Headaches During Fasting

Why fasting causes headaches, and the specific steps that resolve them in most cases within an hour.

The Most Common Cause: Sodium Loss

When insulin drops during a fast, the kidneys excrete sodium at an increased rate. Low sodium causes blood volume to decrease slightly, which can trigger tension-type headaches. This is by far the most common cause of fasting headaches, and it is easily addressed.

The Quick Fix

Dissolve 1/4 to 1/2 teaspoon of sea salt or Himalayan salt in a large glass of water and drink it. For many people, the headache resolves within 30–60 minutes. If you have been avoiding salt because you thought it was universally unhealthy, it is worth knowing that the research on sodium is more nuanced than headlines suggest — and during fasting, you genuinely need it.

Other Possible Causes

Dehydration on its own can cause headaches — make sure you are drinking at least 2 liters of water during your fasted hours. Caffeine withdrawal is another culprit if you normally drink multiple coffees and have cut back. These headaches tend to occur at consistent times and fade after 3–5 days as the body adjusts.

Prevention Going Forward

Add a pinch of salt to your morning water as a daily habit. If you use electrolyte supplements, ensure they contain sodium (many zero-calorie options do). Most people who address sodium proactively never experience significant fasting headaches.

When to Be Cautious

Severe headaches, or headaches accompanied by vision changes, nausea, neck stiffness, or extreme sensitivity to light are not typical fasting headaches. These warrant medical attention regardless of fasting status.