How many hours have you been fasting?
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Hours fasted
0
hours
Important — read before fasting
This calculator estimates typical metabolic phases based on peer-reviewed research. It is not medical advice.
Talk to your doctor first before fasting beyond 24 hours, or at any duration if you take medications (especially insulin, sulfonylureas, or blood pressure drugs), are pregnant or breastfeeding, have diabetes, kidney or heart disease, a history of eating disorders, or any chronic condition.
Stop fasting and seek care immediately if you experience severe weakness, confusion, chest pain, fainting, or persistent heart palpitations.
Explore the fasting map
Tap any phase below to see the research. Enter your hours above to see where you are on the timeline.
— What's activated —
— The 72-hour fasting map —
Digesting
Sugar reserves (glycogen)
Fat burning
Ketones
Growth hormone (HGH)
Cellular cleanup (autophagy)
Deep reset
0h
0h12h24h36h48h72h
Measured in humans
Inferred from animal data
Tap any phase for research and details.
You're in the metabolic valley. Both blood sugar and ketones are low simultaneously — the hardest moment of any extended fast. It passes within 1–2 hours. This is normal. Keep going.
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Should I stop or push through?
Signal reference — green, yellow, red
✓
You reached your goal hours
Planned completion. Break it confidently.
✓
Clear, stable, and calm
The fasting zone. Continue or stop — both are correct.
!
Mild headache
Try salt in water first — wait 20 minutes. Resolves in most cases. If not, break the fast.
!
Hunger around hours 13–16
The metabolic valley. Temporary. Often passes in 1–2 hours.
✗
Dizziness or weakness
Break the fast. These signals override goals.
✗
Heart palpitations or confusion
Break immediately. Eat something. No goal is worth this.
⚠ Severe chest pain, fainting, or persistent palpitations: call emergency services immediately. Do not wait.
What breaks a fast?
Common foods and drinks, and whether they end the fasted state
✓ Fine during fast
✓
WaterAny amount. Zero calories.
✓
Black coffeeNo milk, no sweetener. No insulin response.
✓
Plain teaAny type. Nothing added.
✓
Sparkling waterZero calories, no response.
✓
Salt in waterAddresses sodium depletion. Doesn't break fast.
✗ Breaks fast
✗
Milk or creamFat and protein raise insulin.
✗
Any sweetenerMany raise insulin without calories.
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Juice (any kind)Sugar. Ends fasted state immediately.
✗
BCAAsAmino acids activate mTOR.
✗
Any foodAny caloric intake ends the fast.
Bone broth contains protein and technically breaks a strict metabolic fast, though some use it during extended fasts for electrolyte support and comfort.
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How much protein should I eat to break my fast?
Single-bolus protein timing protocol for muscle protein synthesis
A large single protein bolus after an extended fast sustains muscle protein synthesis for 12+ hours — more effective than spreading protein across multiple smaller meals. Enter your details below.
90
grams
protein target for your first meal
At 175 lbs after a 36-hour fast, aim for 90 grams of protein in your first meal.
Each row below shows how much of that single food alone would reach your target. Choose one source, or combine smaller portions from several to hit 90 grams total.
Deep guides by fast duration
Detailed hour-by-hour breakdowns of the most-practiced fasting lengths
Each guide covers the metabolic timeline, what you'll feel hour by hour, electrolyte needs, and the research behind that specific fast length.
16hour
16-Hour Fast Guide
The daily intermittent fasting protocol. Fat burning, early ketosis, sustainable every day.
24hour
24-Hour Fast Guide
A full-day fast. Established ketosis, early HGH response, once or twice weekly.
36hour
36-Hour Fast Guide
Deep ketosis territory. Significant HGH elevation. The alternate-day fasting protocol.
48hour
48-Hour Fast Guide
Growth hormone surge window. Cellular cleanup beginning. Electrolyte management essential.
72hour
72-Hour Fast Guide
The traditional extended fast. Peak autophagy, stem cell activation territory.
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Common questions about fasting
Everything you need to know about extended fasting
What is the metabolic valley and why does it happen?
