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Your current meal timing
A schedule that lines up with when you already eat is easier to keep than one that constantly forces exceptions.
By ONE Editorial Team · Published: September 4, 2026
There is no single fasting duration that is right for everyone. Some people use a relatively short overnight fasting window; others choose longer time-restricted eating schedules. The right choice depends on your experience, health, nutrition, and how well the routine fits your daily life.
Longer fasting is not automatically better. A fasting window is useful only if it remains practical, nutritionally adequate, and appropriate for the person following it.
12–14 hours
A less restrictive fasting window that may fit more easily into normal overnight meal timing.
16 hours
A commonly discussed time-restricted eating window.
18–20 hours
More restrictive and leaves less time for meals.
24 hours or longer
A substantially longer fast that should not be treated as a routine progression simply because shorter fasting feels manageable.
These are general categories, not prescriptions. Longer fasting is not automatically more effective or appropriate.
A useful answer to “how long should you fast?” is not a single number. People arrive at intermittent fasting with different bodies, routines, and health histories, and each of these factors influences which duration is realistic.[1]
Individual differences that matter include:
Published studies also use different fasting protocols — daily time-restricted eating, alternate-day patterns, 5:2 approaches, and longer fasts have all been studied — which is one reason the evidence cannot be reduced to a single “best hour” target for every reader.[4][5]
Rather than picking a number and forcing it, work through the factors below and see which fasting window they point toward. The key question at the end is: can this schedule fit your life without consistently compromising nutrition, energy, or wellbeing?
A schedule that lines up with when you already eat is easier to keep than one that constantly forces exceptions.
If you have not fasted intentionally before, a shorter window is a more useful reference point than a long one.
The narrower the window, the harder it becomes to meet protein, fiber, and total energy needs. If meals are consistently rushed or skipped, the schedule is likely too restrictive.
Meetings, family meals, travel, and shift work all affect which windows are realistic. A fasting duration that repeatedly clashes with real life is unlikely to last.
Training intensity, session length, and recovery all influence how a given fasting window feels. Athletes and people in heavy training loads may need a broader eating window.
Medications, chronic conditions, pregnancy, breastfeeding, and a history of eating disorders can all change what is appropriate. When in doubt, talk to a healthcare professional.
A less restrictive fasting window is a more useful starting reference than a long one. It lets you observe how hunger changes across the day, whether the routine fits your work and sleep, and whether you can maintain adequate nutrition within your eating window.
Some people prefer to begin with a shorter fasting window and adjust only if the routine feels manageable. Others move to 16 hours fairly quickly. Neither approach is universally right.
For more detail, see our intermittent fasting for beginners guide, or the step-by-step how to start intermittent fasting walkthrough.
These are the three most commonly discussed daily fasting durations. Each represents a decision about how compressed you want your eating window to be.
A note on ONE: fasting history stores completed fasting sessions longer than 12 hours. A 12-hour Custom Timer will still run, but it will not appear as a completed session in your history.
An 18- or 20-hour daily fasting window creates a significantly narrower eating window than the more common options above.
Practical trade-offs may include:
These schedules are best treated as choices for people whose routine and preferences already fit them — not as levels that everyone should eventually reach. More restrictive does not mean more appropriate.
A 24-hour fast is meaningfully different from a routine overnight time-restricted eating pattern. It can be harder to tolerate, creates more opportunities for dehydration or inadequate intake, and can interact more significantly with medications and medical conditions.[2][3]
Longer fasts should not be treated as a required progression simply because shorter fasting has felt manageable. There is no automatic reason to escalate.
Note on ONE's longer Quick Start options
ONE provides 24-hour and 36-hour Quick Start timer options for users who choose those durations. Their availability in the app is not a recommendation that longer fasting is appropriate for every user. Extended fasts warrant more caution and, in many cases, professional guidance.
Not necessarily. Weight change over time depends primarily on overall energy balance and dietary intake, not on the length of a single fasting window.[4]
A longer fasting window does not guarantee a greater calorie deficit, greater fat loss, or better adherence. Some people compensate during the eating window; others find the schedule difficult enough that consistency drops. Systematic reviews comparing intermittent fasting with continuous energy restriction generally show broadly similar outcomes when overall intake is similar.[5]
Fasting duration is one input, not the whole answer. Nutrition quality, total energy, adherence, sleep, and activity all influence outcomes.
