What to expect in the first 24 hours of fasting?

October 23, 2025 •

4 min read

Fasting for a full 24 hours triggers a fascinating metabolic switch, with the body shifting from burning glucose for energy to tapping into its fat stores. Understanding what to expect in the first 24 hours of fasting? can help you navigate the process with confidence and awareness.

Quick Summary

This guide details the physical and mental changes during the initial 24-hour fasting period, outlining the shift from glycogen use to fat burning and discussing potential side effects and benefits.

Key Points

Metabolic Switch: The body shifts from burning glucose from your last meal to using stored glycogen, then finally to breaking down fat and producing ketones for fuel.
Glycogen Depletion: In the first 12–24 hours, your liver and muscle glycogen reserves are used up, signaling your body to find alternative energy sources.
Ketosis Begins: As glycogen is depleted, your body starts producing ketones from fat, a process called ketogenesis that provides an alternative fuel for your brain and muscles.
Common Side Effects: Expect temporary side effects like hunger pangs, headaches, fatigue, and irritability as your body adjusts to the lack of glucose.
Cellular Repair: As you approach the 24-hour mark, cellular autophagy begins, where your body recycles old and damaged cells, promoting cellular health.
Gentle Reintroduction: It is important to break your fast with small, easily digestible, and nutrient-dense foods to avoid overwhelming your digestive system.

For many, the idea of abstaining from food for an entire day seems daunting, but a 24-hour fast is a well-established form of intermittent fasting with notable physiological effects. Your body is remarkably adaptive, and undergoing a full day without caloric intake initiates a series of metabolic transitions. This article provides a comprehensive timeline of the physical and psychological changes you can anticipate, along with practical tips to make the process more manageable.

The Fed State (0–4 Hours)

Immediately after your last meal, your body enters the fed state. At this point, it is busy digesting and absorbing nutrients. Your pancreas releases insulin, which helps move glucose from the bloodstream into your cells for immediate energy or to be stored as glycogen in your liver and muscles. During these first few hours, you will feel normal and energized as your body processes this recent caloric intake.

The Early Fasting State (4–12 Hours)

As your body finishes digesting and blood glucose levels begin to drop, your insulin levels also decrease. Your body then turns to its glycogen reserves for a steady energy supply. During this phase, you might start to feel the first gentle pangs of hunger, but for most people, these initial hunger signals are relatively mild and can be managed with hydration and distraction. Drinking plenty of water is crucial during this stage, as it helps prevent dehydration and can curb some hunger cues.

Transition to Fat Burning (12–24 Hours)

As you move deeper into the fast, your body’s glycogen stores become depleted. Most people's liver glycogen reserves are exhausted somewhere between 18 and 24 hours into the fast. This is the critical moment of the metabolic switch, where your body starts breaking down stored fat (lipolysis) to create an alternative fuel source called ketones (ketogenesis). The physiological changes are more pronounced during this phase:

Entry into Ketosis: Your body shifts from being a sugar-burning machine to a fat-burning one, a state known as ketosis. Your brain and other organs start using these ketones for energy.
Cellular Repair (Autophagy): As you near the 24-hour mark, cellular “housekeeping,” or autophagy, begins to intensify. This process helps your body recycle damaged cellular components and is linked to potential benefits like reduced inflammation and anti-aging effects.
Hormonal Changes: Fasting triggers a significant increase in human growth hormone (HGH), which helps preserve muscle mass and supports fat metabolism. Your body also becomes more sensitive to insulin, which is a major metabolic benefit.

Managing Common Side Effects

While a 24-hour fast offers notable benefits, it can also come with some temporary discomfort, especially for those new to the practice. Being prepared can make a significant difference.

Hunger Pangs: The hunger hormone, ghrelin, can peak around the 16-24 hour mark before subsiding. Staying hydrated and busy can help you push through these waves.
Headaches: Headaches are a common side effect, often caused by dehydration, low blood sugar, or caffeine withdrawal if you are a regular coffee drinker. Drinking water and, if you consume caffeine, having black coffee or tea, can alleviate this.
Fatigue and Dizziness: As your blood sugar levels dip and your body switches fuel sources, you might feel a temporary dip in energy, dizziness, or lightheadedness. Gentle activities like walking can help, but intense exercise is not recommended during this time.
Irritability and Brain Fog: Some people experience irritability or difficulty concentrating as their body adjusts. Paradoxically, others report enhanced mental clarity in the later hours as ketones become the primary fuel source for the brain.

