What's the purpose of dreaming?
We all dream — but why? As with many mysteries of the mind, science doesn't have one neat answer.
'You'll get as many answers to the question 'What is the purpose of dreaming?' as there are dream psychologists,' says Deirdre Barrett, dream researcher at Harvard University and author of The Committee of Sleep.
According to Austrian neurologist and founder of psychoanalysis Sigmund Freud, dreams offered vital clues to unresolved conflicts buried deep within our psyche. But Freud's theory, introduced in his 1899 book The Interpretation of Dreams, sparked plenty of controversy. Critics argued that his dream interpretations were overly focused on sex, highly subjective, and impossible to verify—two analysts might offer entirely different readings of the same dream, with no objective way to know who was right.
In the decades since Freud, other scientists have offered alternative explanations for why we dream. One of the most prominent is the threat simulation theory, proposed by Finnish neuroscientist and psychologist Antti Revonsuo in 2000. According to this view, dreaming is an ancient biological defense mechanism. By simulating dangerous situations, our brains rehearse the skills needed to recognize and avoid threats—a kind of virtual reality training ground for survival. A 2005 study lent support to this theory by examining the dreams of Kurdish children exposed to war and trauma. Compared to non-traumatized Finnish children, these children reported more frequent dreams filled with severe threats, suggesting that their minds were practicing how to cope with danger.
But even the threat simulation theory is debated. A 2008 study comparing residents of high-crime areas in South Africa to those in low-crime parts of Wales found that South African participants, despite facing more real-world threats, actually reported fewer threatening dreams than their Welsh counterparts. This result challenges the idea that the brain uses dreams to simulate danger when exposed to trauma.
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Another theory suggests that dreams are simply a side effect of memory consolidation—the brain's way of replaying and reinforcing new memories while we sleep. As the brain's hippocampus and neocortex work together to file away fresh information, they may also blend it with older memories, creating the often strange mashups we experience as dreams.
Dreams may also help us process and manage emotions, especially negative ones, according to the emotion regulation theory of dreaming. Research focusing on recently divorced individuals experiencing depression found that participants who dreamed about their ex-spouses were more likely to show significant improvement in their mood one year later, particularly if their dreams were vivid and emotionally rich. Another study found that people who dreamed about stressful events they had experienced before sleep woke up feeling more positively about the events the next day, suggesting that dreams can help transform emotional distress into resilience.
Recent brain imaging studies support this idea. People who frequently experience fear-related dreams show reduced activation in fear centers of the brain during waking life, hinting that these dreams may serve as a kind of overnight therapy session, helping us better regulate our emotions when awake.
Ultimately, Barrett suggests that we may be asking the wrong question. 'We'd rarely ask the analogous question: 'What is the purpose of thinking?'' she says. Just as waking thought serves many functions—from planning to problem-solving to daydreaming—dreams likely do too. 'The value of dreaming lies in its difference. It's a distinct mode of thought—one that supplements and enriches our waking cognition.'
In fact, some researchers believe dreams offer a unique mental space for solving problems that stump us during the day. In this altered brain state, regions responsible for imagery become more active, allowing the mind to solve problems requiring visualisation. History is full of famous examples: Mary Shelley reportedly dreamed the central scenes of Frankenstein; German chemist August Kekulé envisioned the ring structure of benzene in a dream; and Russian chemist Dmitri Mendeleev dreamed his final form of the periodic table of the elements.
In the end, dreams may serve many purposes—or none at all—but they remind us that even in sleep, the brain never truly rests.
This story is part of Popular Science's Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you've always wanted to know? Ask us.
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2 days ago
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Solar-Powered Slug Steals Chloroplasts and Stores Them for Emergency Food
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National Geographic
2 days ago
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When climbing the world's tallest mountains, what counts as cheating?
