Is a 13-hour fast effective?

Is a 13-hour fast effective? Direct evidence for that exact schedule is limited. It leaves an 11-hour eating window, which can organize meal times, but studies of longer fasting schedules cannot establish the same results for 13 hours. Food intake, individual circumstances, and the outcome being measured all matter.

For context on different schedules, see our guide to fasting durations. This article explains the 13-hour schedule and the limits of research on its effects.

If you have diabetes, take glucose-lowering medication, are pregnant, or have a history of disordered eating, consult your doctor before trying fasting. Stop and seek advice if fasting makes you dizzy, unusually tired, or preoccupied with food.

Key takeaways

• A 13-hour fast leaves an 11-hour eating window within a 24-hour day.
• The small trial comparing 12-, 14-, and 16-hour fasts did not test a 13-hour schedule (1).
• The older-adult pilot study used an eight-hour eating window, equivalent to about 16 hours of fasting (2).
• The cited research does not establish a universal physiological threshold at hour 13 (1, 3).
• The 12-month trial compared an eight-hour eating window without calorie counting with prescribed calorie restriction; the diets were not calorie-matched (4).
• General time-restricted-eating findings cannot establish the effect of a specific 13-hour schedule (5).
• The clock times and drink table in this article describe a schedule; medical fasting requires separate clinical instructions.

What do studies tell us about fasting duration?

The cited studies do not establish one fasting duration for everyone. Time-restricted eating means eating within a consistent daily window; protocols and study populations vary (3).

One small trial assigned 41 metabolically healthy, untrained adults to 12-hour, 14-hour, or 16-hour fasting schedules, or a control group, for eight weeks (1).

Only the 16-hour group showed significant body- and fat-mass changes in that trial. No group followed a 13-hour fast, and the small study cannot establish a universal ranking of schedules (1).

A separate nine-month pilot study in 90 adults aged 60 or older with overweight or obesity tested an eight-hour eating window, equivalent to roughly 16 hours of fasting. It does not provide evidence for an eight- to 13-hour fasting recommendation (2).

These differences in participants and schedules matter when applying research to a daily routine. The choice also needs to account for medical advice, meals, work, and sleep.

Read more: 72-Hour Water Fast: What No One Tells You About

How does a 13-hour fast work?

A 13-hour fast leaves 11 hours for eating in a 24-hour day. An overnight example is finishing food at 7:00 pm and having breakfast at 8:00 am.

A defined eating window changes when you plan meals and snacks. For example, an overnight schedule can mean:

• Setting an evening time to finish eating.
• Planning breakfast around the next morning’s commitments.
• Checking whether regular meals still fit the available window.

Our 12-hour fasting guide explains a neighboring schedule. A shorter or longer window is a scheduling choice that also needs to fit your nutritional and medical circumstances.

What happens after 13 hours of fasting?

The cited evidence does not establish a universal physiological event at hour 13. Responses to fasting vary, and research on time-restricted eating uses different windows, meal timing, and participant groups (1, 3).

A precise clock time cannot establish that digestion is complete, that someone is in ketosis, or that a particular cellular process has reached a meaningful threshold. Those claims need direct measurements and evidence for the specific context.

Treat claims about a fixed hour for a body “reset” cautiously. The studies discussed here do not define such a threshold (1, 3).

For additional scheduling examples, see our 13-hour fasting schedule guide.

What does the evidence say about weight loss?

The cited research does not establish a specific weight-loss effect from fasting for exactly 13 hours. A systematic review of time-restricted eating discusses the roles of energy intake and meal timing across varied protocols, so its pooled findings cannot be assigned directly to this schedule (5).

Weight change depends on more than the duration of the eating window. A schedule can change meal timing without necessarily reducing overall food intake.

In a 12-month trial involving 90 adults with obesity, an eight-hour eating window without calorie counting was compared with prescribed daily calorie restriction and a control group. The two dietary intervention groups had similar observed weight loss, but their calorie intake was not experimentally matched. This finding does not prove that the approaches are equivalent or establish an effect for a 13-hour fast (4).

