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Not medical advice. The Atlas is not a substitute for a clinician who can examine you.

36-Hour Fast

Monk Fast · Day-Spanning Fast

Evidence grade 2 of 5, Thin. Read how the Atlas grades evidence.

2 human trials, about 14 studies, ~300 participants across dedicated 36h-range studies. The written rationale for this grade is still being prepared.

FDA
Not a regulated intervention
WADA
Not prohibited

Reference only. Not medical advice.

What can go wrong

A 36-hour fast carries real risks that scale with the person and their medications. People taking insulin or other glucose-lowering drugs can experience dangerous hypoglycemia and must not attempt this without clinician supervision and medication adjustment. Dehydration and electrolyte shifts can cause fatigue, dizziness, headache, and, on refeeding after longer or repeated fasts, the metabolic disturbances of refeeding syndrome in vulnerable people. Fasting raises serum uric acid and the associated dehydration can trigger gout flares, so those with gout should be cautious. This protocol is not appropriate during pregnancy or breastfeeding, for children, for people who are underweight, or for anyone with a history of disordered eating, where it can reactivate harmful patterns. If you have any history of an eating disorder, or are using fasting to control weight in a way that feels distressing or compulsive, this is not a do-it-yourself protocol: speak with a qualified healthcare provider or a clinician experienced in eating disorders before fasting. Anyone with diabetes, kidney disease, gout, or who takes regular medication should consult a healthcare provider first.

A 36-hour fast spans a full calendar day, dinner on day one to breakfast on day three, with only water and other calorie-free fluids in between. It sits between the 24-hour and 48-hour protocols, deep enough to drive sustained ketosis yet short enough to schedule around work. Most of the rigorous human data on the exact 36-hour unit is small and mechanistic: a single 36-hour fast measurably remodels circulating lipids and shifts appetite hormones in healthy adults. The clinical results often quoted for '36-hour fasting' are largely borrowed from alternate-day fasting, whose cycles run about 36 hours, and from longer therapeutic fasts. Treated as its own protocol, the 36-hour fast is best described as a popular, plausibly useful metabolic reset whose dedicated evidence is still preliminary.

Skip a full day of eating. Dinner to breakfast two days later, deep enough to matter.

What people take it for

These are the reasons people give for taking it. They are not effects it has been shown to have.

  • Periodic deeper metabolic reset for generally healthy adults seeking a step beyond 24 hours
  • Adjunct, under medical supervision, in structured programs for type 2 diabetes and insulin resistance, where related fasting schedules have shown signal
  • Sustained nutritional ketosis within a single fast for those exploring fat-adapted metabolism
  • A scheduled once-weekly practice (the Monk Fast pattern) used for routine and perceived focus benefits

What people get wrong

  • Often saidA 36-hour fast is just a longer 24-hour fast

    ActuallyThe extra twelve hours extend ketosis and span a full circadian day, and the limited dedicated data, such as a single-fast lipidome study, show measurable metabolic shifts at 36 hours. But the difference is one of degree, and the clinical payoff of 36 over 24 hours has not been tested head to head in an outcome trial.

  • Often said36 hours is the proven autophagy window

    ActuallyAutophagy is the most cited reason people pick 36 hours, yet human evidence for when autophagy peaks is thin and indirect. Most timing claims come from animal work; direct human measurement during fasting is limited, so a specific 36-hour autophagy peak is a hypothesis, not an established human finding.

  • Often saidThe diabetes-reversal results prove the 36-hour fast works

    ActuallyThe widely shared case of patients stopping insulin comes from a 3-person BMJ series that used 24-hour and three-times-weekly fasts, not a 36-hour protocol. It is a striking signal for therapeutic fasting in general, but it does not specifically validate 36 hours, and three patients cannot establish efficacy.

  • Often saidScience has shown 36-hour fasting improves longevity markers in humans

    ActuallyThe longevity and cardiovascular numbers usually attached to this come from alternate-day fasting trials such as Stekovic 2019, whose cycles run about 36 hours but which is a different protocol with a small 4-week randomized arm. Borrowing those results for a standalone 36-hour fast overstates what the dedicated evidence supports.