To reach the highest point on Earth, average climbers need around three to four weeks to let their bodies acclimatize on the ascent and descent. To cut that time to only seven days, mountain climber Lucas Furtenbach is offering a chemical boost with xenon, an inert gas that is mainly used as an anesthetic. Photograph by Cory Richards, Nat Geo Image Collection In 1978, Austrian physician and mountaineer Oswald Oelz was a team doctor on an expedition to Mount Everest when climbers Reinhold Messner and Peter Habeler became the first people to reach its summit without supplemental oxygen. Before then, it was unthinkable that humans, unassisted, could climb 29,032 feet, the height of Everest, where due to drop in atmospheric pressure we inhale only about 30 percent of the oxygen we breath at sea level. Almost half a century later, Oelz's grand-nephew, Austrian climbing guide Lukas Furtenbach, was the architect of a new feat atop Mount Everest. This May, four of his clients, along with five Sherpas, summited the world's tallest mountain only five days after they left London. Usually, it takes an average of 40 days of slow acclimatisation to adjust to the high altitude and scarce oxygen on Everest. The secret to the team's lightning-fast ascent: About two weeks before the expedition, Furtenbach's clients were given xenon through a medical mask. The noble gas is sometimes used as an anaesthetic but is also thought to boost the production of erythropoietin, a hormone that stimulates red blood cell production. The idea, suggested to Furtenbach by German anaesthesiologist Michael Fries, was to artificially accelerate the acclimatisation process. The strategy, however, immediately caused controversy in the mountaineering community. Experts on high-altitude research who spoke to National Geographic mainly questioned whether xenon could actually produce an effect strong enough to mimic acclimatisation. And earlier this year, the International Climbing and Mountaineering Federation issued a statement warning about the absence of scientific studies to prove the safety and efficacy of xenon at high altitude. Then there's the question of whether xenon, banned in professional sport by the World Anti-Doping Agency, makes the climb up Everest so easy that it obscures the line between sportsmanship and tourism. Around 7,000 people climb Everest every year with the help of supplemental oxygen. For others, using supplemental oxygen is considered a cheap shortcut akin to utilizing sherpas and fixed-ropes. Left, a climber scales Mount Everest with the aid of supplemental oxygen. Right, oxygen tanks are seen along a section of Everest called "the Balcony" near the summit. 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Some tour guides who lead these large groups say the controversy ignited by xenon places unfair scrutiny on what's simply the latest of many tools making mountain climbing more accessible and safer. American climber Adrian Ballinger, owner of Alpenglow Expeditions, thinks climbers should just be honest about the style they choose. 'Professional athletes don't use supplemental oxygen when climbing in the mountains because it makes things easier. But for recreational and non-professional climbers who hire guiding companies, it's different,' he says. However, he draws the line at the use of xenon in mountaineering—even in commercial expeditions. 'I don't see any reason,' he says, 'to use a substance banned as doping.' Doing drugs, 29,032 feet high Climbers have a long history of employing different drugs to survive the cold and dangerous conditions of Earth's highest peaks. In 1953, mountaineering legend Hermann Buhl took methamphetamine pills, then known by the brand name Pervitin, to stay awake during a perilous descent after summiting the Himalayan mountain Nanga Parbat in Pakistan. (Buhl made his climb without supplementary oxygen and became the first and only person to achieve a solo first ascent of an 8,000-meter peak, famously surviving the night at 26,000 feet by standing on a tiny ledge.) In the following decades, mountaineers experimented with both banned and legal substances, from amphetamine to Viagra. Two well-known prescription drugs, diuretic acetazolamide (commonly known as Diamox) and corticosteroid dexamethasone (Decadron) are often used to treat high-altitude conditions like acute mountain sickness or cerebral edema—but against expert recommendations, some climbers take them preventatively. Nothing, however, works better to fight hypoxia and enhance performance at high altitude than a steady flow of supplemental oxygen. Hermann Buhl in 1953, after summiting Nanga Parba, the ninth highest mountain in world, located in Pakistan. Under the influence of the drug pervitin, a stimulant similar to methamphetamine, Buhl was able to push on to the summit after the rest of his team was forced to return to camp, making Buhl the first and only person to make a solo-ascent of an 8,000 meter peak. Photograph by Touring Club Italiano/Marka/UniversalA view of Nanga Parbat as seen from Jammu & Kashmir, 1933. Photograph by Royal Air Force/Royal'If you use supplemental oxygen continuously, oxygen delivery to tissues is maintained. You will not develop altitude illness, and exercise performance will not be affected,' explains Martin Burtscher, a long-time researcher in the field of high-altitude medicine and retired professor at the University of Innsbruck in Austria. 