If you got a headache in your first week of fasting, you’re in ordinary company, and it’s very likely to settle.
How common are they?
The landmark study followed hospital staff through a twenty-five-hour fast. Around four in ten of the people fasting got a headache, against a much smaller share of those who weren’t. Risk went up the longer the fast ran, and people who already got frequent headaches were far more likely to get one.1
But across fifteen randomized trials of intermittent fasting, headaches came up at about the same rate as in the control groups. Which suggests that once you’re past the first stretch, it stops being a feature.5
What does one feel like?
A review of the literature found fasting headache usually looks like a tension headache. Both sides, across the front, steady pressure rather than a throb, mild to moderate.2
What causes them?
Here it gets less tidy. The obvious answers are caffeine withdrawal and blood sugar, and both are involved.
But in that first study, caffeine withdrawal didn’t explain the pattern. And the review above says plainly that much about the mechanism is still unknown.21
Caffeine is clearly part of it for some people. In a study of more than seven hundred coffee drinkers starting Ramadan, over half got a headache on the first day, and the heavier their usual habit, the higher the odds.3
How do you head one off?
- Hydrate before you’re thirsty. The easiest cause to remove and the one most often behind it.
- A pinch of salt in water. Especially if you also feel lightheaded.
- Change one thing at a time. Caffeine and your eating window in the same week is asking for it.
- Taper rather than quit. People who cut caffeine gradually reported far less trouble than people who stopped cold.
- Shorten the window. Risk rises with length.4
Ending a fast because your head hurts is a reasonable decision made by a reasonable person.
When should you stop rather than push through?
- It’s getting worse rather than holding steady.
- It comes with visual changes or feeling faint.
- It’s simply bad. That’s enough on its own.
And if headaches keep arriving every time you fast, past the first few weeks, raise it with your doctor rather than treating it as the cost of doing business.