By ONE Editorial Team · Published: September 6, 2026
After around 18 hours without food, some people may be relying more heavily on fat-derived fuels than earlier in the fast, with ketone production potentially contributing more. Insulin is generally lower than after eating, and glycogen may be lower than at 12 or 16 hours.
Eighteen hours is an approximate reference point rather than a universal biological switch. In ONE’s app, this mark transitions the display into a “Deep Fasting” tracking label — that is a product label, not a recognised clinical fasting stage.
ONE’s 18-hour stage transition is a tracking landmark, not a universal biological threshold.
Two columns for each system: what may be more prominent by this point in a fast, and what NOT to assume from a single hour on a fasting timer.
| System | What may be more prominent | Do NOT assume |
|---|---|---|
| Insulin | Lower than post-meal levels; less variation over the fasting window. | That insulin has reached a special "fasting threshold" at hour 18. |
| Glycogen | Liver stores may be lower than earlier in the fast. | That glycogen is fully depleted by 18 hours. |
| Fat oxidation | May be a larger contributor to energy use than at 16 hours. | That fat burning has reached a peak. |
| Ketones | May be more noticeable in some people. | That every person is in ketosis by hour 18. |
| Autophagy | Ongoing at some level, as at other times. | That autophagy has "just been activated" or has reached a maximum. |
| Fuel mix | Weighted further toward stored fat vs recent food. | That the body has fully switched to fat as an exclusive fuel. |
These are directional shifts on average, not events. Individual timing varies substantially.
Because ONE’s app changes its display at the 18-hour mark, it is easy to read that transition as a biological event. It is not.
In the ONE app
A simplified educational marker in the app’s five-phase timeline. The label helps visualise fasting progress based on elapsed time.
In the science
“Deep Fasting” is not a recognised clinical stage. The 18-hour mark does not correspond to a measured onset of ketosis, autophagy, glycogen depletion, or peak fat oxidation.
Not as a universal biological switch. Eighteen hours is a useful reference point because it is a longer fasting interval, it is commonly discussed as part of an 18:6 schedule, and it is where ONE changes its product phase label — but biology remains continuous and highly individualised.[1]
Timing of any specific shift still depends on the last meal, prior carbohydrate intake, glycogen status, recent activity, body composition, insulin sensitivity, fasting history, and individual physiology. Two people at hour 18 can be in meaningfully different underlying states.
With no caloric intake for 18 hours, insulin is generally lower than after eating. That is a directional statement; there is no validated “special fasting insulin threshold” that a person reaches at hour 18, and insulin does not reach zero.
Insulin is one signal in a broader system that also includes energy intake, activity, other hormones, and individual context. A lower insulin reading does not by itself prove fat loss or determine fasting benefits.
Liver glycogen may be lower than earlier in the fast, but exact stores vary substantially. Recent exercise and prior carbohydrate intake strongly affect the picture, and glycogen depletion is not universally complete at 18 hours.
This page does not publish a “glycogen depletion” clock. A person who trained heavily the evening before does not have the same glycogen picture at hour 18 as someone who ate a large carbohydrate meal a few hours before starting the fast.
Fat oxidation may become more prominent as fasting continues, and by hour 18 the fuel mix has usually shifted further toward stored energy for many people. This does not mean fat burning began at 18 hours — fat oxidation was occurring earlier in the fast, and even outside a fast altogether.[1]
Acute fat oxidation during a fasting window is not the same as long-term body-fat loss. Body-weight change depends on energy balance over time. For the broader picture, see our fasting and calorie deficit guide and intermittent fasting for weight loss.
Possibly, but not universally. Some people may have meaningfully higher ketone levels by around this point; others may not. The timing depends on prior carbohydrate intake, glycogen status, exercise, fasting duration, and individual metabolism.
“Ketosis begins at 18 hours” is a claim that goes further than individual physiology supports. For more depth, see our guide to fasting and ketosis.
Current human evidence does not establish 18 hours as a universal autophagy onset. Autophagy is ongoing at some level throughout the day, and fasting affects nutrient-sensing pathways — but that does not translate cleanly into a specific consumer clock hour.[1]
Popular framings such as “autophagy starts at 18 hours,” “Deep Fasting means autophagy,” “18h is optimal for cellular repair,” and “autophagy reaches a maximum after 18h” go further than the human evidence supports. Most precise timing claims come from animal or cellular studies rather than validated human clock-hour thresholds. For broader physiology context across durations, see the fasting hours reference.
No. ONE’s “Deep Fasting” label does not refer to a measured autophagy level. The name is a tracking label for the 18h+ range of the app’s five-phase timeline, chosen to make fasting progress easier to visualise.
It is not a scientific claim that autophagy is greater, that ketones have reached a target, or that fat oxidation is maximal. Reading the display update as evidence of a biological event goes further than the app is designed to signal or than the underlying evidence supports.
Not universally. Eighteen hours is more restrictive than 16, with a longer food-free interval, a narrower eating window, and possibly greater stored-fuel contribution for some people. But more restrictive is not automatically better — adherence, nutrition opportunity, sleep, exercise, and lifestyle all matter more than a strict hour-by-hour comparison.
A schedule someone follows most days for months tends to deliver more than a stricter one dropped within weeks. For the physiology context at the earlier mark, see what happens after 16 hours of fasting.
