Fasting has surged in popularity, praised for its potential health benefits ranging from weight loss to longevity. But what really happens to your body when you stop eating for an extended period—say, 36 hours? A recent simulation, built on existing medical research and metabolic data, reveals surprising insights into how the body shifts, adapts, and even struggles during this intense period of food deprivation.

This isn’t just about willpower. It’s about biology, and the transformation starts within hours.

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0 to 6 Hours: The Digestive Wind-Down

During the first six hours of fasting, your body is still riding the tail end of its last meal. Blood sugar levels remain fairly stable as your digestive system processes the food you’ve consumed. Insulin—the hormone that helps store glucose in cells—is active, and your body is still primarily using glucose (sugar) for energy.

At this stage, you don’t feel much different. Hunger might kick in depending on your usual eating patterns, but it’s mostly manageable.

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6 to 12 Hours: The Shift Begins

As your blood sugar begins to drop, insulin levels decline, signaling your body to start using stored energy. The liver begins breaking down glycogen, the stored form of glucose, to keep blood sugar stable. This process is called glycogenolysis.

During this phase, you may start to feel a dip in energy, irritability, or mental fog. This is especially noticeable if your body is accustomed to frequent meals or high-carb diets. You’re running on reserves now, and your body is preparing to shift fuel sources.

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12 to 24 Hours: Ketosis Creeps In

By the 12-hour mark, glycogen stores begin to deplete significantly, especially if you’re active or under stress. The body then begins the process of switching to fat as its primary energy source, entering a metabolic state called ketosis.

In ketosis, fat cells are broken down into fatty acids and converted into ketone bodies by the liver. These ketones become an alternative fuel source for your brain and body. Many people report increased mental clarity and reduced hunger during this period, despite not eating.

However, for others, this can be the toughest stretch. Headaches, nausea, fatigue, and mood swings can arise—sometimes referred to as the “keto flu.”

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24 to 36 Hours: Deep Fasting State

At this stage, your body is fully in fasting mode. Insulin levels are extremely low, and growth hormone production increases to help preserve lean muscle mass. Autophagy—the body’s natural process of cleaning out damaged cells and regenerating new ones—ramps up, potentially offering powerful anti-aging and cellular repair benefits.

This is when some of the most promising benefits of extended fasting begin to occur:

• Improved insulin sensitivity

• Inflammation reduction

• Cellular regeneration

• Enhanced mental clarity

Interestingly, the sensation of hunger often subsides during this window, as the body becomes more efficient at utilizing fat and ketones.

But it’s not all upside. The 36-hour mark can also stress the body if hydration or electrolyte balance isn’t properly managed. Muscle protein breakdown may begin in some individuals, especially if they’re very lean or physically active.

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The Risks: Not for Everyone

While fasting for 36 hours can trigger a cascade of beneficial effects, it’s not without risks. People with underlying health conditions—especially diabetes, heart disease, or eating disorders—should not attempt extended fasting without medical supervision.

Symptoms such as dizziness, extreme fatigue, heart palpitations, or confusion can indicate more serious issues like electrolyte imbalance or hypoglycemia. Hydration and mineral intake (sodium, potassium, magnesium) become crucial during long fasts.

Moreover, once the fast ends, the reintroduction of food must be gentle. A large or carbohydrate-heavy meal after such a fast can cause digestive distress or spike insulin levels sharply. Breaking the fast with broth, vegetables, or light proteins is often advised.

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The Takeaway: A Double-Edged Sword

The 36-hour fast is a powerful metabolic tool that can reset the body’s systems, trigger cellular cleanup, and deepen fat burning. For some, it’s a path to mental clarity and health renewal. For others, it may be too extreme or carry unnecessary risks.

Ultimately, fasting is not a one-size-fits-all solution. Its impact depends on the individual—age, health, diet, stress levels, and genetics all play a role. Before taking on a prolonged fast, consider speaking to a healthcare provider, and always listen to your body.

A simulation may show us what could happen—but your own experience will depend on how you prepare, how you fast, and how you break it.