Autophagy is a normal cellular recycling process influenced by nutrient availability, energy status, and many other signals. Fasting is biologically relevant to pathways associated with autophagy, but that connection is not the same as a stopwatch.
Current human evidence does not support assigning a universal fasting hour at which autophagy begins, reaches a guaranteed level, or becomes “optimal.” Descriptions along the lines of “autophagy starts at 16 hours” or “peaks at 24 hours” oversimplify a complex, continuous process that varies from person to person.
ONE uses simplified fasting phases to help visualise progress during a fast, but those app ranges are not universal biological thresholds. For more depth, see our fasting stages guide.
“Too long” is answered functionally, not by a fixed cut-off. A fasting duration may be too restrictive if it repeatedly causes:
A reasonable response is to shorten the fasting window, pause fasting altogether, or seek professional guidance if concerns persist. A stable, moderate routine is usually more useful than a longer schedule that repeatedly leaves you feeling unwell.
There is no requirement to keep increasing fasting hours. A stable schedule at a moderate duration can be perfectly reasonable — sometimes better than a routine that constantly changes.
Frequent changes also make it harder to understand whether a routine is actually sustainable, because you never see the same schedule across enough weeks to evaluate it. Practical reasons to adjust include:
Training demands influence whether a given fasting duration feels practical. Longer or more intense sessions typically need to sit closer to a meal rather than deep inside a fasted state.
Practical considerations:
Sleep contributes a large block of fasting time. If your last calories are at 8 PM and your first meal is at 10 AM, that is already a 14-hour interval before any deliberate change to daytime meals.
A fasting schedule that repeatedly disrupts sleep or creates late-night hunger may be poorly matched to your routine. Shifting the placement of the eating window is often more useful than extending its length.
Speaking with a healthcare professional is particularly important for people who:
ONE is intended for adults 18 and older.
Important
ONE is a fasting and wellness tracking tool. It does not diagnose medical conditions or replace advice from a qualified healthcare professional.
The ONE fasting timer helps you track the duration you choose. It does not decide how long you should fast — that is your decision, ideally informed by the factors above and, where relevant, a qualified healthcare professional.
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What ONE does and does not do
You can use the Custom Timer to track fasting durations that are not named Quick Start presets, such as 14 hours, 18 hours, or 20 hours. ONE saves completed fasting sessions longer than 12 hours to your fasting history. ONE does not medically prescribe a fasting duration; the AI Coach reviews the patterns you produce, it does not decide the “correct” length of your fast.
There is no single number that suits every person. Fasting durations from around 12 hours (a longer overnight window) up to 20 or more hours are all used in different intermittent fasting approaches. The appropriate duration depends on your experience, health, daily routine, and whether you can maintain adequate nutrition within your eating window.
A 12-hour fast is a recognised time-restricted eating pattern and a common starting point. Whether it is long enough depends on your goals and how well the routine fits your day. In ONE, fasting sessions of 12 hours or less are not saved to your fasting history — you can still run a 12-hour timer, but it will not appear as a completed session.
There is no universally ideal fasting duration. 16:8 is widely discussed in the popular literature, but studies use many different fasting protocols and 16 hours is not proven to be optimal for every person or every goal.
Not necessarily. An 18-hour fast leaves less time for meals within the eating window, which can make it harder to meet nutritional needs. More restrictive is not the same as more effective.
Not automatically. Weight change over time depends primarily on overall energy balance, not on the length of any single fasting window. Some people find longer fasts hard to sustain, or compensate during the eating window.
Autophagy is a normal cellular process influenced by nutrient availability and many other signals. Current human evidence does not support assigning a universal fasting hour at which autophagy switches on for everyone.
A fasting duration may be too long if it repeatedly causes persistent dizziness, fainting, confusion, significant weakness, sleep problems, difficulty concentrating, difficulty meeting nutritional needs, or a worsening relationship with food. If these occur, shorten the window, pause fasting, or speak with a healthcare professional.
No. ONE is a fasting and wellness tracking tool. It does not medically prescribe a fasting duration. You choose your fasting duration, and ONE helps you track it, view your fasting history, and review your patterns over time.
Prepared by the ONE Editorial Team using guidance from established medical institutions and peer-reviewed research.