36-Hour Fast: Benefits, Safety, and How to Do It

Quick answer: A 36-hour fast means going without food for a full day and a half -- typically from dinner one evening through the entire next day, breaking the fast with breakfast on the third day. This duration activates significant autophagy, deep fat oxidation, and metabolic reset, but requires preparation, electrolyte management, and prior fasting experience.

What Is a 36-Hour Fast?

A 36-hour fast is the shortest form of extended fasting. It bridges the gap between daily time-restricted eating protocols like 16:8 and multi-day fasts of 48 to 72 hours. In practice, it usually looks like this:

Day 1, 7 PM: Eat your last meal (dinner)
Day 2: Fast the entire day -- no food
Day 3, 7 AM: Break your fast with breakfast

During the fast, you consume only water, black coffee, plain tea, and electrolytes. Nothing with calories.

The 36-hour fast is a key component of some alternate day fasting protocols, where the period between dinner on a feast day and breakfast on the next feast day spans approximately 36 hours. But it can also be used as a standalone practice -- done once a week or once a month as a metabolic reset.

What Happens During 36 Hours Without Food

The physiology of a 36-hour fast is significantly different from a 16-hour daily fast. Here is the timeline:

Hours 0-12: Transition

Your body processes the last meal, depletes readily available glucose, and begins tapping into glycogen stores. Insulin drops. This phase is familiar territory if you have experience with daily fasting.

Hours 12-18: The Metabolic Switch

Liver glycogen is substantially depleted. Fat oxidation becomes the primary energy source. Norepinephrine increases, mobilizing fatty acids from adipose tissue. You cross the threshold into genuine fasting metabolism.

Hours 18-24: Fat Burning and Ketone Production

The liver begins converting fatty acids into ketone bodies (beta-hydroxybutyrate, acetoacetate, acetone). Blood ketone levels rise to 0.5 to 1.5 mmol/L in most people. The brain begins using ketones for a significant portion of its energy needs. Many people report a surge in mental clarity during this period.

Growth hormone secretion increases dramatically. Hartman et al. (1992) documented a 2,000% increase in HGH during 24-hour fasts, which supports fat mobilization while preserving lean tissue.

Hours 24-36: Autophagy and Deep Fasting

This is where the 36-hour fast distinguishes itself from shorter protocols. By 24 hours, autophagy -- the cellular self-cleaning process -- is operating at meaningful levels. AMPK is fully activated, mTOR is suppressed, and cells are actively breaking down damaged proteins and dysfunctional organelles for recycling.

Alirezaei et al. (2010) demonstrated significant autophagy activation in neuronal tissue after 24 hours of fasting. By 36 hours, this process has been running for over 12 hours, providing a substantial window for cellular repair.

Blood ketone levels typically reach 1.0 to 2.0 mmol/L. Insulin is at its lowest sustained level. The inflammatory profile improves -- Faris et al. (2012) in Nutrition Research found that a single prolonged fast significantly reduced inflammatory markers including IL-6, CRP, and homocysteine.

Evidence-Based Benefits

Deep Autophagy Activation

The primary differentiator of a 36-hour fast over daily protocols is the depth and duration of autophagy. While daily 16:8 fasting initiates early autophagy signals, a 36-hour fast provides a sustained window of cellular cleanup that shorter fasts cannot achieve. This is relevant for long-term cellular health, immune function, and potentially cancer prevention, though human cancer prevention data remains preliminary.

Enhanced Insulin Sensitivity

Halberg et al. (2005) showed that even short-term intermittent fasting improved insulin-mediated glucose uptake. A 36-hour fast produces a more pronounced insulin sensitization effect because of the prolonged period of zero insulin stimulation. For people with insulin resistance or prediabetes, periodic 36-hour fasts may offer meaningful benefits beyond daily fasting protocols.

Significant Fat Oxidation

During 36 hours without food, the body relies almost entirely on fat stores for energy. Depending on body composition and activity level, you can expect to oxidize 150 to 300 grams of body fat during a 36-hour fast. This is not water weight -- it is actual fat metabolism.

Immune System Reset

Cheng et al. (2014) in Cell Stem Cell found that prolonged fasting (2 to 4 days) triggered regeneration of immune stem cells. While 36 hours is shorter than the fasting periods in that study, it does initiate the immune stress-response pathways that prime the immune system for renewal upon refeeding.

Mental Clarity

Many fasters report their sharpest mental state during hours 18 to 36. This is likely driven by elevated ketones (the brain's preferred alternative fuel), increased norepinephrine, and the absence of postprandial cognitive dips. Mattson et al. (2018) in Nature Reviews Neuroscience reviewed the neuroprotective mechanisms of fasting, including enhanced neuronal stress resistance and BDNF (brain-derived neurotrophic factor) production.

How to Do a 36-Hour Fast Safely

Prerequisites

Do not attempt a 36-hour fast unless you have:

At least 4 to 6 weeks of comfortable daily fasting experience (16:8 or longer)
No medical conditions that contraindicate fasting (see "Who Should Not Fast" below)
A plan for electrolyte supplementation

Step-by-Step Guide

Before the fast:

Eat a balanced dinner with adequate protein, healthy fats, and fiber. Do not carb-load or overeat -- this will make the first 12 hours harder, not easier.
Prepare your electrolyte supplies for the next day.

