Guide

Intermittent fasting windows explained: 14:10, 16:8, 18:6 and 20:4

Updated 2026-09-27 · 10 sources

An intermittent fasting window is the block of hours each day when you eat, with the rest spent fasting. The numbers name the split: 14:10 means 14 hours of fasting and a 10-hour eating window, while 16:8, 18:6 and 20:4 shrink it further. Studies show modest weight loss, usually similar to ordinary calorie cutting.

What is time-restricted eating, and how does it work?

Time-restricted eating means eating within a consistent daily window and fasting for the rest of the day, which stretches your overnight fast ^[1]. It changes when you eat rather than what you eat. In practice, a shorter window can mean eating fewer calories, simply because there is less time to eat ^[2].

It is one of several intermittent fasting styles. Others, like alternate-day fasting and the 5:2 diet, cut calories on whole days instead of limiting hours ^[3].

The 8-hour window, known as 16:8, is the most studied version ^[1]. During the fasting hours, research protocols typically allow zero-calorie drinks such as water ^[4].

A widely cited 2019 review in the New England Journal of Medicine describes how eating within 6 hours and fasting for 18 can trigger a "metabolic switch" from burning glucose to using ketones for energy ^[5]. But much of the research on fasting and longevity comes from animal studies, and those results do not necessarily apply to people ^[2].

How do 14:10, 16:8, 18:6 and 20:4 differ?

They differ in how many hours you fast each day. The 14:10 schedule gives you a 10-hour eating window and is the gentlest, while 16:8 is the most studied. The 18:6 and 20:4 schedules squeeze eating into 6 or 4 hours. Shorter windows can mean eating less, but they have been studied less ^[1]^[4].

Longer fasts are not automatically better. The first trial to compare 4- and 6-hour windows directly found similar weight loss and similar improvements in insulin resistance with both over 8 weeks ^[4].

For people with type 2 diabetes, a 2026 expert review in The Lancet Diabetes & Endocrinology notes that 8-hour windows are more effective than 10-hour windows for weight loss. Evidence that earlier windows work better for blood sugar is still preliminary, so the authors suggest picking the time of day that suits you ^[1].

Common time-restricted eating schedules compared
Schedule	Fasting hours	Eating window	Example eating hours used in trials	What the research shows
14:10	14	10 hours	8 a.m. to 6 p.m.	In a 12-week trial in 120 adults with type 2 diabetes, it lowered HbA1c by 0.9% and body weight by 2.9% versus usual eating ^[1].
16:8	16	8 hours	12 p.m. to 8 p.m.	The most studied window ^[1]. In a 12-week trial of 116 adults, 16:8 alone was not more effective for weight loss than eating throughout the day ^[6].
18:6	18	6 hours	1 p.m. to 7 p.m.	In an 8-week trial in adults with obesity, about 3% weight loss versus controls, with roughly 550 fewer calories eaten per day ^[4].
20:4	20	4 hours	3 p.m. to 7 p.m.	In the same 8-week trial, results were similar to 18:6, not better ^[4].

What does the research say about weight loss and metabolic health?

Time-restricted eating can lead to modest weight loss, but usually no more than ordinary calorie cutting. A 2026 Cochrane review found intermittent fasting may make little to no difference in weight loss compared with standard dietary advice. Effects on blood sugar, cholesterol and blood pressure are mixed, and most trials are short ^[7]^[3].

The Cochrane review pooled 22 randomized trials with 1,995 adults living with overweight or obesity. Compared with no intervention, it found intermittent fasting likely makes little to no difference to weight loss either. Confidence in most findings was low to very low, because many studies were small or not very robust ^[7].

A 2025 BMJ analysis of 99 trials was more positive: every fasting style, and ordinary calorie restriction, reduced weight compared with unrestricted eating. Only alternate-day fasting beat calorie restriction, by about 1.3 kg, and trials lasting 24 weeks or more showed no advantage for any style over the others ^[8].

Head-to-head trials point the same way. In a 12-month trial of 139 adults with obesity, adding an 8 a.m. to 4 p.m. eating window to calorie restriction was not more beneficial for weight, body fat or metabolic risk factors than calorie restriction alone ^[9]. In the 12-week TREAT trial, the 16:8 group also lost slightly more lean mass than the regular-meals group ^[6].

On metabolic markers, a 2022 review found that some studies show better blood pressure, LDL cholesterol, triglycerides, insulin resistance and HbA1c, while others show no benefit ^[3]. In adults with type 2 diabetes, trials report HbA1c drops of about 0.3% to 1.2% compared with usual eating, similar to calorie restriction ^[1].

