How to do a 36 hour fast
Updated
A 36 hour fast, sometimes called a monk fast, is a day and a half without eating. If you finish dinner at 7 pm on Monday, you're done at 7 am on Wednesday, with the whole of Tuesday in between. If you have a medical condition or take medication, check with your doctor first.
Who should talk to a doctor first
Harvard Health's advice on time-restricted eating, a daily eating window rather than a 36 hour fast, lists notable exclusions: people with diabetes, people with eating disorders, pregnant or breastfeeding people, and people who need to take medication with food. Fasting Timer's own disclaimer also asks anyone under 18 to talk to their doctor before fasting.
If any of that is you, or you aren't sure, ask first. If you've never gone a full day without eating, the 24 hour fast guide covers the shorter version, and the 48 hour fast and 72 hour fast guides cover longer ones. For fasts that repeat every week, see the intermittent fasting schedule guide.
The clock plan
Most of the planning is arithmetic. Thirty-six hours is one day plus twelve hours, so a fast that starts in the evening ends the morning of the day after tomorrow. This is the dinner-to-breakfast version:
| Hour of the fast | Clock time | Where you are |
|---|---|---|
| 0 | Monday, 7 pm | Last meal finished |
| 4 | Monday, 11 pm | Bed |
| 12 | Tuesday, 7 am | Awake, first morning |
| 24 | Tuesday, 7 pm | Where a 24 hour fast would end |
| 28 | Tuesday, 11 pm | Bed again |
| 36 | Wednesday, 7 am | First meal |
The times are an example. Shift the whole column to match your own dinner: a 9 pm last meal ends at 9 am Wednesday. Write the end time down before you start, so you aren't doing the sum while you're hungry.
Sleeping through it
In this schedule, two nights fall inside the fast. With bed at 11 pm and a 7 am wake-up, hours 4 to 12 and hours 28 to 36 happen in bed. The one full day awake in the middle is Tuesday, from 7 am to 11 pm. The fast ends when you wake up on Wednesday, and the first meal is breakfast.
What the research says about hours 12 to 36
A review in Obesity (Anton et al., 2018) describes a "metabolic switch" in which the body moves from using glucose to using fat and ketones as fuel. The authors say it typically occurs between 12 and 36 hours after you stop eating. The exact point depends on the glycogen in your liver when you start and on how much energy you burn during the fast.
The same review notes that individuals in some previous trials reported high levels of hunger and potential discomfort on fasting days, and that safety remains a potential concern. It is a review of fasting research in general, not a test of a 36 hour fast on a particular group, so it can't tell you how a 36 hour fast will feel for you.
Water, coffee and blood tests
None of the sources here say what to drink during a 36 hour fast, so there is no sourced rule to give you. The one drink guidance with a source is for blood tests, which is a separate case: MedlinePlus says you usually fast for 8 to 12 hours before a blood test, with nothing but plain water, and your provider confirms the exact time.
For what two registered fasting studies allowed during fasting hours, see can you drink coffee while fasting. Those are study protocols, not health advice.
The first meal
What you can settle in advance is the time of the first meal. With the plan above, that's Wednesday breakfast at 7 am. If you have a condition or take medication, ask your doctor how to end a fast this long.
Doing a 36 hour fast with Fasting Timer
Fasting Timer runs on iPhone. The timer and the daily plans, from 12:12 to OMAD, are free. The 36 hour plan is one of the Extended fasts, which are part of Premium.
- Open the Timer tab and tap the plan chip (it shows the current plan, such as "16:8 · Most popular").
- In Choose a plan, find Extended fasts and tap 36h fast ("Skip a whole day"). Without Premium, it opens the upgrade screen instead.
- Tap Start fasting. The ring fills as time passes, and the Started and Goal boxes show when you began and when you'll finish. The Goal box is your end time from the table above.
- Started late? Tap the Started box to open Edit start time and tap Save.
- Notifications tell you when the fast is complete and when there's one hour to go.
- At the end, tap Complete fast. If you stop early, End fast asks "End your fast?" and offers End and save or Discard this fast.
The Current stage card on the Timer tab names a stage for each point of the fast. Between hour 24 and hour 48 it reads "Deep autophagy", and at hour 48 it changes to "Extended fast", which the app says should only be done with guidance from a doctor. Treat the stages as rough. The app says its timings are approximate and general information, and the source cited here supports only the 12 to 36 hour range above.
The free version shows the fasts from your last 7 days and your current streak. There are also Home Screen and Lock Screen widgets, and a Water today card on the Timer tab to log water. Fasting Timer doesn't count calories, give meal plans or sync across devices; your data stays on your iPhone.
Fasting stages are general information, not medical advice. Talk to your doctor before fasting, especially if you are pregnant, have diabetes or another medical condition.
FAQ
- How do you start a 36 hour fast?
- Pick the time of your last meal, add 36 hours, and write down the result before you begin. A last meal at 7 pm on Monday ends on Wednesday at 7 am. Ask a doctor first if you have a medical condition or take medication.
- How long is a 36 hour fast in days?
- A day and a half: 24 hours plus 12. Starting after dinner at 9 pm, you finish at 9 am two mornings later, which makes it a fast across two nights.
- When does your body switch to burning fat?
- A 2018 review in Obesity says the metabolic switch typically occurs between 12 and 36 hours after you stop eating. The exact point depends on the glycogen in your liver when you start and on how much energy you burn, so 36 hours is the far end of a wide range, not a guarantee.
- Is a 36 hour fast safe?
- None of the sources here tested the safety of a 36 hour fast: the Anton et al. review in Obesity says safety remains a potential concern, and Harvard Health, writing about time-restricted eating, excludes people with diabetes, people with eating disorders, pregnant or breastfeeding people, and people who need to take medication with food. Talk to your doctor before you try it.
- Can you do a 36 hour fast in Fasting Timer?
- Yes. The 36h fast plan ("Skip a whole day") is listed under Extended fasts in the plan picker, and Extended fasts are part of Premium. The Goal box shows your end time once the fast starts.
- Can you fast for 36 hours before a blood test?
- Only if your provider tells you to. MedlinePlus says you usually fast for 8 to 12 hours before a blood test, with nothing but plain water, and your provider confirms the exact time.