When to Break My Fast — Exact Time

This page answers the intermittent fasting version of the question: the hour your fasting window ends and your eating window opens. It is a clock sum — last meal plus fasting hours. Enter both below and the exact time comes back.

The arithmetic, three ways

• Dinner at 20:00, on 16/8 — the fast ends at 12:00 the next day.
• Last meal at 21:00, on 18/6 — it ends at 15:00.
• Dinner at 20:00, on a full 24-hour fast — it ends at 20:00 the following day.

That moment is also the opening edge of your eating window. To see both edges and how much has to fit between them, use the eating window calculator.

How to break it

There is no ritual here, and no food that does anything special. A normal balanced meal with protein in it is the whole recommendation. The two failure modes worth naming:

• Treating it as a reward. A very large meal after a long fast erases whatever deficit the hours created, which is the mechanism the results actually depend on.
• Eating too little. Short windows push some people the other way, into an accidental under-eat followed by a binge later.

The reason to stop waiting for a metabolic switch

A lot of fasting content implies that holding out another hour flips you into fat burning. The physiology does not line up with the schedules people run.

StatPearls’ Physiology, Fasting chapter says the liver carries blood glucose through roughly the first 24 hours of a fast, and that glycogen stores are depleted at around that point, after which the body turns to fat and protein. The current obesity-treatment chapter puts the same depletion at 42 hours. Two chapters, two estimates — and both are far past the end of a 16- or 18-hour fast.

The useful conclusion is not that fasting is pointless. It is that the switch people are waiting for does not occur inside an ordinary overnight-plus-morning fast, so there is nothing to be gained by white-knuckling an extra ninety minutes. The full 24-hour fast page covers what does change on longer schedules, and what does not.

When breaking early is the right call

Fasting is a tool, not an endurance test. Break it early if you get strong dizziness, real weakness or you simply feel unwell. StatPearls also flags a specific and common one: fasting headache, most likely at the start of a fasting habit, with hypoglycemia, dehydration and caffeine withdrawal all proposed as contributors.

One case is not a matter of preference. NIDDK lists fasting as a cause of low blood glucose in people who keep taking glucose-lowering medicines, and names insulin, sulfonylureas and meglitinides as the drugs that can drop blood sugar below 70 mg/dL. If you take any of them, an extended fast is a decision for your clinician, not for a calculator.

For the protocols and what the trials found, see the intermittent fasting calculator.

This content is informational and does not replace evaluation by a physician or dietitian.