16/8 vs. 18/6 vs. 20/4: Which Type of Intermittent Fasting Suits You?

16/8, 18/6 and 20/4 are the most common time-restricted eating models, all based on the same principle: you compress your meals into a shorter part of the day and fast for the rest of it. The difference between them is, essentially, how long the window is in which you're allowed to eat — but that difference of a few hours significantly changes the difficulty, social flexibility and risks of each protocol.

Comparison table

ProtocolFeeding windowDifficultySuitable for beginnersRisk of eating too littleExample schedule16/88 hoursEasy–moderateYes, recommended as a first stepLow12:00–20:0018/66 hoursModerateAfter 2–4 weeks on 16/8Moderate13:00–19:0020/44 hoursHighNot as a starting pointHigh14:00–18:00

16/8 — the reference format

It's the most studied and the easiest to sustain socially (it allows for lunch and dinner, for example). Most beginner guides, including resources from Johns Hopkins Medicine, recommend this format as the entry point into intermittent fasting.

Who it suits: people with a standard working schedule, those who want to try fasting without radically changing their social routine, and anyone starting out for the first time. Example day: first meal at 12:00 (protein + complex carbohydrates), optional snack at 16:00, dinner at 19:30. Advantages: easy to maintain long-term, allows for family meals, low risk of not meeting your calorie needs. Disadvantages: for some, the metabolic effects are slower to notice compared with shorter windows.

18/6 — a balance between results and comfort

It shortens the window by another 2 hours compared with 16/8, which usually means 2 main meals instead of 2 meals plus a snack. It's a natural next step once you've got used to 16/8, not a starting point.

Who it suits: people who have already been through the adaptation stage on 16/8 and want an extra step without reaching the restriction of a very short protocol. Example day: first meal at 13:00, second (and last) meal at 18:30–19:00. Advantages: a clearer feeling of fullness outside the window, suited to those who want somewhat more visible results without the discomfort of a 4-hour window. Disadvantages: requires more meal planning, because each meal has to cover a larger share of your daily protein and fibre needs.

20/4 — for advanced experience

With only 4 hours to eat, the risk of not meeting your daily calorie and protein needs increases significantly, especially for physically active people. Mayo Clinic notes that the long-term benefits of very short windows are not yet as well documented as those of moderate formats.

Who it suits: people with at least a few months of fasting experience, no medical conditions, who closely monitor their nutritional intake. Example day: one large meal, or two meals close together between 14:00 and 18:00. Advantages: a very simple window logistically (essentially a single meal period per day). Disadvantages: higher risk of extreme hunger, headaches, fatigue, irritability, and of not meeting your protein and micronutrient needs within a single short window.

What happens in the body in each window

Regardless of the format, the basic mechanism is similar: as the fast goes on, insulin levels drop, liver glycogen stores start to deplete, and the body gradually shifts towards using fat as its main energy source. Researchers at the National Institute on Aging and the authors of the review published in the New England Journal of Medicine (de Cabo & Mattson, 2019) explain that these metabolic processes become more pronounced as the fasting window lengthens — which is why 20/4 can produce more visible metabolic effects than 16/8, but also more discomfort, especially in the first few weeks.

How to move from one format to another without shocking your body

There's no need, and it's not recommended, to jump straight to your desired window. A gradual transition greatly reduces the risk of extreme hunger and giving up: stay on each format for at least 1–2 weeks before shortening the window by another 2 hours, watch how your energy and sleep respond, and settle on the format where you feel good, even if it isn't the most restrictive one. There's no compulsory "next level" — a steadily maintained 16/8 beats a 20/4 abandoned after a week.

Common mistakes when choosing a format

• Choosing 20/4 straight away, out of a desire for quick results, without going through the intermediate stages
• Changing the format every week, without letting your body adapt to just one
• Choosing a format that's too restrictive for your social or work schedule, which leads to giving up
• Ignoring signs of chronic fatigue or persistent headaches, thinking "that's just normal with fasting"

How to choose

• If you're just starting out, have a standard working schedule, or want to try fasting without radically changing your social life: 16/8
• If you have 4+ weeks of experience with 16/8, feel comfortable and want an extra step: 18/6
• If you have solid experience, no medical conditions, closely monitor your protein and micronutrient intake, and can accept a very short window temporarily: 20/4, but not as a permanent lifestyle — with periodic breaks
• If you work shifts or have an irregular schedule: choose the format with the most flexibility for you (usually 16/8), and shift the window depending on that day's shift

Sources and references

• NIH — Time-restricted eating for metabolic syndrome
• Johns Hopkins Medicine — Intermittent Fasting: What Is It, And How Does It Work?
• Mayo Clinic — Intermittent fasting: What are the benefits?
• de Cabo & Mattson, 2019, NEJM — Effects of Intermittent Fasting on Health, Aging, and Disease
• National Institute on Aging — Research on intermittent fasting shows health benefits