24 Hour Fast: What Happens and How to Do It

Quick answer: A 24-hour fast means one meal to the same meal the next day — dinner Monday to dinner Tuesday — with only water, black coffee, plain tea and electrolytes in between. It is the first step past daily time-restricted eating, and the point where a fast starts to need planning rather than just patience. It is also the longest fast most people ever need to do.

The 24-hour fast is the entry point to extended fasting. It is long enough that your body clearly changes gear, short enough that you sleep through a third of it, and simple enough that you never have to count a window.

This page covers what one day without food does, how much of that is measured in humans and how much is inference, who should not attempt it, and how to run one so it goes well.

What a 24-hour fast looks like

You eat a normal meal, then eat nothing until the same time the next day.

Monday, 7 PM: dinner
Monday night through Tuesday: water, black coffee, plain tea, electrolytes
Tuesday, 7 PM: dinner

Dinner-to-dinner is the version most people find easiest, because it puts only one waking day inside the fast and you never wake up already hungry. Lunch-to-lunch works the same way. Breakfast-to-breakfast is the hardest, since it asks you to go to bed with the fast still running and then face a morning on top of that.

Done once or twice a week, this is exactly the protocol Brad Pilon popularised — Eat Stop Eat is a 24-hour fast on a weekly schedule, and nothing more complicated than that.

What happens over the 24 hours

Fasting charts, including the zone model on this site, put confident hour markers on this. Some of those markers rest on solid human measurement and some are estimates. It is worth knowing which is which.

Hours 0–12: running down the stores

You finish digesting, insulin falls, and the liver hands out stored glycogen to hold blood glucose steady. Nothing dramatic. If you already do 16:8 you have been here hundreds of times.

Hours 12–18: the switch

Liver glycogen runs low and the body shifts its main fuel to fat. Anton and colleagues' review in Obesity names this the metabolic switch and puts it beyond roughly 12 hours without food, at which point fatty acids are mobilised and ketone production begins climbing. This part is well measured. How fast you get there is not fixed — a big carbohydrate dinner, a hard training session, your body composition and how often you fast all move it.

Hours 18–24: ketones doing real work

Ketone bodies become a meaningful fuel, including for the brain. Cahill's review of starvation metabolism is the classic description of that handover: beta-hydroxybutyrate is what lets a human function without eating. Also well measured — though the deepest data on brain ketone use comes from fasts of weeks, not one day, so at 24 hours you are early in that curve, not at the end of it.

What is not well measured: the exact hour autophagy "switches on" in a human. Charts that give you a number are reporting a model, not a measurement, and that includes ours. There is human work on the signalling upstream of it, but no study establishes an hour at which a person's cells start recycling in earnest. Treat any precise autophagy timestamp as an estimate.

What a 24-hour fast has actually been shown to do

The most directly relevant trial is a randomised cross-over study by Horne and colleagues in 30 apparently healthy volunteers doing a single day of water-only fasting. It found acute increases in human growth hormone, haemoglobin, red blood cell count and HDL cholesterol, and decreases in triglycerides and weight. Most of those markers were back at baseline within 48 hours, weight and triglycerides aside.

Two things follow from that, and both matter.

The effects are real. A single day of fasting moves measurable things in healthy people, which is more than can be said for most one-day interventions.

The effects are mostly transient. Values returned to baseline within two days. A 24-hour fast is a stimulus you repeat, not a switch you flip once. Anyone selling a single day as a reset is overselling a study like this one.

On growth hormone

The very large growth hormone numbers that circulate online come from fasts longer than a day. Hartman and colleagues measured a five-fold increase in 24-hour growth hormone production in nine healthy men — on the second day of a fast, not the first. Horne's single-day trial did find growth hormone rose acutely, so the direction holds at 24 hours; the size does not transfer. If you see a percentage in the hundreds or thousands attached to a one-day fast, it has been lifted from a longer study.

On weight

The scale will drop, usually by a pound or two, and most of that is water and the glycogen it was bound to. It comes back with your next few meals. That is not a reason to skip the fast — it is a reason not to read the morning-after number as fat loss.

On whether it beats ordinary dieting

The best long-run comparison is Trepanowski and colleagues' one-year randomised trial of alternate-day fasting against daily calorie restriction. Fasting did not win. Weight loss was similar at 6 and 12 months, and the fasting group had the highest dropout rate at 38%.

Read that as useful rather than discouraging. Fasting works about as well as careful daily restriction, so pick the one you will actually keep doing. For many people that is a weekly 24-hour fast, precisely because it asks for effort one day instead of vigilance seven.

Will you lose muscle?

Not from one day. The direct human evidence on muscle protein breakdown during fasting comes mostly from longer fasts, so at 24 hours we are partly reasoning from the ends of the curve rather than from the middle of it — that is the honest position.

What there is points the same way. Soeters and colleagues put lean healthy men through two weeks of intermittent fasting and found no change in whole-body protein breakdown compared with a standard diet. If a fortnight of repeated fasting does not move proteolysis, one day is not going to strip your legs.

The real muscle risk in fasting is not the fast, it is what surrounds it: too little protein overall, too little resistance training, and doing this too often. Does fasting cause muscle loss goes through that in detail.

