Dehydration is on every “common migraine triggers” list, and the folk wisdom is so entrenched that “drink more water” is the first thing many people try. The actual evidence is smaller and more complicated than the folklore suggests. This page covers what controlled studies show, the confounding direction-of-causation problem, and why Hermly deliberately doesn’t include hydration as a model feature.
What the actual research shows
The cleanest randomised evidence is the Spigt 2012 trial in Family Practice: 102 participants with frequent headache, half randomised to increase daily water intake by 1.5 L over baseline, half to a control. The intervention group reported about 21 fewer hours of headache per 2 weeks (a modest improvement) and a small increase in self-reported quality-of-life scores.
That’s the evidence base, basically. There are observational correlations linking dehydration to attacks in survey research, but those suffer from massive recall bias — people who notice an attack ask themselves “what did I do differently?” and water intake is one of the easier things to remember imperfectly.
The honest summary: hydration probably has a modest effect for some people. It is not in the same evidence tier as sleep, stress, weather, or cycle.
The direction-of-causation problem
A core issue with claiming “I got a migraine because I didn’t drink enough” is the reverse-causation possibility: the prodromal phase of a migraine includes appetite changes and nausea, which can suppress water intake hours before the headache becomes recognisable.
That means a typical pattern of “I drank less yesterday → I got a migraine today” can be entirely consistent with “I was already in prodrome yesterday and that’s why I drank less”. The two are indistinguishable without an experimental design.
Holsteen 2020 — the multi-trigger study that found self-reported triggers in general are poor predictors (within-person C-statistic 0.56) — included hydration as one of the tested features. It did not survive as a meaningful predictor.
Why Hermly doesn’t track hydration
Apple Health has a “Water” category. We could read it. We’ve chosen not to, for three reasons:
- Logging compliance is poor. Most users who log water do so inconsistently. Inconsistent inputs mostly add noise, not signal.
- The signal is small. Even if we had reliable hydration data, the marginal AUC improvement is modest based on the evidence above.
- It would invite a “drink more water” UI nudge. That kind of instructive prompt violates our Honesty over reassurance principle — we’d be implying causation we can’t support.
If hydration is meaningfully driving your attacks, your own journal will surface it faster than an app can.
Sensible dehydration practice (not Hermly’s recommendation)
This isn’t medical advice, but the general consensus around hydration and headache is:
- Drink to thirst plus a small buffer in dry / hot / high-altitude environments.
- Replace fluid losses after exercise; sports drinks are useful when sweat losses are substantial (longer than ~60 minutes of hard exercise) but not for everyday hydration.
- During an active attack, small sips of anything tolerable beat forcing volume.
If you’ve noticed personally that water meaningfully changes your attack frequency, your direct experience is probably more informative than the literature, which is too thin to either confirm or refute the effect at the individual level.
What this page is
A correction to one of the most overconfident claims in migraine folklore. Hydration matters; it matters less than the other triggers Hermly does track. If “I just need to drink more water” has been the conclusion you keep coming back to, the trigger-tracking guide may help you separate the genuine signal from the confounders.