On the 3rd, German scientists published a paper saying they had succeeded in freezing and then thawing a mouse brain to revive its memory circuits. It recreated the situation of a bear or a squirrel waking from hibernation. It was thanks to a special substance that prevented the formation of ice crystals that expand in volume when brain tissue is frozen. The thawed brain tissue even demonstrated the learning functions necessary for memory.
Scientists say that if humans could hibernate like in science fiction (SF), they could drastically reduce the resources needed for long-term space travel and minimize the harm the body suffers in microgravity. The same technology could keep transplant organs viable longer and extend the lives of critical patients until they receive needed treatment. Movie-like scenarios in which you fall asleep and wake up on Mars and an illness is completely cured are drawing closer.
◇Mouse brain thawed, even shows learning function
A team led by Professor Alexander German at Friedrich-Alexander University (FAU) in Germany cooled hippocampal tissue from mouse brains to minus 130 Celsius. The hippocampus governs memory and learning in the brain. After thawing and observing with an electron microscope, they found that synaptic structures, the consolidations between neurons (nerve cells), remained intact without damage.
The thawed neurons began to exchange electrical signals again. Even long-term potentiation (LTP) appeared. Long-term potentiation is a phenomenon in which the efficiency of signal transmission between specific neurons increases over the long term and is a key mechanism by which memory and learning occur. It means the previously frozen brain was not only alive but functionally ready to store and process new information.
The brain withstood the freezing process thanks to the prevention of ice crystals. When water freezes, it expands in volume. This is why earthenware jars filled with water crack in winter. If the water inside cells freezes, sharp ice crystals destroy cell membranes and neural networks. German's team solved this problem by mimicking a natural antifreeze in animal livers.
Siberian newts can survive for decades even at minus 50 Celsius. The secret is glycerol produced in the liver. This component acts as an antifreeze, preventing cells from freezing and bursting. The team used a mimicking substance to perform so-called vitrification. Glass is a solid, but unlike ice it lacks a crystalline structure, so it does not damage tissue.
The study could also aid medical research. Tissue collected during surgery could be safely frozen for a long time and then thawed years later to test newly developed drugs. If organs such as the heart or kidney are frozen in the same way, they can be transported over long distances. German said, "If we could switch a person into an artificial hibernation state and then revive them, patients with incurable diseases could wait until new treatments emerge."
◇Long-distance space travel becomes feasible with hibernation
U.S. and European space authorities are developing artificial hibernation technology for deep-space exploration. With current rocket technology, a round trip from Earth to Mars takes more than two years. It is hard for humans to endure such long missions. In space, gravity scarcely acts, severely weakening muscles and bones. Astronauts who stayed on Russia's Mir space station were found to have about a 20% decrease in muscle protein after one year. Bones also gradually weaken. According to the National Aeronautics and Space Administration (NASA), in space, bone density decreases by 1% each month.
Hibernating animals are different. Bears lose as much as 20% of their body weight while sleeping through winter, but their muscles and bones remain intact so they can move immediately upon waking. Deadly radiation is also a problem in space, but hibernation is thought to improve tolerance to radiation. Above all, energy consumption falls. Normal sleep reduces energy use by only 5%, but squirrels can cut energy consumption by as much as 85% through hibernation. If astronauts enter artificial hibernation, spacecraft can carry that much less water, oxygen, and food.
Scientists are seeking artificial hibernation techniques from animals. In 2022, a University of Wisconsin team wrote in the journal Science that they had found how squirrels lose one-third of their body weight during hibernation yet maintain muscle. The weight loss stems from drawing energy from fat during hibernation. Instead, by using gut microbes to recycle nitrogen in urine to obtain protein, they maintain muscle.
Kelly Drew at the University of Alaska identified substances that induce hibernation. In 2011, the team induced hibernation by injecting synthetic adenosine into ground squirrels. The work was inspired by a prior study released by Japanese researchers that said hibernating hamsters wake if the brain cannot respond to adenosine.
In 2023, a University of Washington team said in Nature Metabolism that it induced one hour of hibernation by directing ultrasound for 30 seconds at the hypothalamus of mouse brains. The hypothalamus regulates bodily homeostasis, including body temperature and sleep. A team at Oregon Health University said last year it had found a switch in the hypothalamus that can put non-hibernating rats into winter sleep. When specific neurons in the hypothalamus activated, metabolism slowed and body temperature fell. The rats showed brain waves and heartbeats characteristic of hibernation.
◇Human hibernation experiments in the 2030s, gains for medicine too
The United States and Europe plan to conduct artificial hibernation experiments on humans in the 2030s. Clifton Callaway, a professor of emergency medicine at the University of Pittsburgh School of Medicine, conducted preliminary experiments from 2024 with NASA support on five Namsung ages 21 to 54. Injecting sedatives that suppress the body's natural shivering lowered body temperature from 37 to 35 Celsius, and metabolism plunged 20%. Ludwig Maximilian University of Munich and Goethe University are conducting similar research with support from the European Space Agency (ESA).
Artificial hibernation could help medicine on Earth as well as space travel. Jürgen Bereiter-Hahn, an emeritus professor at Goethe University with ESA's hibernation research group, said, "If we slow vital activity, we can give doctors time to find treatments," adding, "for example, that time could be used to develop antibodies to treat cancer."
In organ transplants, keeping both patients and whole organs in a hypothermic state can reduce surgical risk. It can also be used to transport organs over long distances. And critical patients who are bedridden for long periods experience declines in muscle and bone like astronauts in microgravity. Artificial hibernation could also be used to maintain musculoskeletal health in critical patients.
The same goes for treating obesity and diabetes. Bears' fat cells resist insulin, the hormone that signals sugar uptake, during hibernation. That allows fat to be used as an energy source. In humans, insulin resistance causes diabetes. Ground squirrels gain weight rapidly before hibernation and suppress appetite during it. Studying this could yield clues to treating human diabetes and obesity. In hard times, winter sleep is a tonic, whether on Earth or in space.
References
PNAS (2026), DOI: https://doi.org/10.1073/pnas.2516848123
Current Biology (2025), DOI: https://doi.org/10.1016/j.cub.2024.11.006
Nature Metabolism (2023), DOI: https://doi.org/10.1038/s42255-00804-z
Science (2022), DOI: https://doi.org/10.1126/science.abh2950
Neuroscience & Biobehavioral Reviews (2021), DOI: https://doi.org/10.1016/j.neubiorev.2021.09.054
Journal of Neuroscience (2011), DOI: https://doi.org/10.1523/JNEUROSCI.1240-11.2011