OMAD, 20:4 or 18:6? How to Choose a Longer Fasting Window

18:6 means fasting for 18 hours and eating inside a 6-hour window. 20:4 means a 4-hour window, and OMAD ("one meal a day", sometimes written 23:1) means one meal, usually inside an hour. They are three points on the same line, and the right one is the longest window you can repeat comfortably without feeling bad, not the longest one you can survive once.

Longer is not automatically better. Reviews of intermittent fasting describe the research as mixed and still developing, and people differ a lot in how they handle it. This guide shows what each pattern looks like on an ordinary day, how people choose between them, and a gentle way to step up. It is general information, not medical advice, and the section on who should not try these windows matters more than the schedules do.

First, the safety part

Longer fasting windows ask more of your body than 12:12 or 16:8 do. Before you move to 18:6, 20:4 or OMAD, read this:

• Talk to a doctor first if you have a medical condition (diabetes especially), take medication, are pregnant or breastfeeding, are under 18, or have ever had an eating disorder.
• MedlinePlus Magazine notes that intermittent fasting is not safe for everyone and names people with diabetes, kidney disease or heart-related issues as needing extra caution. It also advises approaching it individually, with help from a dietitian or physician who knows your health history and medications.
• If you take medication for blood sugar, a long fast can change how that medication works. Your prescriber needs to be part of the plan. The National Institute of Diabetes and Digestive and Kidney Diseases has general diabetes information if you want a starting point for that conversation.
• If you have ever struggled with food, tight rules about when you may eat can pull old patterns back. Early treatment matters, and the NIMH eating disorders page points to where to find help. If a schedule makes you anxious about food, skip it and talk to a professional.
• Stop and get help if you feel faint, confused, unusually weak, or unwell in any way. No streak is worth that.

Harvard Health says in its overview of fasting that long-term benefits are not yet known, and the 2019 review by de Cabo and Mattson in the New England Journal of Medicine is a widely cited summary of what is and isn't established. Keep that tone in mind. Nothing below is a promise about weight, energy or health.

What 18:6, 20:4 and OMAD look like

The numbers are easier to understand on a clock. Here is a typical day for each, assuming you wake around 7 am and sleep around 11 pm. Your own hours will differ.

Pattern	Fasting	Eating window	Example window	Meals people usually fit
16:8 (for reference)	16 hours	8 hours	11 am to 7 pm	Two or three
18:6	18 hours	6 hours	1 pm to 7 pm	Two, or one large and one small
20:4	20 hours	4 hours	3 pm to 7 pm	One main meal plus a snack, or two small meals
OMAD (23:1)	23 hours	About 1 hour	6 pm to 7 pm	One meal

You can see the shape of each on the fasting schedule calculator: pick a plan, add your wake-up and sleep times, and it draws the eating and fasting windows on a 24-hour ring. It also warns you if the window starts before you wake or ends after you sleep, which happens more often than you'd think with the longer patterns.

18:6 on a real day

You skip breakfast and likely lunch, depending on when the window opens. A common version is eating from noon to 6 pm or 1 pm to 7 pm. That fits a normal lunch and an early dinner, with room for a snack between them. For many people it is the first longer window that still looks like regular meals.

The main adjustment is the stretch from waking up to the first meal. If you wake at 7 am, a 1 pm start means six hours of morning with only water, tea or black coffee, depending on what your doctor and you decide counts for you.

20:4 on a real day

A four-hour window is short. Most people use it for one substantial meal plus something smaller, or two modest meals about two hours apart. Eating at 3 pm to 7 pm, for example, leaves the whole morning and early afternoon without food.

This is where practical things start to matter: getting enough food into four hours, how you feel in the late afternoon before the window opens, and whether your work or workouts line up with it. Examine.com has a plain overview of OMAD and 20:4 if you want a longer read, though I couldn't open it from here to confirm its current wording, so treat it as further reading rather than a source for any claim above.

OMAD on a real day

One meal a day means there is exactly one chance to eat. Many people pick dinner because it works with family and social life, which fits the question people often ask about what the eating window looks like at 23:1. In practice it is a meal that takes anywhere from 30 minutes to an hour or so, possibly with a drink or small extra before or after.

OMAD is the pattern where the cautions earlier in this guide apply most strongly. Fitting a day's worth of food, protein and variety into one sitting is hard, and plenty of people report feeling very full, then very hungry, and tired in between. If you try it, treat it as something to ask a professional about, not a challenge to push through.

Is longer better? Why 18:6 isn't "more correct" than 16:8

A common thread in the r/intermittentfasting community is that longer fasts aren't automatically better than shorter ones. People also ask 16:8 vs 18:6 again and again, in thread after thread, and the answers tend to land in the same place:

1. The window you can keep wins. A 16:8 you do five days a week beats an 18:6 you abandon after nine days.
2. What you eat inside the window still matters. A shorter window doesn't change what is on the plate. See why the scale sometimes doesn't move for a calm checklist of what to look at before changing your window.
3. How you feel is data. Headaches, poor sleep, irritability, dizziness, constant food thoughts or obsessing over the clock all say the window may be too long for you right now.
4. Research doesn't show a clear winner. Reviews describe intermittent fasting evidence as mixed and still developing, especially for long-term outcomes. There is no solid basis for saying that 20:4 beats 18:6 for your body.

If you're coming from 16:8 and haven't settled into it yet, the 16:8 schedule guide is a better place to start. Many people stay there happily.

How to choose: a few honest questions

Work through these before you pick a window.

