Could skipping meals for a day and a half really send your body into survival mode? That’s the question stirring debate after a viral simulation video showed what happens inside your body during a 36-hour fast. It’s the same routine British Prime Minister Rishi Sunak claims to follow weekly—but behind the headlines and hype, there’s a deeper, less glamorous story.
While intermittent fasting is often framed as a health shortcut to weight loss and mental clarity, the reality is more complicated. From hormone shifts to headaches, and fat-burning to fatigue, going without food for that long doesn’t just trim your waistline—it puts your entire system into overdrive.
Before you skip your next meal in the name of detox or discipline, here’s what actually happens when you fast for 36 hours—and why it’s not always as beneficial as it sounds.
What Happens to the Body During a 36-Hour Fast?
At first glance, fasting for 36 hours might sound like a tough but manageable discipline—skip a day’s worth of meals, power through with some water or black coffee, and enjoy the supposed fat-burning benefits. But once your body passes the 12-hour mark without food, a cascade of changes begins, and not all of them are as beneficial as they seem.
Phase 1: Glycogen Depletion and Early Symptoms
Within the first 12–16 hours, your body runs out of glycogen—the stored form of carbohydrates in your liver and muscles. Once depleted, it begins converting fat into ketones for energy. This metabolic switch is what many fasting advocates point to as the mechanism behind fat loss and “mental clarity.” But this transition isn’t smooth for everyone.
As blood sugar drops, symptoms like headaches, dizziness, and low energy can start to creep in. This is partly due to hypoglycemia and partly due to shifts in hydration and electrolytes, especially if you’re not actively replenishing them. Some people also experience a spike in irritability or mood swings—a side effect backed by studies showing heightened emotional reactivity during fasting periods.
Phase 2: Ketosis and Metabolic Shift
Around the 24-hour mark, your body is likely fully in ketosis, breaking down fat stores for fuel. Some describe this stage as a “second wind”—where hunger fades and energy stabilizes. That said, it’s also when less visible internal processes, like autophagy, begin to ramp up. Autophagy is your body’s way of clearing out damaged cells to make room for healthier ones. This cellular clean-up is one of the most-cited benefits of prolonged fasting.
But here’s the catch: while autophagy sounds promising in theory, most of the supporting research comes from animal studies. Human trials on fasting and autophagy are limited, short-term, and often inconclusive. As Prof. James Betts from the University of Bath points out, many of the expected benefits of long fasts haven’t been clearly observed in actual human subjects.
Phase 3: Stress on the System
By the 36-hour mark, your body has been under prolonged energy restriction. For some, this can trigger a “reset.” For others, it starts tipping into a stress state—cortisol levels may rise, concentration dips, and the risk of nutrient depletion increases, especially if fasting becomes routine without proper planning.
Digestive issues like constipation or bloating are also common. That’s because the gut slows down during fasting, and abruptly restarting your normal diet afterward—especially with large or rich meals—can overwhelm the system.
The Health Risks and Side Effects You Might Experience
Fasting for 36 hours isn’t just a matter of willpower—it’s a physiological stress test. While some people tout feeling sharper or lighter during extended fasts, many others experience a host of unpleasant side effects that shouldn’t be brushed off. These aren’t just mild inconveniences; they’re signs your body is reacting to a lack of energy, nutrients, and routine biological cues.
1. Fatigue, Dizziness, and Low Energy
One of the most immediate issues people report during a 36-hour fast is a steep drop in energy. As your body shifts from glucose to fat for fuel, the transition can be jarring. You might feel sluggish, foggy, or lightheaded. For some, even standing up quickly can lead to dizziness—a sign of low blood pressure or dehydration.
2. Headaches and “Fasting Brain”
Headaches are common in the first day of fasting. This can be due to several factors: caffeine withdrawal, dehydration, low blood sugar, or disrupted sleep. A review of multiple studies on intermittent fasting found that headaches were one of the most consistently reported side effects, often mild to moderate, but sometimes severe enough to disrupt daily function.
3. Digestive Problems
Despite the absence of food, digestive issues still occur. Bloating, constipation, or even nausea can result from both slowed gastrointestinal motility and changes in gut microbiota. Plus, when people break a long fast with a large or heavy meal, it can trigger cramping, diarrhea, or rebound bloating.
4. Mood Swings and Irritability
Hunger can do more than make you cranky—it can cause genuine mood disturbances. A study involving women fasting for 18 hours showed a measurable increase in irritability. This isn’t just psychological—it’s the body’s response to low blood sugar and disrupted serotonin activity. Add sleep disruption to the mix, and emotional regulation becomes even harder.
5. Sleep Disturbances
Ironically, while some claim fasting helps them sleep better, many report insomnia or fragmented sleep—especially during the first few attempts. This could be due to hormonal shifts, late-night hunger cues, or elevated cortisol levels. A 2019 study tracking over 1,400 people on extended fasts found that 15% experienced significant sleep disruption.
