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By Stephen Beech Why the brain feels "foggy" after a poor night's sleep has finally been explained. Attention lapses due to sleep deprivation coincide with a flushing of fluid from the brain - a process that normally occurs while asleep, say American scientists. Researchers at Massachusetts Institute of Technology (MIT) found that during such lapses, a wave of cerebrospinal fluid (CSF) flows out of the brain. They say that the process normally occurs during sleep and helps to wash away waste products that have built up during the day. The "flushing" is believed to be necessary for maintaining a healthy, normally functioning brain. When a person is sleep-deprived, it appears that their body attempts to catch up on the cleansing process by initiating pulses of CSF flow. But that comes at a cost of "dramatically" impaired attention, according to the study published in the journal Nature Neuroscience. Study senior author Professor Laura Lewis, of MIT, said: “If you don’t sleep, the CSF waves start to intrude into wakefulness where normally you wouldn’t see them. "However, they come with an attentional trade-off, where attention fails during the moments that you have this wave of fluid flow.” She explained that, during sleep, the CSF that cushions the brain helps to remove waste that has built up during the day. In a 2019 study, Lewis and her colleagues showed that CSF flow during sleep follows a "rhythmic" pattern in and out of the brain, and that the flows are linked to changes in brain waves during sleep. That finding led Lewis to wonder what might happen to CSF flow after sleep deprivation. To explore that question, she and her colleagues recruited 26 volunteers who were tested twice - once following a night of sleep deprivation in the lab, and once when they were well-rested. In the morning, the researchers monitored several different measures of brain and body function as the participants performed a task that is commonly used to evaluate the effects of sleep deprivation. During the task, each participant wore an electroencephalogram (EEG) cap that could record brain waves while they were also in a functional magnetic resonance imaging (fMRI) scanner. The research team used a modified version of fMRI that allowed them to measure not only blood oxygenation in the brain, but also the flow of CSF in and out of the brain. They also measured each participant’s heart rate, breathing rate, and pupil diameter. The participants performed two attentional tasks while in the fMRI scanner, one visual and one auditory. Sleep-deprived participants performed much worse than the well-rested ones on the tasks, as expected. Their response times were slower, and for some of the stimuli, the participants never registered the change at all. During the momentary lapses of attention, the research team identified several physiological changes that occurred at the same time. They found, most significantly, a flux of CSF out of the brain just as those lapses occurred. After each lapse, CSF flowed back into the brain. Lewis said: “The results are suggesting that at the moment that attention fails, this fluid is actually being expelled outward away from the brain. "And when attention recovers, it’s drawn back in.” The research team believe that when the brain is sleep-deprived, it begins to compensate for the loss of the cleansing that normally occurs during sleep, even though the pulses of CSF flow come with the cost of attention loss. Study lead author Dr. Zinong Yang said, “One way to think about those events is that your brain is so in need of sleep, it tries its best to enter into a sleep-like state to restore some cognitive functions. “Your brain’s fluid system is trying to restore function by pushing the brain to iterate between high-attention and high-flow states.” The research team also found several other physiological events linked to attentional lapses - including decreases in breathing and heart rate - along with constriction of the pupils. They found that pupil constriction began about 12 seconds before CSF flowed out of the brain, and pupils dilated again after the attentional lapse. Lewis said, “What’s interesting is it seems like this isn’t just a phenomenon in the brain, it’s also a body-wide event. "It suggests that 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 that’s happening throughout the brain and body." She says the "close linkage" between such different events may indicate that there is a single circuit that controls both attention and bodily functions such as fluid flow, heart rate, and arousal. Lewis added: “These results suggest to us that there’s a unified circuit that’s governing both what we think of as very high-level functions of the brain - 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."
