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The rhythm — upcoming fasts
Behind the curves — primary sources
Insulin falls steeply within hours of fasting onset. Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1–22. DOI
Glucose declines and stabilizes in the low-normal range through gluconeogenesis. Cahill 2006 (above); Klein S et al. Progressive alterations in lipid and glucose metabolism during short-term fasting in young adult men. Am J Physiol. 1993;265(5):E801–6. DOI
Ketones (BHB) cross into nutritional ketosis (~0.5 mmol/L) around 16–24h and deepen over days. The brain draws an increasing share of its fuel from ketones across days, not hours — at 3.5 days they still supply only ~25–35% of brain energy; ketone dominance is a multi-day-to-weeks shift. Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1–22. DOI; Hasselbalch SG et al. Brain metabolism during short-term starvation in humans. J Cereb Blood Flow Metab. 1994;14(1):125–31. PubMed
Growth hormone pulse amplitude rises substantially with prolonged fasting. Ho KY et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988;81(4):968–75. DOI
AutophagyMouse-derived · est. The hour-by-hour curve is extrapolated from animal studies — the "ramps at 16h, peaks 24–48h" shape comes from mice (Alirezaei 2010). Human fasting studies measure only indirect surrogate markers at scattered single time-points and disagree (some show muscle markers falling at 36h); a validated method to measure actual autophagic flux in humans appeared only in 2021, and direct human flux data during prolonged fasting is only now emerging (small early studies, including a 2024 three-day water-fast trial, disagree) — no settled hour-by-hour human curve yet exists. Treat hour targets as a rough animal-derived guide, not a measured human fact. Alirezaei M et al. Short-term fasting induces profound neuronal autophagy. Autophagy. 2010;6(6):702–10. DOI
Fat oxidation rises progressively as glycogen depletes — liver glycogen is largely (not fully) depleted by ~24h, with fat the dominant fuel by ~24–48h, not a sharp switch at 16h. Rothman DL et al. Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR. Science. 1991;254(5031):573–6. PubMed; Klein 1993 (above).
Stem cells & immune renewalMouse-derived The 48–72h "immune regeneration / stem-cell activation" claim rests mainly on mice; the only human data was a small, confounded group of cancer patients fasting alongside chemotherapy (lymphocyte counts, not stem cells). Promising but unproven in healthy humans. Cheng CW et al. Prolonged fasting reduces IGF-1/PKA to promote hematopoietic-stem-cell-based regeneration and reverse immunosuppression. Cell Stem Cell. 2014;14(6):810–23. DOI
Cortisol secretory burst mass rises with prolonged fasting (HPA-axis activation) — flat through ~24h, climbing thereafter. Bergendahl M et al. Fasting as a metabolic stress paradigm selectively amplifies cortisol secretory burst mass… J Clin Endocrinol Metab. 1996;81(2):692–9. PubMed
TestosteroneDisputed · declines Contrary to popular claims, short-term fasting in lean men tends to lower testosterone via reduced LH drive — not raise it. Evidence is mixed and body-composition-dependent. Klibanski A et al. The effects of fasting on LH, FSH, testosterone and prolactin in normal men. J Clin Endocrinol Metab. 1981. PubMed