What happens when you fast, hour by hour

Blood sugar usually peaks within an hour or so after a meal.

• Freckmann 2024: in 36 healthy adults wearing glucose sensors, blood sugar peaked about 50 minutes after a meal, on average.
• Freckmann 2007: in 24 healthy adults, the average time to the peak was 46 to 50 minutes, across different meals.
• American Diabetes Association 2025: advises measuring blood sugar 1 to 2 hours after the start of a meal, generally the time of the peak in people with diabetes.

By around 3 hours after a typical meal, it is usually back near its pre-meal level.

• Moebus 2011: in blood tests of 28,024 adults, blood sugar 3 hours after eating differed by less than 0.1 mmol/L from blood sugar after 8 hours or more without food.

Your insulin level keeps falling, so the signal that says “store energy” fades.

• Deru 2021: notes that insulin, and insulin compared with glucagon, fall markedly within the first 12 hours of a fast.

Around 9 hours after your last meal. Your blood sugar now stays steady without any food.

• American Diabetes Association 2025: for a fasting blood sugar test, a person counts as fasting after at least 8 hours without calories.

Your liver keeps it there, partly from its glycogen store and partly by making new sugar from other materials such as amino acids and glycerol. This is called gluconeogenesis.

• Petersen 2017: a review of how the liver makes sugar. After an overnight fast, gluconeogenesis supplies about half of the sugar the liver releases.

After a night without food the two share the work about equally; by the end of the first day, new sugar supplies most of it.

• Landau 1996: in healthy adults, new sugar made up about 47 % of the liver’s output after 14 hours without food, 67 % after 22 hours and 93 % after 42 hours.

As your liver’s sugar reserve runs low, your body starts to rely more on stored fat. Fat cells release fatty acids into the blood, and most of your tissues can burn them for energy.

• Mattson 2018: a review. When liver glycogen runs out during a fast or long exercise, the body makes ketones from fatty acids released by fat cells: a switch in the fuel cells use.
• de Cabo and Mattson 2019: a review of the same switch from sugar to fat and ketones, and of what intermittent fasting does for health, aging and disease.

Scientists call this the metabolic switch. When it happens differs from person to person: anywhere from about 12 to 36 hours, depending on your last meal and how much glycogen your liver held.

• Anton 2018: a review. The metabolic switch typically happens 12 to 36 hours after the last food, depending on how much glycogen the liver holds and how much energy the body uses, for example in exercise.

With sugar reserves low, your liver turns more fat into ketones, small molecules that your brain and muscles can use as fuel. Their level starts to climb and keeps rising over the following day.

• Puchalska and Crawford 2017: a review of ketones as fuel. Their level rises to about 1 mmol/L after 24 hours of fasting, from about 0.1 to 0.25 mmol/L on a normal day.
• Jamshed 2019: in 11 adults who ate only between 8 am and 2 pm, the 18-hour fast that followed raised their morning ketones, compared with eating over 12 hours.

In a study of healthy adults, light ketosis came at around 21 hours on average, or about 17–18 hours when the fast began with a workout.

• Deru 2021: 20 adults fasted for 36 hours twice. Their ketones reached 0.5 mmol/L, light ketosis, after 21.1 hours on average, and after 17.5 hours when the fast began with a treadmill workout.

In studies of healthy adults, growth hormone levels are already several times higher after about a day without food, and stay high on the second day.

• Hollstein 2022: in 47 healthy adults, growth hormone rose about 5-fold during a 24-hour fast.
• Hartman 1992: in 9 healthy men, on the second day of a fast the body made 5 times more growth hormone than on a normal day, in more frequent and larger bursts.

Growth hormone helps your body tap into fat reserves and is thought to help protect muscle while you aren’t eating.

• Nørrelund 2001: when growth hormone was blocked during a 40-hour fast in 8 healthy adults, muscle protein broke down faster, urea-nitrogen excretion (a sign of protein loss) rose by 50 %, and the body burned less fat.

Stress hormones such as norepinephrine also rise over the first days.

• Zauner 2000: in 11 healthy, lean adults fasting for 84 hours, norepinephrine roughly doubled from day 1 to day 4, and energy use at rest went up.

Your cells constantly recycle worn-out parts. This process is called autophagy, from the Greek for “self-eating”. Its mechanism won the 2016 Nobel Prize in Medicine.

• Nobel Assembly 2016: the prize went to Yoshinori Ohsumi for his discoveries of the mechanisms of autophagy.

Fasting is known to boost autophagy in animals.

• Chaudhary 2022: in mice, intermittent fasting raised markers of autophagy in the liver.
• Bensalem 2025: nutrient restriction stimulates autophagy in animal studies.

In people, scientists are still working out how much and how soon, and the exact timing is not yet established.

• Bensalem 2025: in 121 adults over 6 months, time-restricted eating may have raised autophagy in blood cells compared with standard care. It is an exploratory analysis, and the authors say further studies are needed.
• Chaudhary 2022: in 50 women, markers of autophagy in muscle went down, not up, with intermittent fasting.
• Masedunskas 2024: the plan of a trial in people. Its authors note a lack of human studies on fasting and autophagy.