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Photograph by Renan Ozturk, Nat Geo Image Collection Before he became an advocate of xenon, Furtenbach had experimented with having clients sleep at home in tents with reduced oxygen and training with limited oxygen to help simulate the acclimatization process. That shortened the ascent time to three weeks. Knowing this, Fries, the anaesthesiologist, approached Furtenbach back in 2019 with the idea of using xenon and its erythropoietin production ability to accelerate acclimatisation even further. When confronted with limited oxygen at high altitude, the human body gradually releases erythropoietin after several weeks of acclimatisation, as a climber makes rounds up and down the mountain, slowly gaining altitude. Fries, who spent 15 years researching different effects of xenon while working at Aachen University's hospital in Germany, theorized that a one-time low-dose administration of the gas could produce the same results in a matter of days. Fries also contends that xenon can prevent high-altitude sickness due, in part, to its positive effect on the blood vessels that connect the heart and lungs. Furtenbach first tested xenon on himself in 2020 while climbing Argentina's 22,831-foot Aconcagua and, two years later, on Everest. Both times, he says, he felt strong and fast, and didn't experience any negative side effects. Then he crafted a plan for including xenon in the expeditions offered by his self-titled company, Furtenbach Adventures, which facilitates climbs up Everest and other famous mountains. The decision to offer xenon to clients, he says, was done to make climbing safer. 'The fewer rotations you have to do on the mountain, the safer the expedition becomes,' he argues. (Furtenbach also thinks shorter trips could help curtail the large amounts of garbage long expeditions leave behind.) For the first-ever xenon 'powered' expedition, he chose four British clients, who boasted a combination of high-altitude climbing experience and military training. After ten weeks of pre-acclimatisation at home, sleeping and training with limited oxygen, they received a low dose of xenon in a German hospital and two weeks later embarked from London on their five-day-long ascent. No immediate serious side effects from the xenon treatment were observed by Furtenbach or the members of the expedition. The price of the climb was 150,000 euros a person. Furtenbach declined to specify how much xenon, an expensive gas, added to this total. Climbing rope is a ubiquitous tool amongst mountaineers, and learning how to safely build anchors and belay are essential skills. However, on some mountains, ropes may be pre-anchored and left in place for the entirety of the season to aid less experienced climbers. Left, the first Nepali female to climb Manaslu studies ice anchors in a climbing class. Right, a mountaineer descends to camp III during an attempt to summit Hkakabo Razi, said to be Southeast Asia's tallest mountain. Photograph by Aaron Huey, Nat Geo Image Collection (Top) (Left) and Photograph by Renan Ozturk, Nat Geo Image Collection (Bottom) (Right) Not everyone with high-altitude expertise is convinced that xenon is the best way to quickly climb Everest. Some experts argued that a one-week ascent might be possible without a miracle drug like xenon, if only the climbers would use a high enough flow of oxygen right from the bottom. 'If you have a big flow of oxygen, you don't need to work as hard to acclimatise. From an oxygen perspective, you're not going to the summit of Everest, but much lower,' says Mike Grocott, professor of anaesthesia and critical care at the University of Southampton in England and expert on the physiology of hypoxia. This theory, too, was tested this May when Ukraine-born Andrew Ushakov stated that he climbed to the top of Everest in a little less than four days after leaving New York. To achieve this, he used supplemental oxygen and trained in low-oxygen conditions. A team from the Elite Exped company guided Ushakov to the top. He says he used oxygen as soon as he started his ascent from the base camp, starting with a flow of 0.5 liters per minute and slowly increasing it to three to four liters per minute, which he used on the summit day. The xenon team, Furtenbach says, didn't start using oxygen until they reached 19,700 feet, continuing from there with a usual flow of 1 to 2 litres per minute. Higher flow was used only above 26,000 feet. This theoretically means xenon could indeed have some effect on the acclimatisation process, beyond supplemental oxygen. Still, without peer-reviewed studies, it's hard to conclude that the xenon made a difference, warns Peter Hackett, a high-altitude researcher and professor at the University of Colorado Anschutz Medical Campus. 'My question is—why the big rush,' he says. 'These ascents reveal that Everest's challenge is now all about dealing with hypoxia and not really climbing.' For some climbers, no extra help wanted Climbers who abstain from performance-boosting drugs and supplemental oxygen see xenon as just another departure from the purest, and thereby most elite, form of climbing. The Piolet d'Or, the most coveted mountaineering award, perhaps best exemplifies the most prestigious climbing styles. The award currently doesn't consider ascents done with supplemental oxygen or fixed lines, giving the spotlight to imaginative and innovative new routes, doing more with less, and building on experience. One of the winning teams of last year's Piolet d'Or, American climbers Matt Cornell, Jackson Marvell, and Alan Rousseau, spent seven days charting a new route up the steep north face of Jannu in Nepal. To pack lightly, they shared a single sleeping bag. 'Alpinism without the factor of the unknown is only the plain physical activity,' says Slovenian climbing legend Marko Prezelj, four-time winner of Piolet d'Or. 