Who profits, and who pushes back

  • Diabetes-medication clinicians

    Pushes backEndocrinology / safety

    A 36-hour fast can sharply lower blood glucose. For anyone on insulin or sulfonylureas, medication must be adjusted under supervision, because the hypoglycemia risk is real and can be serious.

  • Eating-disorder and refeeding specialists

    Pushes backClinical safety

    Long fasts can reactivate disordered eating and, in vulnerable people, set up refeeding complications. This is not a self-directed protocol for anyone with that history; a provider should be involved.

  • Fasting-physiology researchers

    Pushes backEvidence quality

    The dedicated human studies at 36 hours are small and measure mechanisms, not outcomes. The strongest clinical numbers people quote come from adjacent protocols, so the 36-hour fast should be described as promising rather than proven.

  • Dr. Jason Fung

    Promotes itNephrologist, IDM

    Uses 36-hour fasts clinically for type 2 diabetes in a structured program. Primary revenue clinical.

  • Extended-fasting community

    Promotes itMixed

    "Monk Fast" framing popular for periodic 36-hour resets.

Where the research comes from

Share of published research by country of origin.
PlaceShare of published research
USA46%
Europe30%
Other24%

Research published from: Global.

How we got here

Full-day-plus fasting is old and global, and its non-Western and religious lineages come first. Theravada Buddhist monastics following the Vinaya have for centuries taken no solid food after midday, producing a long daily fast as a discipline of alertness. Jain traditions include some of the most demanding fasts in the world, with multi-day water-restricted observances. Islamic Ramadan, itself emerging within older Near Eastern fasting customs, structures a recurring dawn-to-dusk fast for nearly a month. Eastern Orthodox Christians keep rigorous fasting calendars covering much of the year. Medical interest in supervised prolonged fasting was formalized in Germany when Otto Buchinger, after his own 19-day fast in 1919, opened a therapeutic fasting clinic in 1920 and described the method in his 1935 book, founding a clinical tradition his family continued for four generations. The specific 36-hour framing is recent and consumer-driven: the online community WeFast popularized the 'Monk Fast' as a once-weekly 36-hour protocol, borrowing monastic imagery rather than any single monastic rule. In parallel, clinicians including Jason Fung used structured longer fasts for type 2 diabetes, and researchers such as Stekovic studied alternate-day fasting whose ~36-hour cycle gave the duration a scientific reference point. The 36-hour fast as a named, deliberately chosen protocol is therefore a modern synthesis of ancient practice, twentieth-century clinical fasting, and twenty-first-century online fasting culture.

  1. pre-1900sTheravada Buddhist, Jain, Islamic, and Eastern Orthodox traditions sustain long full-day and multi-day fasts as spiritual discipline
  2. 1920Otto Buchinger opens his therapeutic fasting clinic in Germany, founding a supervised clinical-fasting tradition
  3. 1935Buchinger publishes a systematic description of fasting physiology in The Therapeutic Fasting Cure
  4. 2018Furmli, Ramos and Fung publish a 3-patient BMJ Case Reports series on fasting and insulin cessation, using 24-hour and three-times-weekly schedules
  5. 2019Stekovic et al. publish an alternate-day fasting trial in Cell Metabolism whose fasting arm runs ~36 hours per cycle, often miscited as a 36-hour-fast study
  6. 2023A controlled study (n=20) shows a single 36-hour water-only fast significantly alters 245 of 832 plasma lipid species; a separate BYU randomized crossover (n=20) maps appetite hormones and ketosis timing over a 36-hour fast
  7. 2024Reports that UK Prime Minister Rishi Sunak practices a weekly 36-hour Monk Fast bring the protocol broad mainstream attention

At a glance

Duration
36 hours
Pattern
Skip one full day
Autophagy
Moderate-high
Use
Therapeutic (T2D), reset

What this entry does not yet cover

What follows is unfinished work on our side, not a finding about 36-Hour Fast. A short entry is not a safe entry.

  • Cost and market. We have not documented what it sells for or who sells it.
  • Supply chain. We have not mapped where it is made.
  • Who uses it. We have not documented the user groups.
  • What could change. No forward-looking summary yet.
  • How it compares. We have not set it against the alternatives.
  • Children and teenagers. We have not checked what is known about younger people.

Sources