Why Do I Get Headaches During a Fast? Common Side Effects Explained

Someone is 14 hours into their fast. Everything feels fine until a dull ache starts spreading across their forehead. This happens to many people who try intermittent fasting. There’s a reason for it, and there are ways to fix it.

Research shows that fasting headaches are surprisingly common. Approximately 61% of people experience headaches while fasting. In one study, 39% of people who fasted developed headaches, compared to only 7% who didn’t fast. This means fasting headaches are normal, not a sign that something is wrong.

This article explains why headaches happen during fasting, what they feel like, and what people can do to prevent them. It also covers when someone should worry about their symptoms and seek medical help.

How Common Are Fasting Headaches?

Fasting headaches affect more than half of people who try intermittent fasting. These headaches typically occur after at least 16 hours of fasting. The pain usually appears in the middle or toward the end of the fasting window.

People with a history of headaches are more likely to develop fasting-induced headaches, with 66% experiencing them compared to 29% of those without such history. This makes sense. Someone who already gets tension headaches or migraines will likely be more sensitive to fasting triggers.

The likelihood of developing a fasting headache increases directly with the duration of the fast. A 12-hour fast might not cause problems, but a 16-hour or 18-hour fast increases the chances. During Ramadan fasting, 41% of people who fasted reported headaches.

According to Mayo Clinic, most side effects including headaches go away within a month. The body adapts. People who track their symptoms in a fasting tracker often notice improvements after the first two weeks. Check out our fasting tracker to monitor your progress.

What Does a Fasting Headache Feel Like?

Fasting headaches have distinct characteristics. They’re usually diffuse or located in the frontal region at the front of the head. The pain sits behind the forehead and between the eyebrows.

The pain is nonpulsating and of mild or moderate intensity. This is different from a migraine. Fasting headaches don’t throb or pound. They feel more like pressure or a dull ache. They’re typically bilateral, affecting both sides of the head, and frontal in location.

Someone may also experience dizziness and weakness along with the headache. The combination of these symptoms can make it hard to focus or get through the day. Most people describe it as feeling similar to a tension headache.

Why Do People Get Headaches During a Fast?

Five main factors cause fasting headaches. Understanding these causes helps people prevent them.

Low Blood Sugar (Hypoglycemia)

The brain isn’t getting the energy it needs from blood sugar (glucose) in food. The brain uses about 20% of the body’s total energy. When someone fasts, glucose levels drop. Hypoglycemia has been especially implicated as a causative factor in fasting headaches.

The brain notices this energy shortage first. It sends pain signals as a warning. This explains why the headache often feels like it’s right behind the forehead, where the frontal lobe sits.

Dehydration

This is one of the most overlooked causes. People often forget to drink enough water while fasting. Headaches are a common symptom of dehydration.

When someone fasts, they lose the water content from food. Most people get about 20% of their daily water intake from meals. Without those meals, dehydration happens quickly. Dehydration can disturb the balance of sodium, potassium and magnesium in the body, which makes headaches worse.

Caffeine Withdrawal

Many people have coffee or tea with breakfast. When they skip breakfast during a fast, they accidentally skip their caffeine too. Caffeine withdrawal has been especially implicated as a causative factor in fasting headaches.

Many people rely on caffeine to stay alert or focused. When the body doesn’t get its expected dose, withdrawal symptoms kick in within 12-24 hours. In studies of religious fasting, the most important exogenous-associated factor was caffeine withdrawal.

Low Sodium (Electrolyte Imbalance)

This causes surprises for most people. Fasting provokes increased sodium loss through urine. The body flushes out more salt during fasting periods.

If sodium isn’t replaced, low serum sodium (hyponatremia) can develop, with symptoms including headache, fatigue, weakness, and brain fog. Even mild sodium loss can trigger head pain. This is why athletes drink electrolyte beverages, and the same principle applies to fasting.

Stress Hormones

Fasting can cause a person’s cortisol levels to increase. Cortisol is the body’s stress hormone. Fasting puts mild stress on the body, raising cortisol levels, which can trigger tension headaches.

Higher cortisol makes muscles tense, especially in the neck and shoulders. This tension radiates up into the head, creating that characteristic pressure feeling.

When Do Fasting Headaches Typically Start?

Timing matters. Headaches occur after at least 16 hours of fasting. Someone doing a 12-hour overnight fast might feel fine, but extending to 16 or 18 hours often triggers symptoms.

The highest headache frequency on fasting days 2-4. The first few days are the hardest. The body hasn’t adapted yet. Good news comes after that initial period. There’s a sharp decline after day 10.

