A new study from researchers at the IMT School for Advanced Studies Lucca, published in PLOS Biology, points to an unexpected factor. Dreams, especially those that are vivid and immersive, may actually make sleep feel deeper and more restorative rather than interrupting it.

Rethinking Deep Sleep and Brain Activity

For decades, deep sleep was viewed as a state where the brain is essentially “switched off,” with slow brain waves, minimal activity, and little awareness. Under this traditional view, deeper sleep meant less brain activity. In contrast, dreaming has typically been linked to Rapid Eye Movement (REM) sleep and considered a sign of partial “awakenings” in the brain.

However, this creates a paradox. REM sleep involves intense dreaming and brain activity that resembles wakefulness, yet people often report that this stage still feels like deep sleep.

To explore this contradiction, researchers analyzed 196 overnight recordings from 44 healthy adults. Participants slept in a laboratory while their brain activity was monitored using high-density electroencephalography (EEG). The data came from a broader project funded by a European Research Council (ERC) Starting Grant examining how different types of sensory stimulation influence the experience of sleep.

Dreaming and Perceived Sleep Depth

Over four nights, participants were awakened more than 1,000 times and asked to describe what they were experiencing just before waking. They also rated how deeply they felt they had been sleeping and how sleepy they were.

The results showed that people reported the deepest sleep not only when they had no conscious experience, but also after vivid, immersive dreams. In contrast, shallow sleep was linked to minimal or fragmented experiences, such as a vague sense of presence without clear dream content. “In other words, not all mental activity during sleep feels the same: the quality of the experience, especially how immersive it is, appears to be crucial” explains Giulio Bernardi, professor in neuroscience at the IMT School and senior author of the study. “This suggests that dreaming may reshape how brain activity is interpreted by the sleeper: the more immersive the dream, the deeper the sleep feels.”

How Dreams May Sustain Deep Sleep

Another surprising finding emerged across the night. Even though physiological signs of sleep pressure gradually decreased, participants reported that their sleep felt deeper as time went on.

This perceived deepening closely followed an increase in how immersive their dreams became. The findings suggest that dream experiences may help preserve the feeling of deep sleep even as the body’s biological need for sleep declines. Immersive dreams may also help maintain a sense of separation from the external environment, which is a key feature of restorative sleep, even while parts of the brain remain active.

Dreams as “Guardians of Sleep”

“Understanding how dreams contribute to the feeling of deep sleep opens new perspectives on sleep health and mental well-being,” says Bernardi. “If dreams help sustain the feeling of deep sleep, then alterations in dreaming could partly explain why some people feel they sleep poorly even when standard objective sleep indices appear normal. Rather than being merely a by-product of sleep, immersive dreams may help buffer fluctuations in brain activity and sustain the subjective experience of being deeply asleep.” This idea echoes a long-standing hypothesis in sleep research — and even in classical psychoanalysis — that dreams may act as “guardians of sleep.”

A New Multidisciplinary Approach to Sleep Research

The study was carried out as part of a broader collaboration between the IMT School, Scuola Superiore Sant’Anna in Pisa, and Fondazione Gabriele Monasterio, where a new sleep laboratory has been established to integrate neuroscientific and medical expertise.

This facility supports a multidisciplinary approach to studying sleep and the sleep-wake cycle, enabling researchers to better understand how brain activity interacts with bodily processes. These findings represent an early step in that effort and provide a foundation for future research into how brain-body dynamics shape sleep in both healthy individuals and those with sleep disorders.

In a new study, scientists at Northwestern University investigated how these small, wingless insects, which move across snowy surfaces to find mates and lay eggs, stay alive in freezing conditions. They discovered that snow flies rely on a surprising mix of biological tools. The insects can generate their own body heat like mammals and produce antifreeze proteins similar to those found in Arctic fish.

While most insects cannot survive below freezing, snow flies remain active at temperatures as low as -6 degrees Celsius (or 21.2 degrees Fahrenheit).

These findings provide new insight into how life adapts to extreme environments. They may also help researchers develop new ways to protect cells, tissues and materials from damage caused by cold.

The study was published on March 24 in the journal Current Biology.

“Insects are cold-blooded, so they are at the mercy of external temperatures,” said Northwestern’s Marco Gallio, who led the study. “But they have a mind-boggling ability to adapt to extremes. When it gets cold, a common strategy is to find shelter and become dormant until conditions get better. But instead of slowing down, snow flies actually prefer freezing cold, snowy conditions and hide away when the snow melts and it gets warm. They really push the limit of what’s possible. Now we’ve found snow flies aren’t just tolerating the cold, they have multiple ways to counteract it.”