The table distinguishes the schedule calculation from the limits of the evidence:

The sample times illustrate the arithmetic only. They are not prescriptions or predictions of body changes.

BetterMe can help you add more structure to your daily wellness routine with workouts, meal planning tools, and habit-building features. Explore the app and take a practical step toward a routine that feels more manageable for you.

What can you eat and drink during a 13-hour fast?

For a calorie-free fasting window, water and unsweetened drinks are practical options. During eating hours, plan ordinary meals that provide enough food for your needs.

Examples of drinks without added calories include:

• Water (still or sparkling)
• Black coffee
• Plain tea

Drinks and foods that contain calories belong within the eating window when following this definition:

• Juice or soda
• Milk or creamers
• Protein shakes
• Sweetened coffee drinks
• Alcohol
• Snacks

Milk, cream, sugar, and other additions contribute calories.

Read more: Long-Term Effects of Intermittent Fasting

Food composition and energy density can influence appetite and intake, with substantial variation between people (6). Practical meal components include:

• Protein: eggs, yogurt, beans, fish, lentils, tofu, or chicken.
• Fiber: vegetables, fruit, whole grains, and legumes.
• Other foods you enjoy and can prepare regularly.

Drink options during a calorie-free fasting window:

Plan the meals as well as the clock times. A schedule alone does not tell you whether your food intake meets your needs.

When should you start and end a 13-hour fast?

Choose start and end times by counting 13 hours between the last calories one day and the first the next. The examples below show overnight windows; none is a clinically established preference for everyone.

Here are a few practical schedules to consider:

• One overnight example runs from 7:00 pm to 8:00 am.

Meal timing is a separate question from fasting duration. The studies cited here vary in their timing rules, so they cannot validate each of these sample clock times (3).

Check the schedule against your medication instructions, work hours, and regular meals before making a change.

Choosing your window comes down to your lifestyle:

• An evening cutoff after dinner can leave space for a shared meal.
• To keep an early breakfast, count backward 13 hours for the evening cutoff.
• Check how that timing fits work and other commitments before adopting it.

Choose timing that leaves room for adequate meals and your usual commitments. If the schedule interferes with eating enough, reconsider it.

You can keep regular meal times without setting a fasting target.

How often should you follow a 13-hour fast?

The cited evidence does not establish a recommended weekly frequency for exactly 13 hours. The trials described here used repeated daily eating windows within specific research protocols (1, 2, 4).

A study schedule is not an individual prescription. Discuss frequency with a qualified healthcare professional if you need guidance for your circumstances.

There is no requirement to extend the fasting period or add more days. Regular meals without a fasting schedule are also an option.

A useful practical check is whether the schedule allows adequate meals and fits daily responsibilities. Hunger, mood, and energy alone cannot confirm a physiological benefit.

BetterMe: Health Coaching app can help you build a more structured approach to your wellness routine with meal plans, workouts, and habit-tracking tools. Start using the app to explore options that may support your personal fitness and nutrition goals.

FAQs

• Does a medical fast follow these drink rules?

No. Instructions before blood tests, anesthesia, or other procedures can differ from a self-selected eating schedule. Follow the clinical team’s instructions about food, drinks, and medication.

• Do you need to make up a missed fasting day?

The cited studies do not establish a need to compensate for a missed day with a longer fast. Keep regular meals and avoid extending a fast just to balance a weekly total.

• Should you change medication timing to fit a fast?

Ask your prescriber or pharmacist before changing medication timing. Some medicines must be taken with food, and the eating schedule should follow those instructions.

The bottom line

A 13-hour fast is an 11-hour eating window, and direct evidence for that exact schedule remains limited. If you are considering it, first check whether your meals and medication instructions fit. Use research on longer windows with care, and avoid treating a clock time as a promise of weight loss or a body reset.