The headache resolves within 72 hours after resumption of food intake. Once someone eats, symptoms improve quickly. But waiting three days between fasts isn’t practical for most people.

According to Johns Hopkins, it can take two to four weeks before the body becomes accustomed to intermittent fasting. During this adjustment period, tracking symptoms in a fasting tracker helps people see progress and stay motivated.

How to Prevent Headaches While Fasting

Prevention works better than treatment. These six strategies help most people avoid fasting headaches.

Stay Hydrated

Water is the simplest solution. People should continue to drink water regularly, adding a pinch of salt if needed or drink electrolyte-rich beverages. Drinking water throughout the fasting window keeps the brain hydrated and functioning properly.

Someone should aim for at least 8 glasses of water during their fasting hours. More is better, especially for longer fasts.

Manage Caffeine Intake

Don’t quit caffeine and start fasting at the same time. If planning to eliminate caffeine during fasting, a person should gradually reduce caffeine intake to minimize withdrawal symptoms.

Better yet, drink black coffee or tea during the fasting window. These beverages have almost no calories and won’t break most fasts. They prevent withdrawal headaches while supporting the fast.

Add Electrolytes

Replacing sodium during fasting helps prevent hyponatremia. Someone can add a pinch of salt to their water or use electrolyte supplements. This simple step makes a big difference for many people.

Sugar-free electrolyte powders work well. They provide sodium, potassium, and magnesium without breaking the fast.

Start Slowly

Jumping into long fasts causes more problems. Gradually transitioning by starting with short periods and gradually increasing fasting duration helps the body adapt without triggering headaches.

Johns Hopkins researchers recommend narrowing the time window of eating gradually. Someone might start with 12 hours of fasting for one week, then move to 14 hours the next week, and finally reach 16 hours. This progression gives the body time to adjust. 

Our AI Assistant can help create a personalized progression plan based on individual tolerance.

Eat Balanced Meals

What someone eats during their eating window matters. During the eating window, balanced meals with a mix of good carbs, protein and healthy fats help prevent sugar crashes.

Complex carbohydrates, like potatoes and other starchy vegetables, paired with some protein work especially well. These foods provide steady energy that lasts into the fasting period.

Get Enough Sleep

Lack of sleep may have been contributory in some cases of fasting headaches. Poor sleep makes any headache worse. Someone who is fasting should prioritize 7-8 hours of sleep each night. This helps the body handle the stress of fasting better.

When Should Someone Worry About Fasting Headaches?

Most fasting headaches are harmless. But certain symptoms require medical attention.

Someone should talk to their doctor if they experience unusual anxiety, headaches, nausea or other symptoms after starting intermittent fasting. Specifically, watch for these warning signs:

• Headaches lasting longer than 72 hours
• Severe headaches that come on quickly, which may be a sign of a stroke
• Headaches with vision changes or slurred speech
• Headaches that don’t respond to over-the-counter medications or require more than the recommended medication dose

Some people shouldn’t fast at all. Skipping meals is not recommended for people under 18, those with a history of disordered eating, or those who are pregnant or breastfeeding. Anyone with diabetes or other medical issues should talk with their health care team before starting any type of fasting regimen.

Find detailed safety information in Who Should Avoid Intermittent Fasting? Your Ultimate Safety Guide.

Other Common Side Effects of Fasting

Headaches aren’t the only symptom people experience. Side effects could include hunger, fatigue, insomnia, irritability, decreased concentration, nausea, constipation and headaches.

In Johns Hopkins studies, a few participants reported modest side effects including constipation, loose stools, and occasional headaches. These symptoms are temporary. Most side effects go away within a month as the body adjusts to the new eating pattern.

The first two to four weeks are the hardest. After that, most people feel better than they did before starting to fast. Tracking all symptoms in a fasting tracker helps people see this improvement over time and stay committed to their goals.

The Bottom Line

Fasting headaches happen to about 61% of people who try intermittent fasting. Five main causes create these headaches: low blood sugar, dehydration, caffeine withdrawal, low sodium, and increased stress hormones.

These headaches usually start after 16 hours of fasting. They feel like dull, non-throbbing pressure at the front of the head. But they’re temporary. Most people see improvement within the first month as their body adapts.

Prevention strategies work. Staying hydrated, managing caffeine, adding electrolytes, starting slowly, eating balanced meals, and getting enough sleep help most people avoid fasting headaches completely.

Someone experiencing persistent or severe symptoms should talk to their doctor. But for most people, fasting headaches are a normal part of the adjustment period. 

With the right approach and tools like  fasting trackers and AI Assistants for personalized guidance, people can manage these headaches and enjoy the benefits of intermittent fasting.