A 24-hour fast means eating your last meal, then not eating again until the same time the next day. It sounds simple, but the execution matters. When you start, what you drink, how you handle hunger waves, and what you eat afterward all determine whether the experience feels manageable or miserable. Here’s how to do it well.
Pick Your Fasting Window
The most common approach is dinner-to-dinner: eat your last meal around 6 or 7 p.m., then don’t eat again until dinner the next day. This works for most people because you sleep through roughly a third of the fast and can stay busy during a normal workday. You skip breakfast and lunch, then sit down to a regular evening meal.
A lunch-to-lunch schedule is another option and avoids going to bed hungry on either end. Breakfast-to-breakfast fasts are less popular because they mean skipping dinner, which tends to be more social and harder to skip without friction.
There’s also a metabolic argument for choosing which meal to skip. A crossover trial found that skipping breakfast increased fat burning by about 16 grams per day compared to a normal three-meal day. But it came with a tradeoff: blood sugar after lunch spiked 46% higher and insulin resistance markers jumped 54% compared to skipping dinner instead. Skipping dinner actually produced a larger bump in total energy expenditure (about 91 extra calories burned versus 41 for breakfast skipping) without the same glucose disruption. If you’re concerned about blood sugar stability, a schedule where you eat breakfast and lunch but skip dinner may be metabolically cleaner, even if it’s socially harder.
What You Can Drink
You’re not eating, but you should be drinking steadily throughout the day. Plain water and sparkling water are the baseline. Black coffee and unsweetened tea are fine and can blunt hunger noticeably. Some people add a small splash of milk or cream to coffee; this technically introduces a few calories but won’t meaningfully disrupt the metabolic state of fasting for most people.
Diluted apple cider vinegar (1 to 2 teaspoons in a glass of water) is another option that some fasters use to manage cravings. Bone broth is worth keeping on hand, especially in the later hours, because it provides electrolytes like sodium and potassium that you’d normally get from food. Losing electrolytes through water alone is the main reason people get headaches or feel lightheaded during a fast.
What to avoid: anything with sugar, protein powder, or sweetened supplements. Protein triggers an insulin response that counteracts the metabolic benefits of fasting. Supplements containing maltodextrin, pectin, or fruit juice concentrate will break your fast the same way food would.
What Happens in Your Body
Your body stores about 12 to 24 hours’ worth of glycogen, a form of glucose packed into your liver and muscles. During a 24-hour fast, you gradually deplete those stores and your body begins shifting toward burning fat for fuel. This metabolic switch is the central event of the fast, and it’s why fasting feels different from simply eating less.
One of the more dramatic changes involves growth hormone. A randomized controlled trial measuring hormone levels during a 24-hour fast found that people who started with low baseline levels saw growth hormone surge by a median of 1,225%. Those who already had higher levels at the start saw a more modest 50% increase. Growth hormone helps preserve muscle tissue during periods without food and promotes fat breakdown, which is one reason short fasts don’t cause the muscle loss people worry about.
Fasting also improves insulin sensitivity. A large meta-analysis of fasting studies in people with metabolic syndrome found significant reductions in fasting blood sugar, circulating insulin, and insulin resistance scores. Levels of IL-6, a marker of inflammation, also dropped meaningfully. These effects are most relevant if you fast regularly rather than just once.
Autophagy, the cellular recycling process often cited as a fasting benefit, likely requires more time than 24 hours to fully activate. Animal studies suggest it begins somewhere between 24 and 48 hours of fasting, and there isn’t enough human research yet to pin down the exact timing. A single 24-hour fast may initiate the early stages, but it’s the metabolic and hormonal shifts that deliver the more reliable benefits at this duration.
How to Handle Hunger
Hunger during a fast isn’t a steady climb. It comes in waves driven by ghrelin, a hormone produced in your stomach. Ghrelin levels rise at the times your body expects food based on your usual eating schedule, then fall again even if you don’t eat. If you normally eat breakfast at 8 a.m., expect a wave around then. Another will likely hit around your usual lunchtime. Most people report that the period between hours 16 and 20 is the hardest, and that hunger actually fades in the final stretch.
Practical strategies that help:
- Stay busy. Boredom and hunger are hard to tell apart. Schedule your fast on a day with enough activity to keep your mind occupied, but not so much physical exertion that you feel depleted.
- Drink something warm. Black coffee, herbal tea, or warm water with a squeeze of lemon can satisfy the oral fixation component of hunger and take the edge off a ghrelin wave.
- Add a pinch of salt to your water. Low sodium is often the real culprit behind headaches, irritability, and that shaky feeling people mistake for dangerous low blood sugar. A quarter teaspoon of salt in a glass of water can resolve it quickly.
- Don’t exercise hard. Light walking or gentle movement is fine and can actually suppress appetite temporarily. But intense workouts will drain your remaining glycogen faster and make the back half of the fast significantly harder.
How to Break the Fast
After 24 hours without food, your digestive system has slowed down. Eating a large, heavy meal right away commonly causes bloating, nausea, or stomach cramps. The goal is to ease your gut back into work.
Start with something small and easy to digest. A handful of nuts, a few bites of avocado, a small portion of cooked vegetables, or a cup of bone broth are all good choices. Give yourself 20 to 30 minutes, then eat a normal-sized meal. Lean protein, cooked vegetables, and healthy fats are easier on your stomach than raw vegetables, fried food, or large portions of refined carbohydrates. A plate of grilled chicken with roasted sweet potatoes and olive oil is a better reentry than a pizza, even if the calorie count is similar.
Resist the urge to “make up” for missed calories by overeating. One of the most common mistakes with 24-hour fasting is consuming so much food in the remaining hours that the caloric deficit disappears entirely.
How Often to Do It
Most people who practice 24-hour fasting do it once or twice a week. This frequency is enough to accumulate metabolic benefits like improved insulin sensitivity and reduced inflammation markers without creating the kind of sustained caloric restriction that leads to muscle loss or hormonal disruption.
If you’ve never fasted before, start with a 16-hour fast (skip breakfast, eat lunch and dinner) and see how you feel. Once that’s comfortable, extend to 20 hours, then try a full 24. Jumping straight to 24 hours with no experience often leads to quitting at hour 14 and concluding that fasting “doesn’t work for you.”
Who Should Skip It
A 24-hour fast is not appropriate for everyone. People with a history of eating disorders can find that structured fasting triggers restrictive patterns or binge-restrict cycles. Pregnant or breastfeeding women have caloric and nutrient demands that a full day without food cannot meet. People at high risk of bone loss and falls, particularly older adults, may worsen their risk with regular fasting.
Common side effects even in healthy people include fatigue, dizziness, headaches, mood swings, and constipation. Women may notice changes to their menstrual cycle with repeated fasting. If you take medication for diabetes, fasting can interfere with blood sugar management in ways that require medical guidance before you try it.