The 36 Hour Fast Explained: UK Guide 2026 | Complete Nutrition

Understanding Fasting

The 36 hour fast explained

A 36 hour fast means no calories for a day and a half. Typical pattern: dinner at 7pm one day, skip all food the next day, breakfast at 7am two days later. Liver glycogen fully depletes. Blood ketones reach 0.5 to 1.5 mmol/L entering mild nutritional ketosis. Growth hormone begins climbing. The fast captures more of fasting physiology than 24 hour fasts but is still well below extended fast risks. Done weekly or fortnightly creates substantial deficit with manageable demands.

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Updated:
May 2026

Written by:
Dominic Walton, MD

Reading time:
7 min

The full answer

Structure and physiology of the 36 hour fast

The 36 hour fast goes deeper than a 24 hour fast without entering true extended fasting territory. Four points explain the structure.

1. The schedule

The standard pattern is dinner-to-breakfast. Eat dinner at 7pm on day 1. Skip all food on day 2 (the fasting day). Eat breakfast at 7am on day 3. The fast covers one full calendar day plus the overnight periods either side. This pattern fits a typical work week well: fast on a weekday with normal social meals either side. Some people prefer Sunday dinner to Tuesday breakfast spanning the start of a work week. The exact schedule matters less than the consistency.

2. The physiology hour by hour

Hours 0 to 8: digestion completing, insulin falling. Hours 8 to 16: glycogen breakdown dominant, fat oxidation rising. Hours 16 to 24: liver glycogen exhausted, gluconeogenesis maintaining glucose, fat oxidation dominant, mild ketones starting (0.3 to 0.8 mmol/L). Hours 24 to 36: mild nutritional ketosis (ketones 0.5 to 1.5 mmol/L), growth hormone rising 2 to 3 fold, adrenaline rising modestly, brain shifting partly to ketone fuel. Most of the metabolic adaptations seen with longer fasts are partially present by 36 hours.

3. The experience day by day

Day 1 (full eating): no special preparation needed, eat normally. Evening of day 1 last meal: balanced meal with protein, vegetables and some healthy fat. Day 2 (full fasting day): hunger waves around usual breakfast and lunch times (often the worst around mid-morning), generally manageable through afternoon, energy typically stable, evening hunger less than expected. Night of day 2 sleep may be slightly disrupted. Morning of day 3 (around 36 hours): often a stable but tired state, hunger lower than expected.

4. Why people choose 36 hour fasts

The 36 hour fast captures more fasting physiology than a 24 hour fast (reaches mild ketosis, growth hormone rises, more substantial autophagy in animal models) while staying well below the medical risks of extended fasts. Weekly or fortnightly 36 hour fasts can be done sustainably without medical supervision in healthy adults. The deficit per fast is roughly 2000 to 3000 kcal. Done weekly this can drive meaningful weight loss without daily restriction.

Practical guidance

Doing a 36 hour fast well

Five practical strategies for the 36 hour fast.

Build up gradually

Do not jump from no fasting to 36 hour fasts. Build through 12:12, 14:10, 16:8 and 24 hour fasts first. By the time you attempt 36 hours your hunger hormone patterns have adapted and the fast feels manageable. Going straight to 36 hours from a standard eating pattern produces intense hunger and high abandonment rates.

Pick the right days

Choose a fasting day with moderate demands. Standard work days are usually fine. Avoid days with heavy training, important social meals, important presentations or unusual stress. A typical Tuesday or Wednesday tends to work. Plan around your life rather than trying to force the fast onto a difficult day.

Manage hydration deliberately

Aim for 3 litres of water through the fasting day plus what you would normally drink elsewhere. Plain water, black coffee, plain tea, herbal tea, sparkling water. Some people benefit from a small pinch of sea salt in water for sodium balance on 36 hour fasts. Avoid sweetened drinks completely.

Get adequate sleep

Poor sleep before or during a fast makes everything harder. Prioritise 7 to 9 hours sleep the night before the fasting day. Sleep on the fasting night may be slightly disrupted but is usually adequate. Avoid late-night caffeine which can amplify any sleep disruption.

Break the fast carefully

Breakfast on day 3 should be a moderate balanced meal eaten slowly. Eggs with vegetables, salmon with salad, Greek yogurt with berries and nuts. Smaller than your normal breakfast. Wait an hour before next meal. By lunch you can return to normal eating. Avoid breaking with refined carbohydrates or sugar which produce strong glucose spikes after the fasting state.