6. Dehydration and Electrolyte Loss
A surprising number of people forget to drink enough water while fasting. Add in increased urination—a common side effect during ketosis—and you have a recipe for dehydration. Symptoms like dry mouth, dark urine, muscle cramps, and fatigue often trace back to lost fluids and electrolytes, not just calorie restriction.
7. Nutrient Deficiency and Malnutrition
Fasting for 36 hours once in a while isn’t likely to cause malnutrition, but regular prolonged fasting without nutritional planning can. If your non-fasting meals don’t include adequate protein, fiber, vitamins, and minerals, deficiencies can build up over time. That’s especially risky for people with already restrictive diets or underlying health conditions.
8. Hormonal Disruption and Reproductive Health
Fasting can alter hormone levels—sometimes in ways that aren’t beneficial. For example, it may suppress reproductive hormones in women, potentially affecting menstrual cycles or fertility. While the exact thresholds vary, prolonged energy restriction can trigger the body’s stress response, prioritizing survival over reproduction.
Who Should Avoid Extended Fasting—and Why
While intermittent fasting is often marketed as a universal solution for weight loss or mental clarity, it’s not safe—or appropriate—for everyone. A 36-hour fast pushes the body far beyond regular dietary patterns, and for certain individuals, it can do more harm than good.
Here’s a clear look at who should not engage in extended fasting and why.
1. People With a History of Eating Disorders
Anyone with a current or past eating disorder—such as anorexia, bulimia, or binge eating disorder—should avoid extended fasting altogether. According to The Emily Program, a mental health and eating disorder treatment center, fasting can encourage the habit of ignoring hunger cues, which can spiral into disordered eating behaviors. Even if intermittent fasting doesn’t start as an eating disorder, the mindset of strict control around food can tip someone into dangerous territory.
2. Individuals With Diabetes or Blood Sugar Conditions
For people with diabetes—especially those on insulin or blood sugar-lowering medications—a 36-hour fast can significantly increase the risk of hypoglycemia (low blood sugar), which may lead to confusion, fainting, or even seizures. Any major shift in eating patterns should be done under medical supervision. Extended fasts make blood sugar levels harder to manage and can interfere with how medications work.
3. Pregnant or Breastfeeding Individuals
During pregnancy and lactation, the body needs consistent nutrition to support both the parent and the baby. Long fasting windows can interfere with energy balance, nutrient intake, and milk production. Healthcare professionals consistently advise against extended fasting during these life stages, as it may compromise both maternal and infant health.
4. Children, Teens, and Older Adults
Growing bodies and aging bodies alike need regular nutrient intake. Children and adolescents require steady energy for development, hormone regulation, and brain function. Likewise, older adults may already experience challenges like reduced appetite, muscle loss, or medication interactions. Adding a 36-hour fast on top of that increases the risk of weakness, dizziness, and nutrient depletion.
5. People Taking Certain Medications
Many medications—like NSAIDs, blood pressure drugs, or blood thinners—are meant to be taken with food. Skipping meals while on these medications can lead to stomach irritation, increased side effects, or reduced efficacy. Anyone taking daily medication should consult a healthcare provider before trying any form of extended fasting.
6. People With High Physical or Mental Demands
If your job or daily routine requires consistent focus, stamina, or physical output, a 36-hour fast could undercut your ability to function safely. This applies to shift workers, manual laborers, caregivers, and anyone managing high-stress or high-responsibility environments. Prolonged fasting can impair concentration, reaction time, and energy—none of which are ideal if you’re operating machinery or making important decisions.
What Science Actually Says About the “Benefits”
The promises attached to intermittent fasting—especially extended fasts like the 36-hour “Monk Fast”—often sound impressive: better brain function, lower inflammation, weight loss, and even protection against chronic disease. But separating solid evidence from speculation is key, especially when these claims are echoed across social media without context.
Here’s what the science actually supports—and where the evidence still falls short.
Weight Loss: Yes, But Not Unique to Fasting
Fasting can lead to weight loss, primarily by creating a calorie deficit. Going without food for 36 hours almost guarantees you’re eating less across the week. But this isn’t exclusive to fasting. A 2025 meta-analysis found that intermittent fasting did not outperform traditional calorie restriction when it came to long-term weight loss. In other words: fasting works because it reduces calories—not because there’s something inherently magical about the fasting process.
Dr. Archana Batra, a nutritionist and certified diabetes educator, agrees that fasting can be effective for weight management, but only when it’s part of a broader, well-rounded approach—including nutrition and exercise.
Improved Insulin Sensitivity and Blood Sugar Control
Multiple studies show that intermittent fasting can improve insulin sensitivity and stabilize blood sugar levels—at least in the short term. This is particularly relevant for people with prediabetes or insulin resistance.
But again, this benefit tends to depend on consistency and dietary quality between fasts. And for people with diabetes, especially those on medication, these shifts can be dangerous if not carefully managed under clinical supervision.
