Health and Medicine
PUBLISHED
Dreams are utterly bonkers at the best of times, yet when we’re feeling a little off-color we can get a glimpse of how deep the rabbit hole really goes.
Research has shown that fever dreams tend to be more vivid, more emotionally intense, and considerably weirder than regular visits to the dreamscape – so how do we even begin to make sense of the madness?
In fever, the brain is not functioning properly, which means that if you are sleeping, then of course, the content [of dreams] is more bizarre.
Professor Michael Schredl
Boggling the mind
Scientific interest in feverish delusions began well before the modern age and has kept psychologists and neuroscientists busy for more than a century.
Back in 1892, the esteemed Scottish doctor and father of modern pathology, Joseph Coats, contributed an article to the Glasgow Medical Journal in which he recounted an “attack of typhus fever” that he had suffered 26 years previously.
As his temperature soared, Coats succumbed to “days of delirium”, during which he experienced visions of his sister’s death that were so lifelike that he remained convinced that she had really died even the next day.
What’s particularly striking about all this is that Coats’ fantasies occurred while he was fully awake. Given the absurdity of this conscious malfunction, one can hardly imagine the deceptions the feverish mind is capable of while dreaming.
One scientist who has imagined just that is Professor Michael Schredl, Head of Research at the Sleep Laboratory at the German Central Institute of Mental Health at Heidelberg University.
“In fever, the brain is not functioning properly, which means that if you are sleeping, then of course, the content [of dreams] is more bizarre,” he tells IFLScience.
Having conducted the two largest existing scientific surveys of fever dreams, Schredl discovered that extreme eccentricity is the single biggest attribute separating these ailing apparitions from regular dreams.
In the first of these studies, he and his colleagues found that fever dreams were on average 58 percent more bizarre than control dreams, and that this sense of wackiness was most commonly accounted for by “spatial distortions”. Specific examples of this proportional preposterousness included moving walls, gigantic insects, and “creatures with over-sized arms and legs.”
Seeking to find some sort of explanation for this increase in absurdity, the study authors state that “as waking cognitive processes are affected also by fever, we speculate that the bizarreness and distortions of fever dreams is a result of a brain that doesn’t work properly at high temperatures.”
We don't know whether they actually had a hot body and this stimulus was directly incorporated into the dream, or whether they simply dreamed about having a hot body.
Professor Michael Schredl
This assertion is backed up by several studies that have indicated that high body temperature – or hyperthermia – significantly interferes with brain connectivity patterns and impairs basic cognition.
For instance, it has been shown that having a core temperature of 38.8°C (101.8°F) compared to that of 37.2°C (98.96°F) can impair memory. Being exposed to temperatures of 50°C (122°F) for approximately 1 hour messes with signaling in the temporal, frontal, and occipital lobes. This, in turn, could have a negative impact on cognitive functions such as attention, memory, and information processing.
To date, however, no one has ever tracked the neural activity of feverish participants while dreaming, so it’s impossible to say for sure how the febrile brain produces these maddening visions.
Acknowledging that such a study would be extremely difficult to conduct, Schredl says that if only we could get a bunch of sleeping sick people into an MRI scanner, we might just get to the bottom of the enigma.
“That would be really great,” he says. “What would be especially interesting would be to see the default mode network – which is for daydreaming and mind wandering, and also possibly for dreaming – and whether this has changed [during a fever dream].”
Turning up the heat
Given the deleterious and potentially even dangerous effects of hyperthermia, it seems slightly odd that our bodies would have evolved to overheat as part of their immune response to infectious pathogens.
As it happens, scientists still don’t entirely understand the purpose of fevers, although the most widely accepted theory is that raising the internal temperature inhibits the spread and viability of invading microbes. There’s also some evidence to suggest that lymphocytes and other components of the immune system actually work better at slightly higher temperatures.
Despite the gaps in our comprehension, one thing we do know is that fever is an incredibly ancient, shared feature among life forms across the planet, with the typical hyperthermic response to infection having first evolved around 600 million years ago.
When a trait has been around for that long and inherited by so many different species, you can be pretty sure it does more good than harm and carries a significant survival advantage.
Funnily enough, temperature also pops up as a common theme in fever dreams. In the second of Schredl’s studies, heat-related topics featured in 10 percent of fever dreams compared to just 2.22 percent of control dreams.
And if you are very ill, I think most people don't expect to experience positive emotions, but more the negative emotions. That’s why fever dreams are also more negatively toned, because it's a stressful situation.
Professor Michael Schredl
To give an idea of how the subject manifests in the infected slumberer’s mind, one study participant described a nightmare in which “the air is so hot that breathing hurts [...]. I see a red glow in the corner of my eyes and I turn around to see an enormous lava ball coming down the mountain heading toward the city and me.”
“I am running faster and faster, the air grows hotter and hotter, the lava ball is changing directions in pursuing me and does not affect the houses,” continues the account. “It seems the lava ball has the task of getting me.”
It’s thus very tempting to conclude that the experience of having a high temperature triggers these heat-related dreams, although Schredl and his colleagues are keen to stress that there are currently no studies testing whether real-world heat stimuli are superimposed into sleepers’ dreams in the same way that sounds and other environmental elements are.
“We don't know whether they actually had a hot body and this stimulus was directly incorporated into the dream, or whether they simply dreamed about having a hot body,” says Schredl.
The continuity hypothesis
It goes without saying that dreams are still something of a mystery to us, but most scientists do at least subscribe to the so-called continuity hypothesis, which states that dreams tend to reflect our waking-life experiences, emotions, and thoughts – albeit sometimes in strange and abstract ways.
“For instance, if you have stress, then your dreams are more stressful, even if you don't dream about the actual stressor,” explains Schredl.
“And if you are very ill, I think most people don't expect to experience positive emotions, but more the negative emotions. That’s why fever dreams are also more negatively toned, because it's a stressful situation,” he adds.
Sure enough, almost 95 percent of fever dreams reported by Schredl’s initial study participants were accompanied by negative emotions. A high proportion even included the dreamer being threatened by everything from dogs, stones, and insects to terrorists and “big spheres”.
Sticking with the continuity hypothesis, it suddenly becomes less surprising to learn that themes related to health and illness tend to occur more frequently in fever dreams than regular dreams.
Fever-tinged visions also typically include fewer characters and interactions than your typical dream, which may reflect the experience of social isolation that most people impose on themselves when ill.
What’s less clear, however, is whether the continuity hypothesis can account for the fact that fever dreams tend to be so much more emotionally intense than everyday dreams.
Even more intriguingly, Schredl’s research revealed that around 40 percent of fever dreams are lucid dreams, in which the dreamer is fully aware that they are dreaming.
Unable to explain the remarkable lucidity of these diseased hallucinations, the study authors cautiously hold their tongues, allowing themselves only to remark that “this supports the notion that fever has an effect on the cognitive processes during sleep.”
In some ways, then, our understanding of fever dreams hasn’t progressed all that much since Dr Coats described them as “imperfection[s] in the completeness of sleep.” Fortunately for him (and his sister), these unsettling apparitions can’t do us any harm, no matter how real they might seem.
This Curious Questions first appeared in Issue 31 (February 2025) of our digital magazine CURIOUS. To access exclusive CURIOUS content first and receive the quarterly digital magazine, become an All Access Member.
