Fasting timeline: what happens hour by hour

A fasting timeline shows what your body does from the moment you finish eating. For the first day, the intermittent fasting timeline follows the same stages. The clock starts at your last bite. For the first few hours you run on that meal, then on sugar stored in your liver, and later on more fat and ketones. The hours below are the ones the site’s fasting timer and calculator use, and every one of them is approximate.

The fasting timeline at a glance

Hours since last meal	Stage on this site	What happens	How strong the evidence is
0 to 4	Fed	You digest your meal. Blood sugar and insulin rise, and the body runs mainly on glucose.	Strong (well-established human physiology)
4 to 12	Early fasting	Insulin falls. The liver breaks down glycogen to keep blood sugar steady.	Strong (well-established human physiology)
12 to 24	Metabolic switch	With glycogen running short, more fat is burned and ketone production begins. In studies the timing ranges from 12 to 36 hours.	Strong that it happens; the hour varies
24 and beyond	Longer fast	Fat burning and ketones keep rising, and growth hormone goes up.	Strong for fat and ketones; early for growth hormone (one small study)

Four stages of intermittent fasting are used across this site. The fasting timer shows each stage name live as your fast runs. Autophagy has no row, for the reason set out below.

0 to 4 hours: fed

Right after a meal, food is still being digested and absorbed. Blood sugar rises, insulin rises with it, and your cells use glucose as their main fuel. Some of the spare glucose is stored as glycogen in the liver and muscles.

Nothing here is special to fasting. Everyone passes through this stage after every meal.

4 to 12 hours: early fasting

Once the meal is absorbed, insulin drops back. To hold blood sugar level, the liver draws on its glycogen and releases glucose into the blood. For most people, an overnight fast sits in this stage.

Some people get a mild headache once they pass 8 hours without food. Doctors recognise this as a “fasting headache”. It usually eases after eating, and its exact cause is not known.

12 to 24 hours: the metabolic switch

The liver’s glycogen store is limited. As it runs low, the body draws more on fat. The liver also starts making ketones from fat. Researchers call this the metabolic switch.

In human studies the switch happens somewhere between 12 and 36 hours after eating. The spread is wide because it depends on how full your glycogen store was at the start. That comes down to what you last ate and your activity level. So the site’s 12-hour mark is the start of that range, and a fast in this stage may not have switched yet.

What does 20 hour fasting do? It sits in this stage, with the same uncertainty. The 20:4 schedule page covers that window in detail.

24 hours and beyond: a longer fast

After a full day, fat burning and ketone levels keep climbing. Growth hormone rises too. A study of six men measured this during fasts of 1 day and 5 days, so that evidence is early. In the first days of a long fast, the kidneys also pass more sodium and water, which may add to headaches. How soon that starts differs from person to person.

Fasting past 24 hours is a step to take only with medical advice. That matters most for anyone pregnant, on regular medicines or living with a health condition. Fasts measured in days carry a further risk. Once a fast passes 5 days with very little food, the first meals can trigger refeeding syndrome. Past 10 days, that risk is high.

Where does autophagy fit in the timeline?

Autophagy is how cells take apart and reuse their own worn-out components. Many fasting timelines put it at 16 hours, 24 hours or later. No human study shows when it starts, so this site gives it no hour.

Most of the research is in animals and cells. In people, the evidence so far is a single small study. Eleven adults ate between 8 am and 2 pm for 4 days. A blood marker linked to autophagy went up. That is early evidence, and it says nothing about the hour. Our guide to autophagy and fasting goes through the research.

Where each daily schedule ends on the timeline

Schedule	Fast length	Stage reached by the end
12:12	12 hours	The edge of the metabolic switch stage
14:10	14 hours	Metabolic switch stage
16:8	16 hours	Metabolic switch stage
18:6	18 hours	Metabolic switch stage
20:4	20 hours	Metabolic switch stage
OMAD	about 23 hours	Late in the metabolic switch stage

Every daily schedule ends before the longer-fast stage. The comparison of every schedule covers all of them, including weekly ones such as 5:2.

How intermittent fasting works on your body over weeks

The hourly stages repeat every day. Weight change builds up over weeks. In an 8-week trial, people on a 4-hour or 6-hour window cut their intake by roughly 550 kcal a day with no calorie counting. When trials matched calories, fasting schedules and ordinary calorie cutting gave about the same weight loss. Human trials point this way, so that evidence is moderate.

A 2026 review of 22 trials listed eating fewer calories first among the reasons fasting leads to weight loss. Some of the first week’s drop on the scale is water. Glycogen is stored with roughly 3 to 4 g of water per gram, so a smaller glycogen store weighs less.

To see which stage your current fast has reached, start the fasting timer from the time you finished your last meal.