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For the first few hours, you’re still digesting your last meal. As that supply winds down, your liver breaks down stored carbohydrate to release glucose and also makes glucose, while your body gradually relies more on fat. Hunger may come and go around your usual mealtimes. Without water, dehydration becomes an added risk. If your vrat includes milk, juice or fruit, those foods supply energy, so the pattern depends on what, how much and when you eat. There isn’t one clock that fits every fast.
It matters more than you might think, because food and fluid change the picture. Practices vary between families and traditions, but these are four common patterns:
Milk, juice, fruit, dry fruit and nuts supply calories and nutrients. Your body processes them, so this is different from a period with no calorie intake. That doesn’t make the observance less meaningful; it means the hour-by-hour story below applies most closely to periods without food or calorie-containing drinks. Either way, it all starts with the last meal.
Whether it’s sargi, sehri or the evening Kharna prasad before Chhath’s long fast, the last meal provides food to digest for several hours. Mayo Clinic gives an average of about six hours for food to pass through the stomach and small intestine, while stressing that timing varies with the person, the food and the amount eaten. That estimate covers both organs; stomach-emptying time varies. A familiar meal with whole grains and protein from dal or curd may feel more sustaining than just sweets.
The British Nutrition Foundation’s Ramadan guidance suggests wholegrain starches, adequate fluids when drinking is allowed, fluid-rich foods and less salty food such as pickles. Those are meal-planning ideas; no pre-fast meal can remove the dehydration risk of going without water.
Count from your last food or calorie-containing drink. These are approximate, overlapping stages, not events that happen to everyone at a set hour. Meal size, activity and individual health affect the timing. A religious fast’s start and finish need not match this metabolic clock.
You’re still digesting and absorbing food. Glucose from the carbohydrate in your meal enters the blood, and the hormone insulin helps the body use it and store some as glycogen—a storage form of glucose—in the liver and muscles. How long this fed phase lasts depends on the meal.
As absorption from the meal decreases and insulin falls, the liver helps maintain blood glucose by breaking down glycogen. It also makes new glucose; these processes overlap. Hunger may show up around a familiar mealtime. In a pilot study of six healthy volunteers fasting for 24 hours, ghrelin, a hormone involved in hunger, rose and then fell around customary mealtimes despite no food being eaten. Hunger may come in waves, rather than simply rising all day.
Your gut gets busy in its own way, too. Between meals, a recurring pattern of contractions called the migrating motor complex moves through the stomach and small intestine. Eating interrupts this fasting pattern. It is part of normal gut movement between meals.
Your body is using a mixture of fuels. As the period without food lengthens, fat can make a greater contribution while the liver continues helping supply glucose. This is a gradual adjustment, not the moment when fat burning suddenly begins.
You may notice lower energy, irritability or a harder time concentrating. Fasting can be associated with headache, but a headache during a fast can have several explanations. For regular chai or coffee drinkers, caffeine-withdrawal symptoms typically begin 12–24 hours after the last caffeine. That is a separate clock from the time since your last meal.
With no further calories, liver glycogen falls and the contribution of fat and ketones—fuels made largely by the liver from fat—can increase. A 2018 review describes the “metabolic switch” at roughly 12–36 hours after food intake stops, depending on starting liver glycogen and energy expenditure. It describes a broad transition, not a universal threshold or a reason to extend your fast.
Karwa Chauth ends at moonrise, so its length varies by location and date. The principal Chhath nirjala observance lasts around 36 hours after Kharna prasad. These observances run on different ritual timetables and have different demands.
Without water, the longer fasts raise greater concern about dehydration and kidney problems. Evidence on prolonged dry fasting is limited and does not establish a safe duration. Get individual medical advice in advance and discuss a modified observance if needed. The warning signs below matter more than completing an hour band.
After a day without food, a big, heavy meal can feel like a lot to land. The British Nutrition Foundation suggests fluids and fluid-rich foods when breaking a Ramadan fast, with slower eating and less over-indulgence. Start with water at a comfortable pace, then a modest, familiar balanced meal. Fruit or dates can be part of it if you enjoy them.
A large fried feast can leave you heavy or uncomfortable. Give yourself time to eat at a comfortable pace, and go back for more if you need it. Follow any prescribed fluid limit or diabetes plan.
This is where it’s worth being careful, because the internet is full of big claims. Here’s what the research has actually found.
On weight: in a year-long trial of 100 adults with obesity, an alternate-day regimen that allowed some food on “fast” days produced similar weight loss to daily calorie restriction, with more dropouts. In the 12-week TREAT trial, eating between noon and 8 pm did not produce significantly more weight loss than three structured meals.
A 2025 review of 99 randomised trials found broadly similar weight outcomes with intermittent fasting and continuous energy restriction, with a modest advantage for alternate-day fasting, mainly in shorter trials. The format and person matter. These studies do not establish a weight-loss benefit from avoiding water.
On religious fasts: a review of 70 publications involving 2,947 participants found average weight loss of about 1.3 kg during Ramadan. Much of it was regained in the following two to five weeks, although average weight remained slightly below the starting level at follow-up.
