What is intermittent fasting?

Intermittent fasting is an eating pattern. You eat during a set span of hours, and you do not eat during the rest of the day, or you eat on some days and eat much less on others. This page is about the daily version, usually called time-restricted eating, because that is the version beginners ask about first.

The Mayo Clinic describes intermittent fasting as a pattern of eating based on time limits. Research on longer-term effects is mixed, and some studies find that simply eating fewer calories can show similar short-term changes. This site does not present fasting as medical care, and it does not claim that fasting cures, treats, or prevents disease.

If you are new to the words, read start here first, then come back for the fuller explanation.

Eating window and fasting window

The eating window is when meals happen. The fasting window is when they do not. A person who eats breakfast at 8:00 and finishes dinner at 20:00 is already using a 12 hour overnight gap, even if they never call it fasting.

Intermittent fasting, as people use the term online, usually means choosing that gap on purpose and sometimes making it longer. The clock is the tool. The food during the eating window still matters. A window full of hurried snacks is still an eating window. A window with two calm meals is also an eating window. The clock does not grade the food.

How the names work

Schedules are often written as two numbers that add up to 24. In 16:8, 16 is the fasting hours and 8 is the eating hours. In 14:10, 14 hours are fasting and 10 are for meals. In 12:12, the day is split in half. In 18:6, the eating window is six hours.

You may also see 5:2, which is different. On that pattern, people eat as they usually do on five days and cut calories sharply on two days. This site focuses on daily windows, because they are easier to describe without turning the page into a diet plan. If a pattern asks you to count a very low calorie target, that is a bigger change than moving breakfast later, and it is not where a first reading needs to start.

What it is not

It is not a cleanse. You do not need a special drink to “start” it. It is not a moral test. Ending a window early because you feel unwell is a reasonable act, not a failure.

It is also not a diagnosis tool. Feeling hungry, tired, or clear-headed on a given morning does not tell you that something is wrong or right with your health. Those are sensations. They are easy to over-read.

Public descriptions sometimes list short-term research findings about weight or blood markers. Those findings are not a promise, and they are not a reason to skip medical care. People with diabetes, people on medication, and anyone in the groups named in the safety note should talk with a clinician before they change when they eat.

A calm way to learn it

Start with your current day. Write down the time of the first bite and the last bite on two ordinary days. The gap between last bite and first bite is your overnight fasting window. Many adults already land near 12 hours without trying.

From there, read the shorter examples before the longer ones. Fasting schedules lines them up. 16:8 fasting is the pattern articles mention most often. 18:6 fasting is a step further, and it is easier to misjudge if you have never held a shorter window.

Decide what you will drink before you decide how long to wait. Water, plain black coffee, and plain tea are the usual choices. The details, including why sweeteners are treated as the end of a fast by many people, are in what to drink while fasting.

Hunger, meals, and stopping

Hunger often comes in a wave around the time you usually eat. It can pass if you are comfortable and well, and it can also be a signal to eat. Both can be true on different days. Hunger while fasting stays with that subject so this page does not turn hunger into a slogan.

When the window opens, eat a meal you already know how to make. How to break a fast is deliberately ordinary. There is no required first food on this site.

If the pattern starts to run your mood, your social meals, or your sleep, that is useful information. A shorter window, or no fasting schedule at all, is a valid outcome of learning. The FAQ answers a few of the questions that usually come next, including what this site will not promise about weight.

Safety

Safety. People who are pregnant, breastfeeding, under 18, have an eating disorder or a history of one, have diabetes or hypoglycemia, take medication, or have a heart, kidney, or other medical condition should talk with a healthcare professional before fasting. Stop a fast and get help if you feel faint, confused, or unwell.

Related guides: common mistakes, fasting and exercise, and the beginner guide.