Cellular Repair and Autophagy: Promising, But Incomplete
Autophagy—the process where cells break down and recycle damaged parts—is often cited as one of fasting’s biggest benefits. Fasting does appear to activate this process, particularly after 24 hours or more without food.
However, most of the research supporting autophagy comes from animal models. Human studies are limited and don’t yet show clear, measurable benefits like disease prevention or slowed aging. As Professor James Betts from the University of Bath explains, “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.”
Cognitive Performance and Mental Clarity
There’s growing interest in how fasting affects the brain. During fasting, the body produces ketones, an alternative energy source that some claim sharpens focus and decision-making. Advocates like Rishi Sunak even point to increased mental clarity during fasts.
Some small studies and anecdotal reports support this, but broader evidence is lacking. Mental clarity may improve for some, especially if they’re used to fasting. For others, the opposite is true—brain fog, irritability, and distraction dominate the experience. Again, the outcome varies by individual.
Inflammation and Disease Risk: Still Early Days
There are some early indicators that fasting may help lower markers of inflammation, oxidative stress, and even reduce risk factors for heart disease. But most of this research is short-term and doesn’t prove cause and effect. There’s also a notable absence of long-term, large-scale studies confirming these benefits in humans following prolonged fasts like the 36-hour model.
Real-World Tips to Stay Safe
If you’re considering trying a 36-hour fast after hearing about its supposed health perks, it’s crucial to go into it with more than just willpower and enthusiasm. Extended fasting puts your body through a lot, and doing it wrong can leave you worse off than when you started. Here’s how to approach it safely, realistically, and without setting yourself up for failure—or harm.
1. Don’t Start With 36 Hours
Jumping straight into a 36-hour fast is like trying to run a marathon without training. Start with shorter fasting windows—like the 12:12 or 16:8 method—before pushing into longer durations. This helps your body gradually adapt to running on stored energy without causing a metabolic shock.
2. Prioritize Hydration and Electrolytes
Fasting doesn’t mean fluid restriction. In fact, dehydration is one of the fastest ways to feel awful during a fast. Aim to drink water throughout the day and consider adding no-calorie electrolyte supplements or mineral salts, especially if you’re experiencing headaches, cramps, or lightheadedness. Watch the color of your urine—it should stay a pale yellow.
3. Don’t Ignore Warning Signs
Dizziness, faintness, nausea, brain fog, or extreme fatigue are not signs of “detox”—they’re signs that your body is struggling. If these symptoms don’t improve or become disruptive, break the fast. There’s no prize for pushing through something your body clearly can’t handle. As a rule: fasting should never make you feel miserable.
4. Break the Fast Gently
After 36 hours without food, your digestive system is on pause. Don’t restart it with a massive, greasy meal. Start with something simple and easy to digest—like a small bowl of broth, a boiled potato, or plain rice. Wait an hour or so before having a more complete meal. Jumping straight into heavy food can cause bloating, nausea, or even diarrhea.
5. Focus on Nutrition Between Fasts
Extended fasting isn’t a free pass to eat whatever you want afterward. If your regular meals lack fiber, protein, and essential nutrients, fasting will only increase your risk of deficiencies over time. The fasting period may reset your system temporarily, but what you eat before and after matters far more for long-term health.
6. Talk to a Professional—Especially If You Have Health Conditions
This isn’t just a disclaimer—it’s practical advice. If you have diabetes, thyroid issues, digestive problems, or take any medications, you need medical guidance before trying an extended fast. A qualified professional can help you decide whether fasting is appropriate for your body and, if so, how to do it safely.
7. Listen to Your Body—Not the Internet
Just because a public figure, influencer, or coworker swears by a fasting routine doesn’t mean it will work for you. Some people genuinely feel better with fasting; others crash hard. There’s no one-size-fits-all. Track how you feel, monitor your symptoms, and be willing to change course if it’s not working.
Fasting Isn’t a One-Size-Fits-All Solution
The growing popularity of 36-hour fasts—fueled by headlines, simulations, and personal success stories—makes it easy to believe this approach is universally beneficial. But when you strip away the hype, the truth is more nuanced: fasting affects everyone differently, and what works for one person could be harmful for another.
While some people experience improved focus, fat loss, or metabolic flexibility, others walk away with headaches, mood swings, and digestive issues. And despite promising theories about autophagy or brain health, most of the long-term human data just isn’t there yet.
There’s nothing wrong with using fasting as a tool—but it shouldn’t be your foundation. What matters more than skipping meals is how you eat the rest of the time, how well you sleep, how consistently you move your body, and how you manage stress. These habits do more for your health than any rigid fasting window ever will.
So if you’re drawn to the idea of fasting, be smart about it. Start small. Monitor how your body reacts. And don’t treat it like a shortcut or a test of willpower. If a 36-hour fast leaves you drained, distracted, or dreading food, that’s not discipline—it’s a sign to try something different.
