A 24-hour fast means eating your last meal at a set time, then not eating again until the same time the next day. Dinner to dinner is the most common approach, but lunch to lunch works just as well. The method is straightforward, but the details of preparation, what you drink, and how you break the fast make the difference between a miserable experience and a manageable one.
What Happens in Your Body During 24 Hours
For the first 4 to 6 hours, your body is still processing your last meal. After that, it begins drawing on stored glucose in your liver and muscles. By roughly 12 to 16 hours in, those stores are depleted and your body shifts toward burning fat for fuel.
Insulin levels drop significantly during a full-day fast, which improves your cells’ ability to respond to insulin when you do eat again. Long-term alternate-day fasting has been shown to cut fasting insulin levels by 52% and reduce insulin resistance by 53% over 12 months, far more than standard calorie restriction. A single 24-hour fast won’t produce those results, but it moves the needle in the same direction each time you do it.
Growth hormone rises substantially during a 24-hour fast. A study published in Frontiers in Endocrinology found that people who started with low baseline levels saw a median increase of 1,225%, with some individuals experiencing increases up to 20,000%. People with higher starting levels saw more modest bumps. Growth hormone helps preserve lean muscle tissue and supports fat metabolism, which is part of why fasting doesn’t simply cause muscle loss the way skipping meals might suggest.
Cellular cleanup processes (autophagy) are often cited as a benefit, but the timeline is less clear-cut than popular content suggests. Animal studies indicate autophagy may begin between 24 and 48 hours of fasting, and there isn’t enough human research to pin down a precise trigger point. A 24-hour fast likely initiates early stages of this process, but it’s not guaranteed to reach peak levels.
How to Prepare the Day Before
Your last meal before the fast sets the tone for how you’ll feel over the next 24 hours. A combination of protein, whole grains, and vegetables gives your body a slow, steady source of energy. Good options include scrambled eggs with whole grain toast, oatmeal with yogurt, or a bowl built around farro with roasted vegetables and a protein source. These foods digest slowly, which helps manage hunger in the first half of the fast.
Avoid a last meal that’s heavy on refined carbohydrates or sugar. A big plate of pasta or a sweet dessert will spike your blood sugar and then crash it, leaving you hungrier sooner. Similarly, very salty foods will increase thirst and can make the early hours more uncomfortable than they need to be.
Pick a start time that makes sense for your schedule. If you eat dinner at 7 p.m., you’ll eat again at 7 p.m. the next day. Most of the first 8 to 10 hours will be spent sleeping, which eliminates a large chunk of the fast without any effort.
What You Can Drink (and What Breaks the Fast)
Water is your primary drink throughout the fast. Still or sparkling, both are fine. Plain black coffee and unsweetened tea are also widely considered acceptable because they contain essentially no calories and don’t trigger a meaningful insulin response.
If you want to add sweetness to coffee or tea, artificial sweeteners like stevia, sucralose, and monk fruit don’t raise blood sugar. Sugar alcohols (like sorbitol, xylitol, and mannitol) are a different story. They can raise blood sugar and may cause digestive discomfort, especially on an empty stomach. Skip them during the fast.
Anything with calories breaks the fast. That includes milk or cream in coffee, bone broth, juice, and diet drinks sweetened with sugar alcohols. A splash of cream won’t erase every benefit, but if your goal is a clean 24-hour fast, keep it to zero-calorie options.
Electrolytes matter. Adding a pinch of salt to your water or drinking mineral water can help prevent the headaches and lightheadedness that come from sodium depletion, especially if you’re active during the fast.
Getting Through the Hard Hours
The toughest stretch for most people falls between hours 16 and 22. By this point, your body has burned through its easy fuel, hunger hormones tend to peak, and you’re likely awake and surrounded by food cues. A few strategies help.
Stay busy. Boredom is a stronger hunger trigger than actual caloric need. Schedule your fast on a day when you have work, errands, or projects to focus on. Sitting at home with nothing to do makes every hour feel longer.
Light activity is fine and can actually suppress appetite temporarily. A walk, gentle yoga, or light housework all work well. Avoid intense exercise during your first few 24-hour fasts. Your energy expenditure does drop during a full day without food (one study in lean men found total energy expenditure fell from about 2,630 calories fed to 2,340 calories fasting), so your body is already conserving energy. Pushing hard physically on top of that can cause dizziness, nausea, or unusually poor performance.
Hunger comes in waves. It peaks and then fades, usually within 20 to 30 minutes. If you can ride out a wave with a glass of water or a cup of black coffee, the next stretch often feels easier.
How to Break a 24-Hour Fast
This is where most people make their biggest mistake. After not eating for a full day, the temptation is to sit down to a large, rich meal. Your digestive system has been resting for 24 hours, and hitting it with a greasy cheeseburger, a big bowl of pasta, or a slice of cake can cause bloating, cramping, and nausea.
Start with something gentle and small. Fermented foods like unsweetened yogurt or kefir are easy on the stomach and help reactivate digestion. Eggs, avocado, or a small portion of cooked vegetables are also good first foods. Wait 30 to 60 minutes, then eat a normal-sized meal if you’re still hungry.
Foods that are very high in fiber, fat, or sugar tend to be the hardest to tolerate right after a fast. Raw vegetables, nuts, seeds, and fried foods can all cause discomfort when they’re the first thing hitting an empty stomach. Ease in with cooked, soft foods first.
There’s no need to “make up” for the missed calories. Eat a normal meal. If you try to cram a full day’s worth of food into one sitting, you’ll likely feel terrible and negate many of the benefits you just earned.
How Often You Can Safely Fast
Most guidance suggests doing a 24-hour fast no more than once or twice per week. Healthcare professionals generally caution against doing it more frequently because there isn’t enough long-term research on the effects, and the likelihood of negative symptoms (fatigue, irritability, difficulty concentrating) increases with frequency.
If you’re new to fasting, start with one 24-hour fast and see how your body responds before making it a regular practice. Many people work up to it by first trying 16:8 fasting (eating within an 8-hour window) for a few weeks.
Who Should Skip a 24-Hour Fast
A 24-hour fast is not appropriate for everyone. People with diabetes face real risks from extended periods without food, including dangerous drops in blood sugar. If you take medications for blood pressure or heart disease, prolonged fasting can throw off your sodium, potassium, and other mineral levels. Anyone who takes medications that need to be taken with food to avoid nausea or stomach irritation will have trouble maintaining their medication schedule during a full-day fast.
People who are already at a low body weight or underweight should avoid 24-hour fasts. Losing additional weight can weaken bones, suppress the immune system, and drain energy levels. Pregnant or breastfeeding women, children, and anyone with a history of eating disorders should also steer clear.