On the metabolic switch: the change in fuel use is real physiology. Animal studies suggest wider effects, but translating those findings into human health benefits requires human evidence. A change in fuel use is not itself proof of a health benefit.
On toxins: fasting is not needed to clean the body out. Your liver, kidneys and digestive tract process and remove waste every day.
And fasting without water? Good evidence for extra health benefits from withholding water is lacking. One small exploratory study of 34 healthy Bahá’í volunteers found no clinically relevant dehydration and no apparent change in body water during repeated daytime fasts in March, with food and fluids allowed at night. That finding cannot establish safety for a continuous 36-hour fast, hot-weather fasting or people with medical conditions.
The value of a nirjala vrat can lie in what it means to you; it does not need a promised physiological benefit to be meaningful. Safety depends on the person, the type and length of fast, the weather and activity.
Please speak to your doctor before a fast, especially a nirjala one, if you have diabetes, kidney or heart disease; take regular medicines; are pregnant or breastfeeding; are older; or have a history of an eating disorder. Diabetes UK highlights fasting risks, particularly for people taking insulin or medicines that can cause low blood sugar. Do not change medication or its timing on your own.
If you feel unwell, break the fast. Thirst, headache, light-headedness, dry mouth, tiredness, and dark or reduced urine can be signs of dehydration. If fully alert and able to swallow safely, start drinking water. Our guide to water intake and digestion explains how fluid needs vary.
If you have diabetes and develop low blood sugar, break the fast and follow your usual hypo-treatment plan. Water alone does not treat a hypo.
Unexplained weight loss, blood in the stool, persistent pain or difficulty swallowing need medical assessment. Get urgent advice for abdominal pain with blood in the stool. Sudden, severe abdominal pain or vomiting blood needs emergency care. If symptoms continue or keep coming back, speak to a doctor.
This is general information. For children, during pregnancy, or for older adults, follow a doctor’s guidance before making changes.
Ask Happy Tummy GPT: request general ideas for the meals before and after your fast, using foods you enjoy. Map the week around it with My Meal Plan. Take questions about whether or how to fast with a medical condition to your doctor.
1. Mayo Clinic. Digestion: How long does it take?
2. British Nutrition Foundation. A healthy Ramadan.
5. Anton et al. Flipping the Metabolic Switch. Obesity (2018); 26:254–268.
8. International Headache Society. ICHD-3, 10.5: Headache attributed to fasting.
10. NHS. Dehydration: symptoms, urgent advice and emergency care.
11. NHS. 5 A Day: what counts? Dried-fruit portion guidance.
13. Lowe et al. The TREAT randomised clinical trial. JAMA Internal Medicine (2020); 180:1491–1499.
16. Cleveland Clinic. Detox or Cleanse? What To Know Before You Start.
17. Cleveland Clinic. Dry Fasting: Why You Should Avoid It.
18. Diabetes UK. Religious fasting and diabetes.
19. NHS. Low blood sugar (hypoglycaemia): treatment and swallowing safety.
20. Bihar Tourism. Chhath Puja: Kharna and the sequence of observance.
21. Akashvani News. Chhath Mahaparv: the 36-hour fast following Kharna prasad.
22. NHS. Stomach ache: warning signs and medical care.
23. NHS. Dysphagia (swallowing problems).
Disclaimer: This article is for general information only and is not a substitute for medical advice, diagnosis or treatment. The timings are approximate and do not establish a safe fasting duration. If you have a health condition, take regular medicines, are pregnant or breastfeeding, or have a history of an eating disorder, speak to your doctor before fasting. Follow any prescribed fluid or dietary restriction and seek medical help for the warning signs described above.
Your body continues digesting the last meal, then relies more on liver glycogen and other stored fuel as food absorption falls. The liver also makes glucose, and fat use can increase gradually. The timing varies; 12 hours is not a universal switch. Without water, dehydration is an additional risk.
Liver glycogen falls further and fat and ketones can contribute more energy. A review describes a broad transition around 12–36 hours without food, rather than a precise 24-hour event. Longer fasting is not automatically healthier. Going without water for this long raises additional safety concerns.
It increases dehydration risk, and good evidence of extra health benefits from withholding water is lacking. Safety depends on the type and length of fast, heat, activity, medicines and individual health. Get medical advice before a dry fast, especially a prolonged one or if you have a health condition.
Milk and juice break a zero-calorie fast because they supply energy and nutrients. The metabolic effect depends on how much and when you have them. They may still be allowed within your religious observance; that is a different question from whether you are consuming calories.
Some intermittent-fasting schedules can help with weight loss, generally with results similar to continuous calorie restriction; some trials show a modest advantage for alternate-day fasting. Those findings do not prove benefits from avoiding water. Weight lost during Ramadan is often partly regained afterwards.
Start with water at a comfortable pace, then a modest, familiar balanced meal. Fruit or dates are an option. Eat slowly and avoid making the first meal a large fried feast. Follow any individual medical, fluid or diabetes plan.