An 18-hour fasting window paired with a 6-hour eating window is what “18:6” describes. This page focuses on what may be happening physiologically at that duration; the method itself — how to place the eating window, practical examples, tips — is covered by our 18:6 intermittent fasting guide.
It can help some people if the overall pattern creates a sustainable energy deficit and stays consistent. Eighteen hours by itself does not guarantee weight loss, and it is not clinically established as superior to 16 hours for fat loss.[5]
“More fasting hours means more body-fat loss” is not a reliable rule. Total intake, activity, adherence, and consistency over time do the work. For the broader picture, see our intermittent fasting for weight loss guide and how calorie deficit works with intermittent fasting.
Not automatically. A 24-hour fast is substantially longer than an 18-hour one, with a meaningfully longer food-free interval, potentially more hunger, more nutrition-planning considerations, and greater disruption for some people.
This page does not present 18 → 24 as a progression ladder that every fasting person needs to climb. For a broader read on duration choices, see how long you should fast. For the fuller picture, see our 24-hour fast guide.
Conceptually: liver glycogen availability may be lower, fat contribution may be greater, and ketone production may be higher for some people. As with every stage transition on a fasting timer, the changes are gradual and vary substantially between individuals. Longer does not automatically mean superior.
Training tolerance at hour 18 varies with the session, the person, and recent nutrition. Intensity, hydration, meal timing, prior training volume, and recovery all interact. Some people train comfortably in a longer fasted state; others feel meaningfully better with food closer to the session.
There is no strong evidence that fasted training at hour 18 produces better long-term body-fat loss than fed training when overall diet and volume are matched. This page does not prescribe a training pattern.
ONE’s simplified timeline transitions from Fat Burning (12–18h) to Deep Fasting (18h+) at the 18-hour mark. That transition is a product tracking landmark, not a biological event.
Important
ONE’s “Deep Fasting” phase is a simplified tracking label based on elapsed fasting time. It is not a medical measurement and does not mean ONE has detected ketosis, autophagy, glycogen depletion, or a maximum level of fat oxidation.
For the full five-phase model in context, see the fasting stages guide.
Yes. ONE saves completed fasting sessions longer than 12 hours to your fasting history, so a completed 18-hour fast is eligible to appear there.
Eighteen hours is not one of ONE’s Quick Start options. Quick Start durations are 16h, 19h, 24h, and 36h. An 18-hour fast is tracked with a Custom Timer rather than a preset. A saved fast in the app is a record of elapsed time, not a medical measurement of what happened during those 18 hours.
An 18-hour fast is not a Quick Start preset in ONE, but it can be tracked directly with the Custom Timer. Eligible completed sessions are saved to your fasting history alongside your other data.
Custom Timer & core tracking (Free)
Premium features
What ONE does and does not do
ONE saves completed fasting sessions longer than 12 hours to your fasting history. The AI Coach analyses the patterns you record; it does not measure insulin, glucose, glycogen, fat oxidation, ketones, or autophagy, does not determine whether 18 hours is optimal, does not confirm whether you are in a “deep fasting” state biologically, and does not judge medical safety.
By around this point, some people may be relying more heavily on fat-derived fuels than earlier in the fast, with ketone production potentially contributing more. Insulin is generally lower than after eating, and liver glycogen may be lower than earlier. The changes are gradual and vary with prior meals, activity, and individual physiology — 18 hours is not a universal biological switch.
It may become a larger contributor as fasting continues, but fat oxidation was occurring earlier in the fast too. Eighteen hours is not the start point, and acute fat oxidation during a fasting window is not the same as long-term body-fat loss.
Possibly, but not universally. Some people may have meaningfully higher ketone levels by around this point, while others may not. Timing depends on prior carbohydrate intake, glycogen status, exercise, and individual metabolism. This page does not publish a fixed hour threshold for ketosis onset.
Current human evidence does not establish 18 hours as a universal autophagy onset. Autophagy is ongoing at some level, and most precise timing claims come from animal or cellular models rather than validated human clock-hour thresholds.
"Deep Fasting" is ONE's simplified tracking label for the 18h+ range in the app's five-phase timeline. It is a product and educational label, not a recognised clinical fasting stage, and it does not mean the app has measured a specific ketosis, autophagy, or fat-oxidation state.
Not universally. Eighteen hours is more restrictive than 16, with a longer food-free interval and a narrower eating window, but more restrictive is not automatically better. Adherence, nutrition opportunity, sleep, and lifestyle matter more than a strict hour-by-hour comparison.
It can help some people if the overall pattern creates a sustainable energy deficit and stays consistent. Eighteen hours by itself does not guarantee weight loss, and it is not clinically established as superior to 16 hours for fat loss.
Not automatically. A 24-hour fast is substantially longer, with more hunger, more nutrition planning, and more disruption for some people. It is not a required next step in a progression ladder — different goals point to different durations.
ONE's simplified timeline transitions from "Fat Burning" (12–18h) to "Deep Fasting" (18h+) at the 18-hour mark. That transition is a product tracking landmark, not a biological measurement — the app does not detect a change in insulin, glycogen, fat oxidation, ketones, or autophagy at hour 18.
Yes. ONE saves completed fasting sessions longer than 12 hours to your fasting history, so a completed 18-hour fast is eligible to appear there. Eighteen hours is not one of ONE's Quick Start options (16h, 19h, 24h, 36h) — it is tracked with a Custom Timer.
Prepared by the ONE Editorial Team using peer-reviewed research and guidance from established medical institutions.