'If somebody prepares the mountain for you by putting in fixed lines and you climb together with 500 people, there is nothing unknown.' The Everest massif from Camp I on Pumori. Photograph by Cory Richards, Nat Geo Image Collection Famous American alpinist Steve House, best known for his bold 'alpine style' first ascent on the Rupal face of Nanga Parbat in 2005, sees alpinism as a process of stripping away excesses to get closer to the experience. 'There is nothing inherently wrong with the ascents done with supplemental oxygen and xenon, but we need to understand these climbs as tourism, not alpinism,' says House. And Mingma Gyalje Sherpa, the first Nepali to climb all 14 of the world's 8,000-meter peaks without supplemental oxygen and founder of the Nepal-based guiding company Imagine Nepal, says there should be a limit to what tour companies offer. He thinks that the traditional way of doing proper acclimatisation is more valuable. 'I would always suggest to clients to do at least one rotation on the mountain up to Camp 2, before the summit push, so they can understand their body at high altitude. We also don't take clients without previous experience,' he says. But even if assisted climbs and medical aids become more common, Alpenglow Expeditions' Ballinger thinks there will always be an interest in unassisted alpine climbing. 'There are endless new route opportunities for alpinism in the Himalaya. And I don't think the fact that we have commercial guiding on a handful of routes on the world's most popular mountains gets in the way of the cutting-edge side of the sport,' says Ballinger. Peter Hackett, the high-altitude researcher, is less optimistic. 'The improved access, safety, and success on Everest have led to a new 'generation' of high-altitude tourists with high ambition but little climbing experience, and more money than time,' he says. 'It's all about— how I can bag this summit and miss as little work as possible.'
Scientific American
2 days ago
- Scientific American
This Solar-Powered Slug Steals Photosynthetic Machinery for Emergency Food
' Solar-powered' sea slugs have specialized depots in their cells that store photosynthetic equipment looted from algae, a study reports. These depots provide just the right chemical environment to keep the stolen apparatus, called chloroplasts, alive and working to turn sunlight into nutrients. 'It was the wildest thing that we had seen,' says study co-author Nicholas Bellono, a biologist at Harvard University in Cambridge, Massachusetts. The authors also found that, in lean times, the slugs can raid these compartments to consume chloroplasts. The compartment 'is basically like a moving refrigerator of chloroplasts where, after a period of starvation, the slugs can switch from storage to consumption to survive', Bellono says. On supporting science journalism If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. The findings were published in Cell. Green grazer Scientists discovered decades ago that certain species of sea slug store chloroplasts from the algae they eat, a diet that can turn the slugs bright green. But no one understood how the slugs keep these foreign organelles alive without the support of the rest of the algal cell. Bellono and his team added chemical tags to proteins newly made by the slugs' own cells. They found that most of the proteins in a slug's chloroplasts were made by the slug — not by the original algae. That meant the slug was helping to maintain the chloroplasts. When the scientists looked at the chloroplasts under a microscope, they found that the organelles were housed in special compartments in the slugs' guts. Each compartment was surrounded by a membrane that tested positive for markers typically found in cellular structures called phagosomes, which fuse with other structures called lysomes to digest unneeded organelles. The researchers named this structure the kleptosome, after a Greek word that means to steal. Life of crime The team also found that these organelles contained ion channels — receptors that convert chemical messages into electrical signals. Among them is one called P2X4, which opens in response to the presence of ATP, an energy-carrying molecule produced during photosynthesis. When Bellono and his team blocked this channel in slugs' kleptosomes, oxygen production from photosynthesis dropped, showing that the kleptosome is actively involved in keeping the chloroplasts functioning. Having discovered the existence of the kleptosome, the researchers wondered whether it helps the slugs to resist starvation. They compared the solar-powered slug Elysia crispata with Aplysia californica, a non-photosynthetic slug that lacks kleptosomes. Aplysia died after three to four weeks without food, whereas Elysia could survive for up to four months. Yet, after four weeks, the Elysia slugs lost their green colour, turned orange — just as leaves do in autumn — and stopped photosynthesizing. Microscopy revealed that the Elysia 's kleptosomes had begun fusing with lysosomes and that the colour change was caused by the degradation of the chloroplast. The study is 'remarkable,' says cell biologist Elena Oancea at Brown University in Providence, Rhode Island. Studying the molecular and cellular processes of creatures as small as sea slugs is extremely challenging, she says. 'It takes a lot of courage to do that.' The discovery of the kleptosome could help to answer broader questions about organelle evolution and other cellular processes that we don't understand yet, Oancea says. All life is built on cells, she adds: 'It's the basic principle of nature.'
