A 36-hour fast usually means finishing dinner one evening, eating nothing the next day, and breaking the fast with breakfast the morning after. This page covers a sample schedule, an approximate hour-by-hour timeline of what many people notice, how to handle fluids and electrolytes, how to break the fast, and how often people typically repeat it. It is an intermediate-length fast, generally attempted only after shorter fasts (such as 16:8 or an occasional 24-hour fast) feel comfortable.
Safety first: a 36-hour fast is not appropriate for everyone. Do not attempt it if you are pregnant or breastfeeding, under 18, underweight, have a history of an eating disorder, or take insulin, sulfonylureas, or blood-pressure medication, unless your healthcare provider has approved and is supervising it. See fasting safety and fasting and medications.
Table of Contents
What a 36-Hour Fast Is
The most common structure is dinner to breakfast, skipping one full day of meals:
- Finish dinner on day 1 at 7 PM
- Eat nothing on day 2 (water, plain coffee or tea, and electrolytes only)
- Break the fast on day 3 at 7 AM with breakfast
- Total fasting time: 36 hours
Other ways to fit in 36 hours:
- Breakfast to dinner: finish breakfast at 8 AM on day 1, break the fast at 8 PM on day 2
- Lunch to late evening: finish lunch at 1 PM, break the fast at 1 AM two days later (rarely practical)
Dinner to breakfast is the most popular because two nights of sleep fall inside the fast, so only one full waking day is spent without food.
Hour-by-Hour Timeline (Approximate)
The timeline below describes what is generally understood to happen and what people commonly report. Timings are rough: they vary with your last meal, body size, activity level, and how used to fasting you are. Human research does not support precise hour markers for fat burning, ketosis, or autophagy, so treat these as broad phases rather than checkpoints.
- Hours 0-4 (evening of day 1): You are still digesting and absorbing your last meal. Blood glucose and insulin rise after eating and then gradually come back down.
- Hours 4-12 (overnight): The body increasingly relies on stored liver glycogen to keep blood glucose steady, and fat use starts to rise. Most of this phase happens while you sleep.
- Hours 12-18 (morning of day 2): Liver glycogen is being drawn down and the share of energy coming from fat keeps rising. Hunger often shows up around your usual breakfast or lunch time, then tends to fade after an hour or so.
- Hours 18-24 (afternoon and evening of day 2): Many people enter mild ketosis somewhere in this window, though some reach it later. This is often the hardest stretch: usual dinner time can bring strong hunger, low energy, irritability, or a headache (often linked to low sodium or caffeine changes).
- Hours 24-30 (night of day 2): Ketone levels typically continue to rise gradually. Sleep may be lighter or more broken than usual. Magnesium and adequate fluids earlier in the day help some people.
- Hours 30-36 (early morning of day 3): Many people report that hunger is lower than it was the previous evening. Some feel clear-headed; others feel cold, lightheaded on standing, or tired. You then break the fast with breakfast.
About autophagy: autophagy (the body’s process for recycling cell components) is increased by fasting in animal studies, but there is no reliable way to measure it in people at home, and there is no established hour at which it “switches on” or peaks in humans. Be cautious of charts that promise specific autophagy milestones. More detail is in our autophagy and fasting article.
When to stop early: end the fast if you feel faint, confused, have heart palpitations, severe dizziness, or vomiting, or if you simply feel unwell. Stopping early is not a failure.
Who It May Suit
- Healthy adults who already fast comfortably for 16-24 hours
- People who prefer an occasional longer fast to a daily eating window
- People using it as one tool to create a calorie deficit, alongside a balanced diet the rest of the week
It is not a good fit if longer fasts trigger binge eating, if you do hard physical work or intense training on the fasting day, or if any of the safety exclusions above apply.