Who should not do a 24-hour fast

Do not fast for a day without talking to a clinician first if any of these apply:

You have diabetes, or take any medication that lowers blood sugar. Insulin and sulfonylureas can drive blood glucose too low, and the NIDDK's guidance on hypoglycemia lists skipping meals as a direct cause.
You take any prescription medication that has to be swallowed with food.
You are pregnant or breastfeeding.
You have any history of an eating disorder. Not "current" — any.
You are under 18.
You are underweight, or have been losing weight without meaning to.
You have a heart, kidney or liver condition, or a history of fainting or low blood pressure.

That is the same screen the extended fast planner runs before it will draw anything, and it is a full stop rather than a warning label. If one of them applies, the answer is a conversation with your doctor, not a shorter fast.

How to run one well

Eat a normal dinner first. Not a feast. Loading up on carbohydrate before a fast makes the first twelve hours harder, because you spend them on the blood-sugar rollercoaster you just built.

Sleep through the middle. Dinner-to-dinner means roughly eight of the twenty-four hours cost you nothing at all.

Drink more than you think. Two to three litres of water across the day is a reasonable target for most adults.

Take electrolytes. This is the single change that separates an easy 24-hour fast from a miserable one. When insulin falls, the kidneys flush sodium, and potassium and magnesium go with it. That is why a headache on a fast is usually a mineral problem rather than a hunger problem. The fasting electrolyte calculator gives amounts for your weight and fast length; electrolytes during fasting explains the mechanism.

Expect hunger in waves, not a ramp. It peaks near your usual meal times and passes. If you can sit through twenty minutes, the wave usually breaks on its own.

Keep moving gently, train lightly or not at all. A walk helps. A heavy session at hour 20 is a bad trade.

Stay busy. Boredom is the strongest hunger signal most people have.

Breaking a 24-hour fast

You do not need a ceremony, but you should not open with the biggest meal of your week either. Your gut has been idle and a large load lands badly.

Start with something moderate — protein and fat rather than a wall of fast carbohydrate — wait a while, then eat normally. The how to break a fast tool will build the sequence around your fast length.

On refeeding syndrome: you will see this raised in fasting forums. For a healthy, well-nourished adult it is not a 24-hour-fast problem. The criteria NICE uses to flag high risk start at little or no intake for more than 10 days on its own, or more than 5 days alongside another risk factor such as very low body weight or alcohol dependence. Refeeding syndrome, and who is actually at risk sets out the full list — worth reading if you are in one of the risk groups above, and worth ignoring the panic about if you are not.

How often should you do one?

Once a week is the common rhythm and a sensible ceiling for most people. Twice a week is the upper end of what a normal week absorbs, and past that you are effectively on alternate day fasting, which is a different commitment with its own dropout problem.

There is no evidence that more frequent 24-hour fasts pay better. There is evidence that adherence falls as the fasting gets harder, which is the practical argument for keeping this to a weekly habit you never miss rather than a schedule you abandon in a month.

How Fasted helps

Fasted runs a 24-hour timer with the metabolic phase shown as you pass through it, so the back half of the day has a shape rather than just a number. Checkpoint notifications tell you where you are, electrolyte logging keeps the minerals on schedule instead of remembered at hour 19, and your weekly fasts land in the same history as your daily ones — which is where you will see whether one day a week is actually doing anything for you.

Frequently Asked Questions

Is a 24-hour fast safe?

For a healthy adult with no contraindications, one day without food is a normal thing the body handles well — humans hold enough stored fuel to run far longer than that. It stops being routine if you have diabetes or take blood-sugar medication, take drugs that need food, are pregnant or breastfeeding, are under 18, or have any history of an eating disorder. For those groups a 24-hour fast is a medical decision rather than a lifestyle one.

What can I drink during a 24-hour fast?

Water, black coffee, plain tea and zero-calorie electrolytes. Nothing with calories, and nothing with sugar. Electrolytes are not optional on a fast this long for most people, since sodium loss is what causes the headaches and cramps people blame on hunger.

How much weight will I lose from one 24-hour fast?

Usually one to two pounds on the scale, and most of it is water and glycogen rather than fat. It returns as you eat again. The value of a weekly 24-hour fast is the repeated calorie deficit and the metabolic stimulus over months, not the number on the morning after.

Can I exercise during a 24-hour fast?

Walking, easy cycling and gentle mobility work are fine and often make hunger easier to sit with. Heavy lifting and high-intensity sessions are better placed on eating days: performance falls late in a fast, and the risk of feeling faint rises at exactly the point you are least able to judge it.

Is a 24-hour fast better than 16:8?

They do different jobs. Daily 16:8 is a habit that quietly shapes how much you eat; a weekly 24-hour fast is a bigger, less frequent stimulus that goes further into fat and ketone metabolism. Neither has been shown to beat careful daily calorie restriction for weight loss over a year, so the better one is whichever you will still be doing next spring. Plenty of people run both.

Will one day of fasting slow my metabolism?

No. Resting energy expenditure does not fall in the first day of a fast — measured work on short-term starvation shows it rising over the first few days rather than dropping, driven by noradrenaline. The metabolic slowdown people worry about belongs to sustained undereating over months, not to a single day without food.