August 20, 2025—How the brain removes toxic build-up while you sleep. Plus, a new moon around Uranus, and mathematicians behind bars.
—Andrea Gawrylewski, Chief Newsletter Editor
TODAY'S NEWS
Artist’s impression of small devices soaring on sunlight at the edges of Earth’s atmosphere. Schafer et al. Nature (2025)
Scientists have devised tiny featherweight disks that could float on sunlight in Earth’s mesosphere or the thin air of Mars. They require no fuel or engines, and could potentially carry payload. | 7 min read
The center of Hurricane Erin is staying well off the coast of the U.S. but could cause hazardous rip currents. Here's how they work. | 5 min read
Because of budget cuts, the National Science Foundation will stop operating the Nathaniel B. Palmer icebreaker and slash polar science funding by 70 percent, devastating Antarctic research. | 8 min read
Scientists have discovered a new moon around Uranus, making it the 29th natural satellite known to orbit the ice giant. It's so small you could walk around it in a few hours. | 2 min read
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TOP STORIES
Brain Washing
While awake, the human brain generates waste—excess proteins and other molecules that can be toxic if not removed. Those include the proteins amyloid beta and tau, which are key drivers of Alzheimer’s disease. Scientists have found that electrical waves that sweep through the brain during sleep and store memories, seem to also propel cerebrospinal fluid (CSF) in and out of the brain, clearing away toxins.
What they found: Building on previous mouse studies, researchers injected a tracer into the cerebral spinal fluid of human participants. One group slept normally through the night; the other was kept awake for 24 hours. All participants underwent multiple MRI scans in the evening and again the next day. Participants who had not slept had dramatically more amounts of the tracer in their CSF, meaning it hadn't been cleared from the brain.
How it works: Sleep occurs in four stages, each characterized by a distinct pattern of electrical activity. Each full cycle of a sleep stage lasts for about 90 minutes. During stage 2 and 3 sleep, large waves of electrical activity sweep across the brain, helping to integrate memories. These waves also pulse blood and CSF within the arterioles. The CSF enters astrocyte cells through channels in their end feet and exits these cells into the gray matter of the brain. After sweeping up debris, the CSF reenters a perivascular cavity (donut-shaped spaces surrounding blood vessels), this time one surrounding a venule, or small vein. The CSF and its waste travel alongside the venule to the periphery of the brain and, ultimately, leave it.
Jen Christiansen
Why this matters: If clearing out waste from the brain is an essential purpose of human sleep, then a dysfunction in this system probably relates to many neurological and psychiatric disorders, including Alzheimer’s. How well this brain-cleaning system functions may someday be like hypertension, something to be treated before it turns into a more serious condition.
WHAT WE'RE READING
RFK Jr., promised to find the cause of autism by September. But his agency has cut funding to the very programs investigating it. | ProPublica
Ant smugglers in the U.S. have been emboldened by widespread government staffing cuts. | Wired
For oncologists who incorporated AI into their practice, their ability to spot tumors fell by around 20 percent within just a few months of adopting it. | Bloomberg
A fascinating nuance from some of the research on brain cleaning during sleep: For participants who didn't sleep one night, even after they had a good night sleep the next night, their brains weren't as efficient at clearing the tracker in the cerebral spinal fluid (and therefore toxic build-up). “You don’t compensate by having a good night’s sleep” after a sleepless night, one researcher told Scientific American. The takeaway for me is that we need to prioritize sleep. Whatever that means for you will be personal—perhaps less caffeine, less screen time before bed, a new meditation practice—but my guess is that you have a good idea of why you might not get enough sleep. Here are some solutions to investigate. Sleep well.
Please send any feedback on this newsletter to: newsletters@sciam.com. Thanks for reading.
—Andrea Gawrylewski, Chief Newsletter Editor
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