Headaches happen because pain-sensitive tissues inside your head get irritated, stretched, compressed, or inflamed. Your brain itself has no pain receptors at all. The pain you feel comes from the structures surrounding the brain: membranes, blood vessels, nerves, and muscles. About 35% of the global population experiences a headache disorder in any given year, making it one of the most common human experiences. Understanding what’s actually generating that pain helps explain why so many different things, from skipping water to changes in weather, can leave you reaching for your temples.
Your Brain Can’t Feel Pain
This is the central paradox of headaches. The organ that processes all pain signals is itself completely numb to touch. Surgeons have confirmed this during awake brain surgeries, where they can touch, cut, and electrically stimulate brain tissue without the patient feeling a thing. The pain-sensitive structures are everything around the brain: the dura mater (the tough membrane lining the inside of your skull), blood vessels feeding that membrane, and the first few centimeters of the large arteries entering the brain.
More recent research from awake craniotomies has expanded this picture. The pia mater, a thin membrane that sits directly on the brain’s surface, and even the small blood vessels running along it also produce pain when mechanically stimulated. So does the lining of the brain’s internal fluid chambers. All of this pain travels primarily through the trigeminal nerve, the large nerve responsible for sensation across your face and forehead. When pain-sensitive structures inside your skull are disturbed, the trigeminal nerve carries that signal, and you feel it as pain in your head, face, or behind your eyes.
How Tension Headaches Work
Tension-type headaches are the most common variety, and they feel like a tight band squeezing around your head. For decades, the explanation was simple: your neck and scalp muscles tighten up, and that causes pain. The reality is more complicated.
For occasional tension headaches, the muscle explanation holds up reasonably well. Stress, poor posture, jaw clenching, or eye strain can keep muscles in your head and neck contracted for hours, irritating local pain receptors. Environmental factors like sleep deprivation, skipped meals, and emotional stress play an outsized role in triggering these episodes.
Chronic tension headaches, the kind that show up more days than not, involve something different. Over time, the nervous system itself changes. Pain pathways become sensitized, meaning they fire more easily and in response to stimuli that shouldn’t normally hurt. The brain’s built-in pain suppression systems also become less effective. In other words, chronic tension headaches are partly a problem of the brain’s pain processing going haywire, not just tight muscles. Genetics appear to play a meaningful role in who develops the chronic form, while occasional tension headaches are driven more by lifestyle and environment.
What Happens During a Migraine
Migraines involve a specific cascade centered on the trigeminovascular system, a network of trigeminal nerve fibers that wraps around the blood vessels in the brain’s membranes. When this system activates, those nerve endings release signaling molecules, most importantly one called CGRP (calcitonin gene-related peptide). CGRP dilates blood vessels and triggers inflammation in the membranes surrounding the brain. This creates a self-reinforcing cycle: inflammation activates more nerve endings, which release more inflammatory signals.
The trigeminal nerve acts as a final common pathway for migraine pain regardless of what originally set it off, whether that’s hormonal shifts, certain foods, bright lights, or stress. This is why newer migraine treatments specifically target CGRP. Antibodies designed to neutralize CGRP or block its receptor have proven effective because they interrupt the cycle right at its source.
Many migraines also involve cortical spreading depression, a slow wave of intense electrical activity that moves across the brain’s surface. This wave is what produces the visual disturbances, tingling, or other sensory changes known as aura. It also activates the trigeminal nerve fibers in its path, linking the aura directly to the pain that follows.
Cluster Headaches and the Brain’s Clock
Cluster headaches are rarer but extraordinarily painful, often described as a burning or stabbing sensation behind one eye. They arrive in bouts (clusters) that tend to occur at the same time of day, often waking people from sleep. This clockwork pattern points to the hypothalamus, a small region deep in the brain that regulates circadian rhythms, hormone cycles, and sleep-wake patterns.
Brain imaging studies using PET scans have identified the posterior hypothalamus as the key area driving cluster headaches. Structural differences in this region have been found in people who get them. The hypothalamus appears to trigger activation of the trigeminal nerve on one side of the face, which is why the pain is always one-sided and accompanied by autonomic symptoms on that same side: a watering eye, nasal congestion, or a drooping eyelid.
Common Triggers and Why They Cause Pain
Dehydration is one of the most straightforward headache triggers. When your body loses fluid, the brain physically shrinks slightly and pulls away from the skull. This tugs on the pain-sensitive membranes and nerves surrounding the brain, producing a dull, aching pain that typically improves within one to three hours of rehydrating.
Changes in barometric pressure, the kind that come with approaching storms, affect the air-filled sinus and nasal cavities in your skull. A drop in atmospheric pressure creates a mismatch between the pressure inside those cavities and the environment, forcing fluid shifts in the tissues. These fluid shifts extend to the blood vessels surrounding the brain, and the resulting stretch or swelling of those vessels activates pain receptors. Some researchers also believe the pressure changes affect how the brain modulates pain signals.
Caffeine withdrawal, alcohol, irregular sleep, strong smells, bright or flickering light, and hormonal fluctuations (particularly the drop in estrogen before menstruation) are all well-established triggers. They work through different mechanisms but share a common endpoint: activating the trigeminal nerve or sensitizing the pain pathways it feeds into.
When a Headache Signals Something Else
The vast majority of headaches are “primary” headaches, meaning the headache itself is the condition. But headaches can also be symptoms of something else entirely. The International Classification of Headache Disorders lists eight major categories of secondary headaches, caused by everything from head trauma and infections to blood vessel problems and medication overuse.
Certain patterns deserve immediate attention. A sudden, explosive headache that reaches maximum intensity within seconds, sometimes called a thunderclap headache, can signal bleeding around the brain. A headache with fever, stiff neck, and confusion may point to an infection of the brain’s membranes. New or worsening headaches after age 50, headaches that change dramatically in character, and headaches accompanied by neurological symptoms like vision loss, weakness, trouble speaking, or altered consciousness all warrant urgent evaluation.
Headaches that only appear in certain positions, get worse with coughing or exercise, or develop after a head injury also carry a higher probability of having a structural cause. Overuse of pain medication itself can paradoxically cause headaches. Using acute headache treatments more than two or three days per week for an extended period can lead to a rebound cycle where the medication withdrawal triggers the next headache.