When to Schedule It
- Choose a day with a predictable workload and no meals with colleagues, friends, or family
- Avoid days with hard training sessions or physically demanding work
- Avoid days when you need to drive long distances or operate machinery while tired
- Many people find a normal workday easier than a free day, because being busy takes attention off food
- A common choice is dinner on Sunday, fasting on Monday, and breakfast on Tuesday
The Protocol
The day before
- Eat normally; there is no need to overeat in preparation
- Make dinner a balanced meal with protein, vegetables, and some carbohydrate or fat
- Drink water through the evening
During the fast
- Drink water regularly; let thirst and urine color guide you (pale yellow is a reasonable sign)
- Plain black coffee, plain tea, and sparkling water are generally accepted
- Consider electrolytes (see the next section)
- Avoid anything with calories
- Light movement such as walking is fine; skip heavy training
Ending the fast
- Plan your first meal in advance so you are not choosing food while very hungry
- Break the fast with a moderate breakfast, then return to normal meals
Fluids and Electrolytes
Headaches, lightheadedness, and fatigue during a day without food are often related to fluid and sodium losses. Common approaches for a 36-hour fast include:
- Sodium: a small amount of salt in water once or twice during the fasting day (for example, a pinch to a quarter teaspoon in a glass of water), or a cup of salted broth if you accept a few calories
- Potassium: optional; supplements should be used cautiously and avoided without medical advice if you have kidney disease or take medications that raise potassium
- Magnesium: some people take a standard supplement dose in the evening; it can loosen stools
If you have high blood pressure, heart or kidney disease, or follow a sodium-restricted diet, ask your healthcare provider before adding salt or supplements. More detail: electrolytes and fasting.
Breaking the Fast
A 36-hour fast does not usually need a special refeeding plan for healthy adults, but a gentle first meal is more comfortable:
- Start with a moderate-sized meal rather than a very large one
- Include protein and some vegetables; eggs with avocado and toast, or yogurt with fruit and nuts, are common choices
- Eat slowly and stop when comfortably full
- Return to your normal eating pattern for the rest of the day
Very large meals, lots of sugary food, or heavy, greasy food right away can cause bloating or stomach discomfort. See how to break a fast and refeeding meal ideas.
How Often Can You Do a 36-Hour Fast?
There is no research-based ideal frequency. Common patterns among healthy adults are:
- Occasionally: once a month or less, as a change from a daily fasting pattern
- Regularly: up to once a week for people who tolerate it well and eat enough the rest of the week
- More than once a week: this becomes a form of alternate-day fasting, which is harder to sustain and needs closer attention to nutrition
Signs to fast less often or take a break include ongoing fatigue, poor sleep, feeling cold much of the time, missed periods, hair shedding, preoccupation with food, or overeating after fasts. See when to take a break from fasting.
Frequently Asked Questions
Is a 36-hour fast better than a 24-hour fast?
Not necessarily. A 36-hour fast is longer, so ketone levels are usually higher by the end and the calorie deficit is larger, but there is little human evidence that the extra 12 hours brings meaningful health benefits beyond that. Many people get similar results from 24-hour fasts, which are easier to fit into life.
Will I lose muscle on a 36-hour fast?
A single 36-hour fast is unlikely to have a noticeable effect on muscle in a healthy adult, especially if you eat enough protein and keep up resistance training on your eating days. Frequent long fasts combined with low protein intake are more of a concern. See fasting and muscle.
How much weight will I lose?
The scale often drops noticeably after a 36-hour fast, but much of that is water, stored glycogen, and food in the digestive tract, and a good part of it usually returns within a day or two of normal eating. Actual fat loss is limited by the calorie deficit of the missed meals. Long-term results depend on your overall eating pattern.
Can I exercise on a 36-hour fast?
Walking, easy cardio, and mobility work are generally fine. Heavy lifting, sprints, and intense intervals are better saved for eating days. Stop if you feel dizzy or weak.
Can I drink coffee during a 36-hour fast?
Plain black coffee is generally accepted. Some people find caffeine on an empty stomach makes them jittery or nauseous later in the fast, so a smaller dose may help. Avoid late-day caffeine if sleep on the fasting night is already light.
Should I do this if I have never done a 24-hour fast?
It is sensible to build up gradually: get comfortable with a daily 16:8 schedule, then try an occasional 24-hour fast, and only then consider 36 hours.
The Bottom Line
A 36-hour fast typically runs from dinner one evening to breakfast two days later. The hour-by-hour changes are gradual and vary from person to person: glycogen use gives way to more fat use and, for many, mild ketosis during the second half. Hunger usually peaks around normal mealtimes rather than climbing steadily. With fluids and electrolytes handled, a gentle first meal, and a sensible frequency, it can be one option for healthy adults who already tolerate shorter fasts. Check with your healthcare provider first if you have a medical condition or take medication.