Headaches and Dehydration: The Brain-Shrinking Truth

Yurino Hisamori
5 days ago
3 min read

Before you start Googling your headache symptoms and exacerbating your health anxiety, try something simpler: drinking a glass of water.

Oftentimes, we get headaches when we’re dehydrated—and this is not a mere coincidence. When your body lacks sufficient water, it doesn’t just affect your mood or energy levels; it affects your brain. In fact, dehydration can literally cause your brain to shrink—triggering a cascade of neurological and vascular changes that may leave you reaching for an aspirin.

This article explores how dehydration affects both brain function and structure, why it results in pain, and what your nervous system is really trying to tell you when you get that all-too-familiar ache.

How Dehydration Affects the Brain

The human brain is composed of approximately 75-80% water, and it relies on this high-water content for key neurological functions. For instance, proper hydration ensures:

Stable electrolyte gradients across neurons (critical for action potentials)
Efficient neurotransmission
Functioning of the glymphatic system, which clears metabolic waste during sleep
Adequate cerebral blood flow

When you become dehydrated, your brain loses fluid volume, particularly in the extracellular space—causing a slight shrinkage of brain tissue. As the brain contracts slightly, it pulls away from the cranial vault—the rigid structure of the skull. This movement places tension on the meninges: three layers of connective matter (dura matter, arachnoid mater, and pia mater) that surround the brain.

While the brain tissue itself is insensate (i.e. it has no pain receptors), the meninges are innervated with nociceptors, or pain-sensitive neurons, primarily via the trigeminal ganglion (TG). These neurons are capable of detecting both external and internal triggers. In the case of dehydration, the meningeal nociceptors become activated both due to the physical tension from the shrinking brain, as well as the reduced cerebral perfusion (blood flow to the brain), which deprives neurons of oxygen and nutrients.

The trigeminovascular system—a key pathway in headache physiology—relays these pain signals through the trigeminocervical complex (TCC) to the thalamus and eventually the cortex, where they are consciously perceived as pain.

What Dehydration Headaches Feel Like

Dehydration-induced headaches vary by individual, but often appear as a dull ache throughout the head, or more localised pain in the frontal or occipital regions. These headaches may also be motion-sensitive—worsening with walking, bending over, or standing up quickly.

Commons Triggers for Dehydration Headaches

Though the mechanism is internal, the causes are usually behavioural or environmental. This can include:

Not drinking enough water throughout the day
Excessive sweating, especially in hot environments or during exercise
Caffeine or alcohol consumption, which both have mild diuretic effects
Illnesses that cause fever, vomiting, or diarrhea

Prevention and Management

Fortunately, there are measures you can take in preventing these headaches—and put simply, it is to do the opposite of the above triggers. Staying consistently hydrated throughout the day, increasing water intake before and after exercise, limiting caffeine and alcohol intake, and considering electrolyte-rich fluids if you’re sweating heavily or ill.

Your brain is highly sensitive to even small changes in hydration. We often hear people joke, “I forgot to drink water all day,” but dehydration is no joke when it comes to your nervous system—it can alter brain volume, blood flow, and the very mechanics of pain. So next time your head starts pounding, don’t panic—maybe your neurons just need a drink.

Reference list

Christensen, S.L. and Levy, D. (2024). Meningeal brain borders and migraine headache genesis. Trends in Neurosciences, [online] 47(11), pp.918–932. doi:https://doi.org/10.1016/j.tins.2024.08.012.

‌LeWine, H.E. (2022). Can dehydration cause headaches? - Harvard Health. [online] Harvard Health. Available at: https://www.health.harvard.edu/diseases-and-conditions/can-dehydration-cause-headaches [Accessed 14 Jul. 2025].‌

Water, T.A. (2015). Texan Alkaline Water. [online] Texan Alkaline Water. Available at: https://texanalkalinewater.com/news/2015/3/11/is-your-brain-dehydrated [Accessed 14 Jul. 2025].