16:8 is the schedule most people mean when they say intermittent fasting. You eat within an eight-hour window and you do not eat for the other sixteen. That is the whole rule.
It is popular because it is the version that survives contact with an actual week. Most of the sixteen hours are hours you were asleep anyway.
How the window works
Pick the eight hours you eat in. Everything outside them is the fast. If your window is midday to 8pm, your fast runs 8pm to midday — and you spent eight of those sixteen hours in bed.
The clock starts at your last bite, not when you finished cooking, not when you sat down. That is why a fasting timer matters more than it sounds: most people misremember by forty minutes in the direction that flatters them.
Why sixteen hours
Not because something magic happens at exactly sixteen. It is a practical number that lands after the point where liver glycogen is largely used up and the body has shifted meaningfully toward burning fat — usually somewhere in the twelve-to-sixteen-hour range, varying with your last meal, your activity, and you.
Sixteen gives most people that shift with a margin, while leaving an eating window wide enough for two real meals. Fourteen works. Eighteen works. Sixteen is the one that people keep doing in month three, which matters more than the number.
Picking your eight hours
There is no correct window, only the one you will actually keep.
- Midday to 8pm — skip breakfast. Easiest for most people, because eating socially in the evening stays possible.
- 10am to 6pm — earlier window, better aligned with circadian rhythm, harder if you eat dinner with other people.
- 9am to 5pm — suits early risers and anyone who sleeps badly after a late meal.
Move it around when life requires it. A schedule you break once and abandon is worse than one you shift by two hours on a Friday.
What the first two weeks feel like
Days one to four are the hard ones, and the difficulty is habit rather than hunger. You will notice how much of your eating was clock-driven — 10:30am is not hunger, it is 10:30am.
Hunger in a fast comes in waves rather than a climb. It rises, and if you do not feed it, it goes away again. This is the single most useful thing to know before you start, because the intuition that it will keep getting worse is what ends most first attempts.
By week two, most people report that the morning wave has flattened out.
What to do about hunger, thirst and coffee
Water, black coffee and plain tea are fine — see what breaks a fast for the detail on the edge cases. Drink more than you think you need: a lot of what registers as hunger at hour fourteen is thirst.
When 16:8 is not for you
Do not start if you are pregnant or breastfeeding, if you have a history of disordered eating, if you have diabetes or take medication that must be taken with food, or if you are under 18. Talk to a clinician first if you have any chronic condition. This applies whether or not you feel fine.
Fasting is a tool, not a virtue. Stopping early is not a failure of character.