Around hours 13–17, both blood sugar and ketone levels are simultaneously low. Glucose stores are nearly depleted but ketone production hasn't fully ramped up — creating a brief window of low available fuel. You may feel hunger, brain fog, or irritability. This is the hardest moment of any extended fast, and the number one reason people quit unnecessarily. It passes within 1–2 hours once the liver fully shifts to ketone production. Knowing it's coming is what makes the difference between quitting and pushing through.
Does black coffee break a fast?
No. Black coffee contains no calories and does not raise insulin. It does not break a metabolic fast. Some research suggests it may enhance fat oxidation. The strict rule: no milk, no cream, no sweetener of any kind — including zero-calorie sweeteners, which can still trigger an insulin response in some people. Plain black only.
When does ketosis start during a fast?
Detectable ketones (0.5 mmol/L or higher) typically appear between hours 18 and 24. The exact timing depends on your last meal — a high-carbohydrate meal delays entry while a low-carb meal accelerates it. People who regularly follow a ketogenic diet often enter ketosis in 12–14 hours. By hours 24–36, deep ketosis is established. A blood ketone meter gives the most accurate confirmation.
When does autophagy start during fasting?
Autophagy begins initiating around hour 24 and becomes significantly elevated between hours 36 and 72. It is the cellular process of breaking down and recycling damaged proteins and organelles — sometimes described as the body running its cleanup crew. The 2016 Nobel Prize in Medicine was awarded to Yoshinori Ohsumi for discovering its mechanisms. Peak autophagy occurs beyond 72 hours. Protein intake, especially BCAAs, suppresses autophagy by activating mTOR.
Will fasting cause muscle loss?
Not significantly during typical 24–72 hour fasts. Peer-reviewed research documented Human Growth Hormone increasing roughly 5-fold during a 24-hour fast (Vinales 2022, p < 0.001), with similar results for 2-day fasts in Hartman 1992. This adaptive hormonal response preserves lean muscle during food scarcity. Ketone bodies also have direct anti-catabolic effects. A protein-forward first meal at the end of the fast further protects and rebuilds muscle. Significant muscle loss only becomes a concern with very prolonged fasting (well beyond 72 hours) or inadequate protein in the refeeding period.
How long do I have to fast to burn fat?
Active fat burning begins around hour 12 of a fast as glycogen stores deplete and your body shifts to fat oxidation. By hour 16, you are reliably in a fat-burning state — which is why the 16:8 intermittent fasting protocol is the most common and effective for fat loss. Fat burning intensifies as you move into ketosis around hour 18–24, where your liver begins producing ketone bodies from stored fat. The longer the fast, the deeper the fat oxidation — though most of the practical fat-loss benefit is captured in the 16–24 hour window, which is sustainable daily.
What should I eat to break my fast?
Lead with protein. For fasts under 24 hours: eggs, Greek yogurt, or chicken work well. For fasts of 24–72 hours: start gently with a small protein-forward meal (eggs, Greek yogurt, or a protein shake), wait 30–60 minutes, then eat your full refeeding meal. The key principle: do not break an extended fast with large amounts of simple carbohydrates or sugary foods. After prolonged insulin suppression, a large carbohydrate load can cause nausea, fatigue, and reactive hypoglycemia.
What is the difference between a 16-hour and a 36-hour fast?
A 16-hour fast delivers reliable fat burning and early ketosis and can be done daily — the most common intermittent fasting protocol. A 36-hour fast delivers deep ketosis with a significant hormone spike, the beginning of meaningful autophagy, and more substantial metabolic benefits, but requires electrolyte management and is typically done once or twice per week at most. A 72-hour fast adds peak autophagy and cellular renewal. Each level adds biological depth but requires more careful preparation.
Is fasting safe for women?
Most historical fasting research was conducted on men, and individual responses vary. The core fasting physiology above (fat oxidation, ketosis, hormonal shifts) applies to all adults. However, women who are pregnant, breastfeeding, trying to conceive, or who have a history of eating disorders should not fast without medical supervision. Some women report that aggressive repeated extended fasting affects their menstrual cycle — if that happens, reduce fasting intensity or work with a licensed healthcare provider. This site does not offer cycle-specific fasting protocols because the primary research supporting specific day-of-cycle prescriptions is limited.