Preparing for and Completing Your Fast

To optimize your 24-hour fast, preparation is key.

Before the fast:

Eat a nutritious, balanced meal rich in protein, healthy fats, and fiber to promote satiety.
Choose a day with a less demanding schedule to minimize stress and exertion.
Make sure you are well-hydrated before starting.

During the fast:

Drink plenty of water throughout the day. You can also have black coffee or unsweetened tea, which may help with appetite suppression.
Stay busy with work, hobbies, or light activities to distract yourself from hunger.
Listen to your body. If you feel unwell, it's okay to break the fast early.

How to Break Your Fast Gently

Ending your fast thoughtfully is as important as the fast itself to avoid digestive upset or binge-eating.

Start small and light: Begin with a small portion of easy-to-digest food, like bone broth, steamed vegetables, or a small smoothie.
Avoid heavy, processed foods: Steer clear of high-sugar, high-fat, and highly processed foods, which can overwhelm your digestive system and cause bloating or nausea.
Prioritize nutrition: Your first meal should be nutrient-dense. Consider lean protein (fish, eggs) and healthy fats (avocado, nuts) along with low-glycemic carbs.

24-Hour Fast: Timeline Comparison


Phase	Duration	Primary Energy Source	Common Feelings
Fed State	0-4 hours	Dietary glucose from meal	Satiated, normal energy
Early Fasting	4-12 hours	Glycogen from liver and muscles	Mild hunger, slight energy dip
Late Fasting	12-24 hours	Stored fat (lipolysis), ketones	Hunger fluctuates, potential fatigue, mental clarity or fog
Post-Fast	First meal after 24 hrs	Gentle reintroduction of nutrients	Potential digestive sensitivity, renewed energy

Conclusion

The first 24 hours of fasting is a period of significant metabolic adaptation for your body, marked by the depletion of glucose stores and a shift toward using fat for fuel. While common side effects like hunger and fatigue may arise, they are a normal part of the process and are often manageable. As your body adapts, you can begin to experience benefits like increased insulin sensitivity and the initiation of cellular repair. Proper preparation, hydration, and a gentle approach to breaking your fast are crucial for a positive experience. Always consider your personal health and consult with a healthcare professional before starting any new fasting regimen, particularly if you have pre-existing health conditions or are new to the practice. For more in-depth scientific context on the body's response to fasting, the National Center for Biotechnology Information offers comprehensive resources on the physiological changes involved.

Frequently Asked Questions

Hunger levels often fluctuate during a fast. You may experience strong hunger signals around the 12-24 hour mark as the hunger hormone ghrelin peaks, but this feeling typically subsides as the body shifts into ketosis.

Headaches are a common side effect, especially for first-timers. They can be caused by low blood sugar, dehydration, or withdrawal from caffeine. Staying well-hydrated with water can often help mitigate these symptoms.

You should drink plenty of calorie-free beverages, including water, black coffee, or unsweetened tea, to stay hydrated. Some people also include electrolyte-enhanced water.

Contrary to popular belief, a 24-hour fast is unlikely to cause significant muscle loss. The body prioritizes burning fat and conserving muscle, partly due to the release of human growth hormone (HGH).

Break your fast gently with a small portion of easily digestible food, such as a broth-based soup, steamed vegetables, or a small smoothie. Avoid large, heavy meals or sugary, processed foods to prevent digestive discomfort.

Certain individuals, including pregnant or breastfeeding women, children, teenagers, and people with a history of eating disorders or certain medical conditions like type 1 diabetes, should avoid fasting without medical supervision.

Autophagy is a cellular recycling process where the body breaks down and removes old or damaged cells. It is initiated during fasting and begins to intensify as you approach the 24-hour mark.