During the fast (Day 2):

Drink plenty of water -- at least 2 to 3 liters throughout the day.
Supplement electrolytes: sodium (1/4 teaspoon of salt in water 2-3 times), magnesium (200-400mg supplement), and potassium (from a lite salt or supplement). Electrolyte depletion is the most common cause of fasting side effects.
Black coffee and plain tea are permitted and can help manage hunger and maintain energy.
Stay lightly active. Walking, gentle yoga, or light stretching are beneficial. Avoid intense exercise.
Keep busy. Boredom amplifies hunger. Plan a full day of engaging activities or work.

Breaking the fast (Day 3 morning):

Start with a small, easily digestible meal. Scrambled eggs, avocado, and a small portion of cooked vegetables is a good choice.
Wait 30 to 60 minutes before eating a larger meal.
Do not break the fast with a large, high-carb, or heavily processed meal. Your digestive system needs time to restart, and a sudden glucose spike after 36 hours of fasting can cause discomfort, bloating, and reactive hypoglycemia.

Managing Side Effects

Hunger: Comes in waves, typically peaking at habitual meal times. It does not build continuously. If you can wait 30 minutes, the worst usually passes.

Headache: Almost always caused by dehydration or electrolyte imbalance. Increase water and sodium intake.

Dizziness or lightheadedness: Stand up slowly. Take electrolytes. If it persists, break the fast.

Irritability: Common in the first 36-hour fast. It improves with experience as your body adapts to using ketones efficiently.

Cold sensation: Metabolic rate adjusts slightly during extended fasts. This is normal and temporary.

How Often to Do a 36-Hour Fast

Once per week: This is the most common frequency and aligns with the alternate day fasting rhythm. A weekly 36-hour fast combined with normal eating on other days provides a strong metabolic stimulus without chronic restriction.

Twice per month: A moderate approach that provides periodic autophagy activation and metabolic reset with minimal lifestyle disruption.

Once per month: Appropriate for those who primarily use daily time-restricted eating and want occasional deeper fasting benefits.

There is no evidence that more frequent 36-hour fasts (more than twice per week) provide additional benefits, and they increase the risk of nutritional deficiency and metabolic adaptation.

Who Should Not Do a 36-Hour Fast

People with Type 1 diabetes or on insulin
Pregnant or breastfeeding women
Anyone underweight or with a history of eating disorders
Children and adolescents
People on medications that require food intake
Anyone who has not built up fasting experience with shorter protocols

If you have Type 2 diabetes, cardiovascular disease, or other chronic conditions, consult your doctor before attempting a 36-hour fast.

36-Hour Fast vs. Other Protocols

Compared to daily time-restricted eating, a weekly 36-hour fast provides deeper autophagy and more pronounced metabolic reset at the cost of one challenging day per week. Compared to multi-day fasts (48 to 72+ hours), the 36-hour fast is safer, requires less medical supervision, and is easier to integrate into normal life.

For most people, the combination of daily 16:8 or 18:6 fasting with a periodic 36-hour fast offers the best of both worlds: daily metabolic benefits plus periodic deep fasting.

How Fasted Helps

Fasted lets you set up a 36-hour fast with a dedicated timer that counts down the full duration. The app sends checkpoint notifications so you know where you are in the fasting process, and the streak tracker records your extended fasts alongside your daily fasting practice. Weight logging before and after your fast gives you concrete data on the results, and the insights dashboard helps you identify the optimal frequency for your extended fasts.

Frequently Asked Questions

Will I lose muscle during a 36-hour fast?

Minimal muscle loss occurs during a single 36-hour fast, especially when growth hormone is elevated. Studies on alternate day fasting (which includes regular 36-hour fasting periods) show preserved lean mass when protein intake is adequate on eating days. The risk increases if 36-hour fasts are done frequently without adequate refeeding.

Can I exercise during a 36-hour fast?

Light to moderate activity (walking, yoga, easy cycling) is fine and can actually help manage hunger. Avoid high-intensity training or heavy lifting during the fast. Save intense workouts for eating days.

How much weight will I lose from a single 36-hour fast?

You will likely see a 1 to 3 pound drop on the scale, but most of this is water and glycogen depletion. Actual fat loss from a single 36-hour fast is approximately 0.3 to 0.5 pounds. The real benefits accumulate with consistent practice over months.

Is a 36-hour fast safe?

For healthy adults with fasting experience, yes. A 36-hour fast is well within the range that humans have evolved to tolerate. The key safety measures are adequate hydration, electrolyte supplementation, and knowing when to break the fast early if you feel unwell.

Can I drink bone broth during a 36-hour fast?

Technically, bone broth contains calories and breaks a strict fast. However, it provides electrolytes and may help with compliance. If you are doing a 36-hour fast for autophagy, avoid bone broth. If your goal is fat loss and the broth (under 50 calories) helps you complete the fast, it is a reasonable compromise.