What are electrolytes and why do they matter during fasting?
Electrolytes are minerals — primarily sodium, potassium, and magnesium — that regulate critical body functions. When insulin drops during a fast, your kidneys excrete more sodium, and potassium and magnesium follow. This is why fasting headaches, muscle cramps, fatigue, and brain fog are almost always electrolyte depletion rather than hunger. A pinch of sea salt in water addresses sodium immediately. Most fasting discomfort that makes people quit is electrolyte-related and entirely preventable.
What is HGH and why does it increase during fasting?
Human Growth Hormone is produced by the pituitary gland. It plays a key role in muscle preservation, fat metabolism, and cellular repair. Insulin suppresses growth hormone release, so when insulin drops during a fast, GH secretion rises through increased pulse frequency and amplitude. Peer-reviewed research (Vinales 2022, Hartman 1992) documented roughly 5-fold increases during 24–48 hour fasts. This is one reason extended fasting can be more effective for body recomposition than simple caloric restriction: the hormonal environment actively preserves muscle while accelerating fat oxidation.
Can I exercise while fasting?
Yes. Light to moderate exercise — walking, Zone 2 cardio, yoga — pairs well with any phase of fasting. High-intensity exercise during the early phases is harder but manageable. During deep ketosis, energy typically stabilizes and exercise performance normalizes. Strength training during a fast can be effective for body recomposition due to the elevated hormonal environment. Avoid heavy lifting during the metabolic valley at hours 13–17. Electrolytes are especially important during fasted exercise.
Sources & research
Primary peer-reviewed research behind the phase timings
FastingPhases is a neutral educational reference. All phase timings and physiological claims above trace to the peer-reviewed studies below. Where the science is uncertain (notably autophagy timing in humans), the chart marks those phases with hatched bars and "emerging evidence" labels rather than false precision.
- Vasim I, Majeed CN, DeBoer MD. "Critical Assessment of Fasting to Promote Metabolic Health and Longevity." Endocrine Reviews, 2025;46(6):856. academic.oup.com
- Ho KY, Veldhuis JD, Johnson ML, et al. "Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man." Journal of Clinical Investigation, 1988;81(4):968–975. jci.org
- Vinales K, et al. "Effects of Short-term Fasting on Ghrelin/GH/IGF-1 Axis in Healthy Humans." 2022. 24-hour fasting induced ~5-fold GH increase (p < 0.001). PMC9387714
- Hartman ML, Veldhuis JD, Johnson ML, et al. "Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men." Journal of Clinical Endocrinology & Metabolism, 1992;74(4):757–765.
- Cahill GF Jr. "Fuel metabolism in starvation." Annual Review of Nutrition, 2006;26:1–22.
- Bagherniya M, et al. "The Beneficial and Adverse Effects of Autophagic Response to Caloric Restriction and Fasting." PMC, 2023. PMC10509423
- Mizushima N, Komatsu M. "Autophagy: renovation of cells and tissues." Cell, 2011;147(4):728–741.
- Cheng CW, Adams GB, Perin L, et al. "Prolonged fasting reduces IGF-1/PKA to promote hematopoietic-stem-cell-based regeneration and reverse immunosuppression." Cell Stem Cell, 2014;14(6):810–823.
- Longo VD, Mattson MP. "Fasting: molecular mechanisms and clinical applications." Cell Metabolism, 2014;19(2):181–192.
- Klein S, Sakurai Y, Romijn JA, Carroll RM. "Progressive alterations in lipid and glucose metabolism during short-term fasting in young adult men." Metabolism, 1993.
Framework references: Jason Fung MD, The Complete Guide to Fasting; Thomas DeLauer, Intermittent Fasting Made Easy; Dominic D'Agostino PhD, KetoNutrition. These authors popularize fasting science for general audiences. Primary research citations above take precedence on any factual claim.
FastingPhases is not affiliated with any of the researchers or authors cited. All information is for educational purposes only and is not medical advice. Consult a licensed healthcare provider before any extended fast, especially if you are pregnant, breastfeeding, taking medication, or have a history of eating disorders or metabolic conditions.
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