What studies measured in healthy adults, not a reading of your own body. Move the hour to see the app's Stages tab at that point in a fast; below it, every stage, day by day, opens with how it may feel, what to do and its papers. Stages to 36 hours are free; the stages from 72 hours belong to the supervised plans, though any fast left running past 72 hours shows Protein-sparing adaptation, and from 96 hours Whole-body shift, until you end it or the 120‑hour ceiling does.
Glycogen largely spent, 24–36 h.
Stop card. On any fast over 24 hours, a "Not feeling right?" link sits under the ring and on the Lock Screen.
Your last meal is still being absorbed. Insulin is high, and the body is storing energy rather than drawing on it.
Sources: Deru 2021 · Cahill 2006
Insulin has fallen to its baseline. The liver is releasing stored sugar, and already making new glucose from other sources.
Sources: Rothman 1991 · Deru 2021
Fat is being released from stores in larger amounts. Ketone levels are usually still low at this point.
Sources: Deru 2021 · Service and O'Brien 2005 · Ho 1988
In studies of healthy adults, blood ketones usually pass the nutritional ketosis threshold around 18 to 24 hours. Timing varies from person to person.
Sources: Deru 2021 · Service and O'Brien 2005
24 hOn any fast over 24 hours, a "Not feeling right?" link sits under the ring and on the Lock Screen. It opens the stop card, with your region's emergency number one tap away. What the stop card says
Most of the liver's stored sugar has been used. The body now makes almost all its glucose fresh, mainly from fat-derived glycerol and amino acids.
Sources: Rothman 1991 · Nilsson and Hultman 1973 · Owen 1998
Ketones are now a major fuel. Blood sugar settles in the low-normal range rather than falling further.
Sources: Service and O'Brien 2005 · Zauner 2000 · Jordan 2019
Nesteia Pro from here. Free covers every daily window, and plans and stages to 36 hours.
Ketones are high, stress hormones like noradrenaline rise, and resting metabolism is slightly higher, not lower. Growth hormone pulses are larger.
In Nesteia. The last stage of any unsupervised plan.
Sources: Service and O'Brien 2005 · Zauner 2000 · Ho 1988
Plans over 72 hours only, with a clinician's supervision
The body shifts to protect muscle: ketones reduce protein breakdown. Much of the early lean-mass drop is water, largely regained after eating.
In Nesteia. Reached at 72 hours, on any fast that passes it. The 96 and 120‑hour plans run only for people fasting under a clinician, confirmed before every such fast.
Sources: Owen 1998 · Felig 1969 · Pietzner 2024
Plans over 72 hours only, with a clinician's supervision
Beyond about three days, measurable changes spread across many organ systems. This is also when electrolyte losses and blood-pressure drops need attention.
In Nesteia. Reached at 96 hours, on the supervised 96 and 120‑hour plans. The studies run to day 7; in Nesteia a running fast ends itself at the 120‑hour ceiling.
Sources: Pietzner 2024 · Wilhelmi de Toledo 2019 · Ogłodek 2021
120 hThe ceiling: 120 hours on every timer, plan, edit, import, link and widget. A running fast ends itself there.
Fasts this long are studied only under medical supervision, with electrolyte monitoring and slow refeeding. Ketones plateau; blood pressure and uric acid change noticeably.
In Nesteia. Reference only. No plan in Nesteia reaches it; a running fast ends itself at 120 hours.
Sources: Wilhelmi de Toledo 2019 · Owen 1969 · NICE CG32
Refeed gradually. After longer fasts, electrolytes shift when eating resumes, and about half the initial drop returns as water and glycogen.
In Nesteia. A plan's refeed days: one after a 24 or 36‑hour fast, two after 48 or 72 hours, three after 96 hours and four after 120 hours. A fast ended early gets the days its length supports.
Sources: ASPEN 2020 · NICE CG32 · Pietzner 2024
In the app these sit under the stages, stacked in full. Each is an explainer card, never a stage, and each has a page of its own with its papers.
Fasting increases autophagy in mice after roughly 24 to 48 hours. In people, autophagy cannot yet be measured non-invasively, human muscle and blood-cell studies give mixed results, and no study has shown a specific hour at which it switches on.
Sources: Mizushima 2004 · Chaudhary 2022
The hunger hormone ghrelin keeps its usual mealtime rhythm during a fast and does not climb steadily. Hunger tends to peak in the first day or two and then ease.
Sources: Espelund 2005 · Natalucci 2005
Sleep disturbance is the most common mild complaint early in a multi-day fast. Later, people in supervised fasting studies often report better sleep quality and mood; controlled evidence is limited.
Sources: Michalsen 2003 · Wilhelmi de Toledo 2019
A day-long fast lowers circulating inflammatory monocytes in healthy volunteers. Short intermittent-fasting regimens have not reduced CRP in meta-analyses.
Sources: Jordan 2019 · BMJ 2025 network meta-analysis
Fasting extends lifespan in laboratory animals. There is no evidence that it does so in humans. Across 99 trials, intermittent fasting forms are roughly equal to continuous daily restriction for body size, and observational data in older adults show higher mortality with very short eating windows.
Sources: Cochrane 2021 · BMJ 2025 network meta-analysis · Mao 2025
All sources, searchable, with the stages that cite each paper.
Sources: 24, primary studies and reviews plus two clinical guidelines (ASPEN 2020, NICE CG32), linked on each stage above, listed under Sources, and cited on each stage and explainer card in the app. Reading all eleven stages and the Sources screen earns the Outcrop badge (the Explorer family) on the Strata page. How the stages are written: the confidence words, how papers are chosen, who writes them and the evidence edition.
Free: every daily window, and plans and stages to 36 h. Pro from £4.99 a month at launch.