Going a full day and a half without eating might sound like self-inflicted torture, but for some, the 36-hour fast has become the latest wellness trend promising everything from mental clarity to “cellular repair.”
A new simulation by YouTube channel Wellness Wise — which is not an accredited medical institution and stresses that its content is “not a substitute for professional advice” — claims to show how the human body responds over a day and a half without food.
But how much of that science actually checks out? And could fasting for 36 hours really be the “full body reset” it’s marketed as?
From Stored Sugar To Fat-Burning Mode
According to the Wellness Wise simulation, the first few hours of fasting set off subtle but measurable changes inside the body.
After about four hours, insulin levels drop, forcing the body to start burning stored sugar for fuel. By the eight-hour mark, blood sugar levels fall further, and the body begins to use glycogen — its short-term energy reserve, stored in the liver and muscles.
By 12 hours, glycogen stores are largely depleted, and the body begins to burn fat instead, entering a process called ketosis — a metabolic state where fat becomes the primary energy source instead of carbohydrates.
This is where fasting fans believe the magic happens. Supporters say ketosis triggers sharper focus, sustained energy, and a jumpstart in fat loss.
By 16 hours, the simulation claims that the body begins autophagy — a kind of cellular self-cleaning mechanism where damaged cells and toxins are broken down for reuse.
At 24 hours, the video says inflammation decreases, insulin sensitivity improves, and deeper “cellular repair” begins.
At around 30 to 36 hours, the simulation reaches what it calls “maximum autophagy.” Growth hormone spikes, fat burning accelerates, and tissues are said to regenerate in what Wellness Wise describes as a “full body reset.”
Again, these findings come from a digital model, not peer-reviewed science.
While intermittent fasting might seem like a modern fad, it’s rooted in human history. Dr Valter Longo, a longevity researcher at the University of Southern California, told News in Health that fasting used to be unavoidable:
“They were forced to fast,” he said, explaining that early societies often went without food due to weather, harvests, or economic hardship.
Now, food is accessible almost everywhere — and we eat accordingly. Dr Vicki Catenacci, a nutrition researcher at the University of Colorado, said that “people now eat, on average, throughout a 14-hour period each day.”
The question is whether our ancestors’ survival adaptations can be deliberately mimicked today for health benefits.
Experts Divided On The Benefits
Researchers agree that fasting changes metabolism, but opinions differ on whether short fasts — like a 36-hour stretch — are meaningfully beneficial.
Adam Collins, Associate Professor of Nutrition at the University of Surrey, told The Guardian that stricter fasts do accelerate the body’s shift from burning carbs to fats. But he warned that most research supporting fasting benefits comes from studies on animals, not humans.
“Whether you can get those effects with just a 36-hour fast once a week, [I’m] not sure,” Collins said.
Professor James Betts, an expert in metabolic physiology at the University of Bath, echoed the caution.
“There [are] a lot of proposed benefits to [running on fats],” he told The Guardian. “But a lot of the research hasn’t really [been borne out in] human beings. So we don’t see dramatic health benefits, certainly in the short term.”
He also noted that extended fasting can sap energy and make people less active — potentially offsetting any metabolic gains.
Heart And Brain Health
Supporters of fasting claim it can boost cardiovascular health and even sharpen brain function. The theory goes that ketosis — the fat-burning state triggered by fasting — provides the brain with a more efficient energy source, improving focus and cognitive performance.
However, research on this remains mixed. Some studies show modest benefits for patients with neurological conditions like epilepsy, multiple sclerosis (MS), or Alzheimer’s disease, but experts say there’s “no clear evidence of a positive short-term effect” on healthy adults.
As for the heart, the science is just as conflicted. Some studies link fasting with reduced inflammation and improved blood pressure, but others — including a large study of 19,000 adults — found that eating within narrow time windows (such as eight hours per day) could actually increase cardiovascular disease risk by 135 percent.
The Science Of Autophagy — And Its Limits
One of the most talked-about “benefits” of fasting is autophagy, a cellular recycling process where old or damaged parts of cells are broken down and reused.
According to the Cleveland Clinic, autophagy can help repair and restore cells — a bit like taking out the biological trash. It’s a key reason some fasting advocates promote going beyond 24 hours.
But again, most of that evidence comes from animal studies, not humans. Professor Betts notes that the timing of autophagy’s onset — usually 24 to 48 hours — hasn’t been reliably confirmed in people.
Johns Hopkins Medicine also cautions that fasting for more than a day may backfire, stating: “Going too long without eating might actually encourage your body to start storing more fat in response to starvation.”
So, Is Fasting For 36 Hours Safe?
The short answer: it depends. For some, especially those with certain health conditions, skipping meals for that long could be risky.
Doctors warn that fasting can cause dehydration, dizziness, muscle loss, or disordered eating behaviours. And while some may feel a mental or spiritual lift, others may simply feel exhausted.
Professor Betts adds that weight loss during a 36-hour fast may come from muscle, not fat — “which would not be ideal.”
Experts stress that anyone considering fasting — whether for longevity, focus, or fat loss — should first talk to a qualified healthcare professional.
As Dr Catenacci puts it, modern humans may not need to fast at all: “People now eat throughout a 14-hour period each day,” she said. “That’s already a long feeding window. What’s important is balance, not deprivation.”
The Wellness Wise simulation offers an intriguing look at how the body might respond to fasting, from insulin dips to fat burning to autophagy. But the consensus among scientists is clear: these effects remain theoretical for most healthy adults, and the risks of prolonged fasting may outweigh the rewards.
So if you’re tempted to try the 36-hour fast, the smartest first step might not be skipping breakfast — it’s calling your doctor.
Featured image credit: YouTube/WellnessWise