What does your day look like? Fasting windows work best when they follow your life. If your biggest meal is a family dinner at 7 pm, a window that ends at 7 pm fits and a window that closes at 4 pm doesn't. If you work nights, anchor to your own wake-up time, not the clock. The night shift guide covers that.

What do you do for exercise? People who lift weights or train hard often ask whether 20:4 gets in the way, and the honest answer is that it depends on the person and on how they plan meals around workouts. Both timing and total food matter, and a sports dietitian or doctor can help with specifics. If you notice weaker workouts, slow recovery or dizziness, widen the window.

How hungry do you get, and when? Some people feel fine through the morning and struggle at 2 am or in the evening. A window that moves your first meal closer to your natural hungry time is easier to hold than one that fights it.

How is your sleep and stress? If you are already short on sleep or under heavy stress, a longer fast may feel harder than it needs to. Consider doing less, not more.

What is your history with food? If you have ever had an eating disorder, or counting hours makes you anxious about eating, these patterns are a poor fit, and that is a reason to talk to a clinician rather than a reason to try harder.

A gentle way to step up

If you have a doctor's okay and want to try a longer window, move in small steps and give each one a couple of weeks.

1. Hold 16:8 comfortably first. If it already feels like a struggle, a longer window won't fix that.
2. Try 18:6 two or three days a week. Keep the other days at 16:8. Notice sleep, mood and energy.
3. Move to 18:6 most days only if the first step felt fine.
4. Treat 20:4 as an occasional option, not a target. Try it on a day when your schedule is calm.
5. Treat OMAD as something to discuss with a professional. If it is right for you, build it with their help, including what the one meal should contain.
6. Step back whenever you want. Going from 20:4 back to 16:8 is not failure. It is adjusting.

One more practical point: most guidance on these windows assumes you eat enough overall. A short window with too little food is just under-eating on a timer. If you are not sure how much food you need, a registered dietitian can help you work that out.

Women, periods, perimenopause and PCOS

This question comes up all the time in the community: how longer windows interact with menstrual cycles, perimenopause and PCOS. Some people report feeling fine, others report feeling worse during certain phases or when stress is high, and the research is still developing. NIH-supported trials are examining fasting in women with PCOS, which tells you the questions are open, not settled.

So the safe general advice is simple. If you have PCOS, irregular cycles, are in perimenopause or menopause, are pregnant, are trying to conceive or are breastfeeding, talk to your doctor before trying 20:4 or OMAD. Hunger waking you at 2 or 3 am, missed or changed periods, or feeling run down are signs to loosen the window and call your doctor, not to push through. This is not a place for advice from strangers online, including from a website like this one.

Where Fastly helps

Fastly is a simple fasting timer made by the same people who made this site. It doesn't tell you which window to use. It helps you keep whichever one you and your doctor pick:

• Presets for the patterns in this guide: 12:12, 14:10, 16:8, 18:6, 20:4 and OMAD (23:1), plus a custom goal if your hours don't fit a standard plan.
• One tap to start, with a progress ring that counts your time, and you can edit the start time if you began earlier than you tapped.
• Streak and weekly activity so you can see how consistent you've been over time. That's a good way to judge whether a window is sustainable.
• Weight and water logs with charts, if you want them.
• Reminders and a home screen widget, so you know when your window opens.
• No account. Your data stays on your phone, and the timer works offline.
• Fasting stages, labelled educational only. They describe what people commonly say about each stretch of hours. They are not a measurement of your body.

It's free to download. Some things, such as the full history and CSV export, need Fastly Pro.

What Fastly doesn't do: it doesn't give medical advice, set your window for you, check whether a plan is safe for you, or promise any result. It won't notice if you feel unwell. You do.

If you only want to try the numbers first, you don't need an app at all. The fasting schedule calculator and the when can I eat timer are free in your browser, with no sign-up. If you later want to compare options, best free fasting apps lists what to check before you subscribe to anything.

Questions

Is 20:4 better than 18:6?

Not in any way that research has clearly shown. Both are longer windows, and 20:4 asks more of you. Pick the one you can keep up while still sleeping, working and eating well. If you can't keep 20:4 for a couple of weeks, 18:6 is a fine place to be.

What does the eating window look like with OMAD?

Usually one meal, often dinner, finished within about an hour. Some people add a drink or a small snack at either end. Because everything has to fit in one sitting, it is the pattern most worth talking over with a doctor or dietitian first.

How do I handle a social dinner on 18:6?

Put your window around the dinner. If dinner is at 8 pm, a 2 pm to 8 pm window works. People who ask about social dinners on 18:6 mostly move the window for that day, and accept that one off day doesn't ruin anything.

Can I lift weights while doing 20:4?

Some people do and some find it hard. It depends on when you train, how much you eat and how you feel. If your strength drops, you feel dizzy, or you recover slowly, widen the window and consider a sports dietitian. This is a question for a professional who knows your training.

I'm a woman with irregular cycles or PCOS. Should I try OMAD?

Ask your doctor first. Cycles, hormones, perimenopause and PCOS all change how people respond to long fasts, and the research is still developing. Missed periods, 2 or 3 am hunger or constant fatigue are reasons to ease off and get medical advice.

What if I try a longer window and feel awful?

Stop and widen it. Feeling faint, shaky, confused or sick is a reason to eat and, if it keeps happening, to call a doctor. Going back to 14:10 or 16:8 is a normal adjustment.

This is general information, not medical advice. Talk to a doctor before fasting if you have a medical condition (such as diabetes), take medication, are pregnant or breastfeeding, are under 18, or have ever had an eating disorder. Stop and get help if you feel unwell.