Gallio studies how temperature shapes biology and is the Soretta and Henry Shapiro Research Professor in Molecular Biology as well as a professor of neurobiology at Northwestern’s Weinberg College of Arts and Sciences. He co-led the study with Marcus Stensmyr, a biology professor at Lund University in Sweden. Other Northwestern contributors include William Kath of the McCormick School of Engineering and Alessia Para from Weinberg. Gallio and Kath are also affiliated with the NSF-Simons National Institute for Theory and Mathematics in Biology (NITMB).

Unusual Genes and Antifreeze Proteins

To understand how snow flies survive such harsh conditions, researchers first examined their genetic makeup. Gallio and his team were the first to sequence the snow fly genome and compare it with related insects that are not adapted to cold environments. They also analyzed RNA to identify which genes are actively used for survival in freezing temperatures. These complex comparisons were carried out by Richard Suhendra, a Ph.D. student working with Kath.

The results were unexpected.

“We couldn’t find many of the genes within any database,” Gallio said. “Initially, I thought we must have sequenced some alien species. It’s very rare for an active gene, which makes a protein, to not have a match.”

Further investigation showed that these unusual genes produce antifreeze proteins. Like those found in Arctic fish, these proteins attach to ice crystals and prevent them from growing. This process protects cells from damage during freezing.

“Remarkably, some of the antifreeze proteins we found are actually structurally related to those of Arctic fish,” Gallio said. “That suggests evolution came to the same solution for a common problem.”

Heat Production Helps Snow Flies Stay Active

The team also identified genes linked to energy use and cellular processes involved in producing heat. This suggested another unexpected ability. Snow flies do not just resist freezing, they also generate their own heat.

“We found genes that in larger animals are associated with mitochondrial thermogenesis in brown adipose tissue,” Gallio said. “Many animals like marmots and polar bears have brown fat, which is there to produce heat. When they go into hibernation, they burn this stored fat to produce heat rather than to produce chemical energy. So, in some ways snow flies use a combination of the strategies used by polar bears and by Arctic fish.”

Blocking Ice and Creating Warmth

To test how the antifreeze proteins work, Matthew Capek, a Ph.D. student in the Gallio Lab, modified fruit flies to produce one of the snow fly proteins. He then exposed them to freezing temperatures in a lab freezer. The modified flies survived at much higher rates than normal fruit flies, confirming that the proteins act as barriers that stop ice from spreading.

In another experiment, researchers tested whether snow flies actually generate heat. They measured the insects’ internal temperature while gradually lowering the surrounding temperature below freezing. During this process, snow flies consistently remained slightly warmer than expected by a couple of degrees Celsius compared to other insects.

“Other insects, like bees and moths, shiver to increase their heat,” Stensmyr said. “But we found no evidence of shivering. Snow flies instead likely produce heat at the cellular level, more similar to how mammals and even some plants generate heat.”

Even a small increase in temperature can be critical for survival in such extreme conditions. This brief warmth may give snow flies enough time to find shelter and avoid freezing when temperatures suddenly drop.

Reduced Sensitivity to Cold Pain

Snow flies also appear to be less sensitive to the painful effects of extreme cold. Most people recognize the sharp sting of touching ice or cold metal. This sensation is triggered by reactive molecules in cells that signal the body to avoid harm. In snow flies, this response is significantly reduced.

Gallio and his team found that a key sensory protein involved in detecting harmful stimuli is much less responsive in snow flies than in other insects. As a result, these insects can tolerate higher levels of cold-related stress and continue functioning in conditions that would overwhelm most species.

“It turns out that a specific irritant receptor is 30 times less sensitive in snow flies than in mosquitoes and fruit flies,” Gallio said. “So, they can cope with higher levels of noxious irritants produced by cold exposure.”

Future Research on Extreme Cold Survival

Next, the researchers plan to explore in greater detail how snow flies generate heat at the cellular level and to identify the full range of antifreeze proteins they produce. This work could reveal whether other organisms use similar strategies to survive in extreme cold environments.

The study, “Coordinated molecular and physiological adaptations enable activity at subfreezing temperature in the snow fly Chionea alexandriana,” will appear in the April 6 volume of the journal Current Biology and feature on the cover. The work in the various labs was partially supported by the National Institutes of Health, the Pew Scholars Program, the McKnight Foundation, the Paula M. Trienens Institute for Sustainability and Energy, the Crafoord Foundation, the National Science Foundation, the Simons Foundation and NITMB. External collaborators included the DNAzoo project and Olga Dudchenko and Erez Lieberman Aiden, who are both faculty members at Rice University and at the Baylor College of Medicine.