewis, Athinoula A. Martinos Associate Professor of Electrical Engineering and Computer Science, member of the MIT Institute for Medical Engineering and Science and the Research Laboratory of Electronics, and associate member of the Picower Institute for Learning and Memory. Lewis is the senior author of the study, which was published in Nature Neuroscience. MIT postdoctoral fellow Zinong Yang is the lead author.

How Sleep Cleanses the Brain

Sleep is essential for survival, but scientists still don’t fully understand why it plays such a crucial role. What is clear is that sleep is necessary to stay alert, and that sleep deprivation has been shown to impair attention and other mental abilities. On a physical level, sleep serves to regenerate: cells and tissues are repaired, growth hormones are released, and the immune system is strengthened. Sleep also plays a central role in regulating hormones and metabolism, such as controlling hunger, blood sugar, and energy balance.

An important function of sleep involves the cerebrospinal fluid that surrounds and cushions the brain. During sleep, cerebrospinal fluid helps flush out waste products that accumulate during waking hours. In a 2019 study, Lewis and her colleagues found that this fluid moves in a rhythmic pattern during sleep that is closely related to changes in brain wave activity.

This earlier discovery raised a new question: What happens to this fluid system when sleep is disrupted? To find out, the researchers recruited 26 volunteers who completed tests twice, once after a night of sleep deprivation in the lab and once after a night of adequate rest. The next morning, participants performed a standard task designed to assess the effects of sleep deprivation, while the researchers tracked a variety of brain and body signals.

Measuring Attention and Fluid Flow in the Brain

During the experiment, each participant wore an electroencephalogram (EEG) cap to monitor brain activity while lying in a functional magnetic resonance imaging (fMRI) scanner. The team used a special version of fMRI that could track both blood oxygen levels and the movement of cerebrospinal fluid (CSF) in and out of the brain. Heart rate, respiratory rate, and pupil size were also recorded.

The participants completed two attention tests in the scanner, one visual and one auditory. In the visual task, they observed a fixed cross on a screen that occasionally changed into a square. They were instructed to press a button whenever a change occurred. In the auditory task, the visual signal was replaced by a sound. As expected, participants who were sleep deprived performed significantly worse than those who were well-rested. Their responses were slower, and in some cases, they did not perceive the signal at all.

When these brief lapses in attention occurred, the researchers observed several physiological changes that occurred simultaneously. Most notably, cerebrospinal fluid moved out of the brain during the lapse and then flowed back in as soon as attention returned. “The results suggest that the moment attention lapses, this fluid is actually expelled from the brain. And when attention returns, it is drawn back in,” says Lewis.

The team believes this pattern reflects the brain’s attempt to compensate for sleep deprivation by activating a cleansing process that normally occurs at night, even if this temporarily disrupts attention. “One way to explain these events is that your brain needs sleep so badly that it tries its best to enter a sleep-like state to restore some cognitive functions,” says Yang. “Your brain’s fluid system tries to restore function by causing the brain to switch between states of high alertness and high flow.”

A Collaborative System of Brain and Body

The study also found that attention lapses are related to changes outside the brain itself. During these moments, breathing and heart rate slowed, and pupils constricted. Pupil constriction began about 12 seconds before cerebrospinal fluid drained from the brain and reversed after attention returned. “Interestingly, this appears to be not just a phenomenon in the brain, but a process that affects the entire body. This suggests that these systems are closely coordinated. When attention wanes, you feel it perceptually and psychologically, but it also reflects a process that takes place throughout the brain and body,” says Lewis.

These findings suggest that a single control system may coordinate both attention and basic bodily functions such as fluid flow, heart rate, and alertness. “These findings suggest that there is a unified circuit that controls both the brain functions we consider highly sophisticated—our attention, our ability to perceive and respond to the world—and very basic physiological processes such as brain fluid dynamics, blood flow throughout the brain, and blood vessel constriction,” says Lewis. While the researchers were unable to identify the specific circuit involved, they point to the noradrenergic system as a strong candidate. This system, which uses the neurotransmitter norepinephrine to regulate cognition and bodily functions, is known to fluctuate during normal sleep.