What the two numbers mean
In 16:8 the first number is hours fasting, the second is hours eating. A 16:8 day has an eight hour eating window, for example noon to 8pm, and nothing but water, black coffee or tea outside it.
The same reading works for14:10,18:6and20:4. The bigger the first number, the shorter the window.
What you eat inside the window is not part of the method. That is why fasting sits in the catalog as a schedule rather than a diet.
The daily windows, from easy to hard
14:10is the gentlest: a ten hour window such as 9am to 7pm, which for many people is close to how they already eat. The catalog rates it low effort.
16:8is the most common. It usually means skipping breakfast or dinner, and people report that mornings are the hard part at first, then routine.
18:6and20:4shorten the window to six and four hours. Both are rated high effort because they take a real bite out of a social day.
OMAD, one meal a day, is the shortest window of all. It is a single sitting, and the catalog lists it as hard.
The weekly patterns
5:2is a different shape: five ordinary days, and two non consecutive days kept very low, around 500 to 600 kcal as commonly described.
Alternate day fastingturns that into every other day.Eat stop eatuses one or two full 24 hour fasts a week with ordinary eating in between.
Weekly patterns leave most days untouched, which some people find easier and others find harder, because the low days stand out.
Early or late window
Early time restricted eatingputs the window early in the day, for example 8am to 4pm, and skips dinner instead of breakfast.
Which end of the day you cut is mostly a question of your life. Family dinners push the window late; early starts and quiet evenings push it early.
What a fasting day looks like
Pick the window, write it down, and keep the same hours most days. Water, black coffee and plain tea are the usual exceptions outside it.
Most people report the first week as the adjustment, and that hunger at the usual mealtimes fades once the routine settles. The catalog page of each protocol carries the same note in neutral words.
Tracking a fasting period
In Undiet a fasting protocol is marked like any program, and it becomes a period on your timeline. You log weight and how the day felt, not what you ate.
That matters with fasting, because the feeling is most of the story. A week of light days on 16:8 says something a scale alone does not.
If you move from 16:8 to 18:6, or back to 14:10, each stretch stays as its own period, so you can see what each window did for you.
Before you start
Fasting is not for everyone, and the catalog does not say who it is for. Talk to your doctor if you take medication, are pregnant or have a condition that depends on regular meals.
Not sure which window fits?Find my dietasks five questions and narrows the catalog, including the fasting entries, to a shortlist.