Safety

Who should avoid 36 hour fasts

Standard contraindications apply with slightly more caution than shorter fasts.

History of eating disorders. Contraindicated.
Pregnancy or breastfeeding. Not appropriate.
Type 1 diabetes or insulin dependent type 2 diabetes. Significant hypoglycaemia risk. Specialist supervision required.
BMI under 18.5. Not appropriate.
Children, adolescents or adults under 18. Contraindicated.

Anyone over 65, on medications or with significant medical conditions should discuss with GP first. The mild ketosis at the end of a 36 hour fast can trigger gout flares in susceptible individuals. Anyone with kidney disease should be cautious. Stop the fast immediately for severe symptoms (dizziness, palpitations, severe headache, chest pain). End with water and a balanced meal.

For the wider picture on fasting from the gentlest protocols to extended fasts plus the science behind hunger, metabolism and refeeding, our Understanding Fasting hub brings every guide together in one place.

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Back to the Fasting Hub

This article sits inside our complete knowledge base on fasting covering protocols, physiology, safety and practical guidance. Head back to the hub for the full index.

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Keep reading

Compare with other fast lengths

Several pages cover different fast durations. Our piece on the 24 hour fast explained covers the entry to longer fasting. The 48 hour fast explained covers the next step up. And alternate day fasting explained covers a related daily-equivalent protocol.

Frequently asked

36 hour fast questions

What is a 36 hour fast?

A 36 hour fast means no calories for 36 hours. Typical pattern: eat dinner at 7pm one day, skip all food the next day, eat next at 7am two days later. The fast spans one full calendar day plus the overnight period either side. Water, black coffee and plain tea are allowed throughout. This protocol is sometimes called Monk Fasting because it skips a full day of meals.

What happens at 36 hours of fasting?

By 36 hours liver glycogen is fully depleted. Fat oxidation dominates fuel use. Blood ketones rise to 0.5 to 1.5 mmol/L reaching mild nutritional ketosis. Insulin is at minimum sustained level. Growth hormone begins to rise (2 to 3 fold by 36 hours, continuing higher). Adrenaline rises modestly. The body has fully transitioned into fasting metabolism. Hunger has typically subsided substantially after the second-day expected meal times.

How does a 36 hour fast feel?

First 24 hours like a 24 hour fast: hunger waves around usual meal times, generally tolerable, stable energy. Hours 24 to 36: hunger typically reduces (ghrelin patterns subside), some people report slight clarity or alertness, others report fatigue and mild headache. Sleep on the fasting night can be slightly disrupted. Morning of day 2 (around 36 hours) often feels stable but tired. Breaking the fast typically feels welcome rather than urgent.

Is a 36 hour fast safe?

For healthy adults yes. 36 hour fasts in well-nourished people without contraindications carry minimal acute medical risk. Standard contraindications apply: eating disorder history, pregnancy or breastfeeding, type 1 diabetes or insulin dependent type 2 diabetes, BMI under 18.5, children, adolescents and adults under 18. Anyone on medications should discuss with GP first. Hydration and electrolyte attention are slightly more important than for 24 hour fasts.

How often should I do a 36 hour fast?

Once per week or every 2 weeks is typical. More than once per week starts demanding significant recovery between fasts. Some people incorporate occasional 36 hour fasts into a base of daily 16:8 practice. Others use 36 hour fasts as their primary protocol (one per week). Avoid stringing multiple 36 hour fasts back to back without recovery days. The protocol should be sustainable not exhausting.

Will a 36 hour fast trigger autophagy?

Probably some autophagy upregulation occurs by 36 hours based on animal data. Direct measurement of human autophagy is technically difficult so magnitude is uncertain. Compared to 16 hour fasts the additional autophagy effect of 36 hours is plausible but the clinical importance is unclear. Most evidence-based benefits of fasting (weight loss, modest metabolic improvements) come from caloric deficit and hormonal shifts not specifically from autophagy.

How do I break a 36 hour fast?

Start with a small balanced meal eaten slowly. Protein (eggs, fish, meat), vegetables and some healthy fat. Smaller than your normal portion. Eat at normal pace. Wait an hour or two before the next meal. The second meal can be closer to normal portion. Avoid breaking with refined carbohydrates, sugar or alcohol which produce strong glucose spikes after the fasting state. Resume normal eating over the rest of the day.