Most trials run for weeks or months, so long-term effects are poorly understood ^[3].
Many studies are small, and confidence in the results is often low ^[7].
Trials have often enrolled adults aged 60 or younger, so results may not apply to children or older adults ^[2].
When calories are matched, adding an eating window has not shown a clear extra benefit ^[9].

Who should not try intermittent fasting without medical supervision?

Some people should skip it or only try it with their doctor. A 2026 expert review advises against intermittent fasting during pregnancy or breastfeeding, for adolescents, for people with a history of eating disorders or severe low blood sugar, and for anyone at risk of frailty. There is not enough data for type 1 diabetes ^[1].

NIH MedlinePlus also notes that intermittent fasting is not safe for everyone, especially people with diabetes, kidney disease or heart problems. It recommends talking with your doctor, who can review your health history and medications, before you start ^[10].

Pregnant or breastfeeding: safety has not been adequately studied ^[1].
Children and teens: safety has not been established ^[1].
A history of eating disorders: fasting may not be appropriate ^[1].
A history of severe hypoglycemia (very low blood sugar): the review recommends avoiding it ^[1].
Older adults with or at risk of muscle loss (sarcopenia) or frailty: use caution, because effects on muscle are unclear ^[1].
Type 1 diabetes: there is not enough data to know whether it is safe or effective ^[1].
Type 2 diabetes treated with insulin or sulfonylureas: doses may need to be lowered to prevent low blood sugar, so only with medical guidance ^[1].

How do you choose a fasting window and build up to it?

Choose the window you can keep most days, not the most extreme one. Eight-hour windows seem to beat 10-hour windows for weight loss, but the best time of day is unsettled, so fit it around your life ^[1]. A cautious approach is to start with 14:10 and shorten the window only if it feels manageable.

There is little research on the best way to ramp up. Treat the steps below as a practical approach rather than a tested protocol.

If you want help staying consistent, Balnc’s fasting tracker has 16:8, 18:6 and 20:4 presets plus custom lengths, lets you choose which days of the week you fast, and can start each fast automatically in your time zone. Either way, this guide is general information, not medical advice. Talk to your doctor or a registered dietitian before starting any fasting plan, especially if you have a health condition or take medication.

Talk to your doctor first, especially if you take regular medication or have diabetes, kidney disease or a heart condition ^[10].
Pick a window that fits your routine. People with erratic or inflexible schedules, such as long shifts without breaks, tend to struggle most ^[1].
Start with a longer window, such as 14:10, and move to a shorter one only if you feel well.
Aim for 5 to 7 days a week, and choose your break days in advance ^[1].
Keep meals balanced, with fruits, vegetables, legumes and whole grains. Fasting is not permission to eat anything ^[1].
Expect a short adjustment period. In the 18:6 and 20:4 trial, mild dizziness, nausea and headaches peaked around week 2 and faded by week 3 ^[4]. Get medical advice if symptoms are severe or do not settle.

Quick answers

Is 16:8 better than 14:10?

For weight loss, 8-hour eating windows appear more effective than 10-hour windows, according to a 2026 expert review. But in a 12-week trial, 16:8 on its own was not more effective than eating throughout the day. The better window is usually the one you can keep 5 to 7 days a week.

Can I drink water or coffee while fasting?

Research protocols typically allow zero-calorie drinks, such as water, during the fasting hours. Drinks with calories, like coffee with milk or sugar, count as eating in these studies because participants are asked to avoid all calories outside the window. If you take medication with food, ask your doctor how to time it.

How much weight can you lose with intermittent fasting?

Short trials usually report mild to moderate weight loss of about 3% to 8% over 8 to 12 weeks, similar to daily calorie restriction. A 2026 Cochrane review found little to no difference compared with standard dietary advice, so expect results similar to other ways of eating fewer calories, not a shortcut.

Is 20:4 fasting safe?

In an 8-week trial in adults with obesity, a 4-hour eating window caused no serious adverse events. Mild dizziness, nausea and headaches peaked around week 2, then faded. It worked about as well as a 6-hour window. Long-term safety data are limited, so talk to your doctor before trying it.

Does intermittent fasting lower blood sugar?

In adults with type 2 diabetes, trials report HbA1c reductions of about 0.3% to 1.2% compared with usual eating, similar to calorie restriction. People taking insulin or sulfonylureas may need lower doses to avoid low blood sugar, so only try fasting with your doctor involved. Data for type 1 diabetes are insufficient.

Sources

This guide is general information, not medical advice. Talk to your doctor or a registered dietitian before changing how you eat, fast or supplement, especially if you are pregnant, take medication or manage a medical condition.

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