Methodology
Every phase boundary on FastingPhases traces to a peer-reviewed source. Here's the data behind each one — and where we mark uncertainty rather than claiming false precision.
Phase boundary reference
The calculator and chart on the homepage map fasting hours to one of seven sequential metabolic phases. The boundary times below are derived from the published research listed in the Sources section, with tolerance ranges reflecting the natural variation reported across studies and individuals.
| Phase | Hours | Primary citation | Confidence |
|---|---|---|---|
| Fed state (digesting) | 0–4 | Cahill 2006[5] | Well-established |
| Early fast (blood sugar stabilizing) | 4–12 | Klein et al. 1993[10] | Well-established |
| Glycogen depletion / fat-burning onset | 12–18 | Longo & Mattson 2014[9] | Well-established |
| Ketosis ramp | 18–36 | Vasim et al. 2025[1] | Strong evidence |
| Growth hormone elevation peak | ~24h spike | Ho et al. 1988[2]; Vinales et al. 2022[3] | Well-established |
| Autophagy ramp (cellular cleanup) | 24–72 (uncertain) | Bagherniya et al. 2023[6]; Mizushima & Komatsu 2011[7] | ⚠ Emerging — see note |
| Stem cell / immune system effects | 72+ | Cheng et al. 2014[8] | ⚠ Emerging — see note |
Where we flag uncertainty
Two phase boundaries on the chart are marked with hatched bars and "emerging evidence" labels rather than solid timing claims:
Autophagy onset in humans
Most popular fasting content cites a precise autophagy onset around 16-24 hours of fasting. The actual research is much messier. Most autophagy timing studies have been conducted in cell culture or in rodents, not in fasting humans. The few human studies that exist (notably the work cited in Bagherniya et al. 2023) suggest autophagy markers begin rising during prolonged fasting but provide wide ranges rather than precise hour markers. We show autophagy as a hatched ramp from ~24 hours rather than a hard onset.
Stem cell regeneration and immune "reset"
The widely-shared "72-hour immune reset" claim derives largely from a single 2014 mouse study by Cheng et al. that showed prolonged fasting triggered hematopoietic stem cell regeneration in mice and a small group of cancer-chemotherapy patients. The cellular mechanisms identified are real, but extrapolating "immune reset" timing from that work to general human fasting outcomes is speculative. We present this phase as emerging evidence, not established physiology.
Calculator logic
How the hours-to-phase mapping works
The calculator takes an input of fasting hours (0 to ~120) and returns the matching phase per the boundaries above. Each phase has:
- A descriptive name (e.g., "Glycogen Depletion")
- A short physiology summary in plain English
- An "expected feeling" note (e.g., "energy may dip" or "mental clarity often peaks")
- A confidence flag (well-established, strong evidence, or emerging)
What the calculator does NOT do
- It does not customize for body composition, sex, age, prior fasting experience, or medications. Phase timings vary by individual; the chart shows population-level ranges.
- It does not adjust for refeeding history. Someone fasting for the first time will hit ketosis later than someone who is metabolically adapted.
- It does not predict outcomes. Phase entry is not the same as benefit realization.
- It does not provide medical advice or individual recommendations.
Safety content
The "Should I stop or push through?" section on the homepage uses a green / yellow / red signal model. The triggers in each band are derived from clinical fasting safety guidance and the consensus warnings in the published research:
- Green (continue): Reached your goal hours; clear, stable, and calm.
- Yellow (try a remedy first): Mild headache (try electrolytes); hunger around hours 13–16 (often passes in 1–2 hours).
- Red (break the fast): Dizziness or weakness; heart palpitations or confusion.
- Emergency: Severe chest pain, fainting, or persistent palpitations — call emergency services.
What the chart does NOT promise
Update cadence
- Phase boundaries: Reviewed annually for new published research; meaningful changes update the chart and these tables
- Sources list: New peer-reviewed research is added when it changes a phase boundary or strengthens an emerging-evidence flag to strong evidence
- Safety guidance: Reviewed when new clinical fasting consensus statements are published
Sources
- 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.
This methodology page was last reviewed in April 2026. For background on the editorial team and funding model, see the About page.