Headaches happen when pain-sensitive structures in and around your skull send distress signals to your brain through a network of nerves. The brain tissue itself has no pain receptors at all. Instead, the pain originates from surrounding structures: the membranes wrapping the brain (called meninges), blood vessels, muscles, and nerves in the head and neck. About 65% of adults worldwide experience at least one headache episode per year, and the underlying mechanism differs depending on the type.
Why Your Brain Can’t Feel Pain
This is the central paradox of headaches. The organ that processes all pain perception is itself incapable of feeling pain. Brain tissue contains no pain-sensing nerve endings. For decades, the protective membrane surrounding the brain (the dura mater) and the major blood vessels running through it were considered the only pain-sensitive structures inside the skull. More recent research has shown that smaller brain arteries and the delicate inner membrane closest to the brain may also be pain-sensitive.
All of these structures are wired into the trigeminal nerve, a large nerve with three branches that covers your forehead, cheeks, and jaw. It acts as the main highway carrying pain signals from inside your skull down to the brainstem. From there, the signal travels to the thalamus (a relay station deep in the brain) and finally to the cortex, where you consciously experience the sensation of pain. A separate set of nerves at the base of the skull covers the back of the head, which is why some headaches feel concentrated there instead.
How Tension Headaches Work
Tension-type headaches are the most common variety, affecting roughly 33% of adults globally in any given year. They feel like a dull, pressing band around the head rather than a sharp or throbbing pain. The exact mechanism is still debated, but both peripheral and central factors are clearly involved.
One piece of the puzzle involves trigger points in the muscles around the skull. Sustained contraction of these muscles, whether from stress, poor posture, or jaw clenching, can restrict blood flow to the muscle tissue and trigger the release of pain-promoting substances. This creates a localized pain signal that feeds into the trigeminal nerve system. People who get occasional tension headaches tend to show higher levels of this peripheral muscle sensitivity.
When tension headaches become chronic (occurring 15 or more days per month), the problem shifts. Brain imaging studies show increased activity in several areas involved in pain processing, including the prefrontal cortex, the thalamus, and the cerebellum. This pattern suggests that the brain’s pain-processing system has become sensitized, essentially turning up the volume on pain signals that wouldn’t normally register. Sleep disruption may also play a role: a molecule called orexin, which normally helps suppress pain signaling in the trigeminal system, appears to decrease with inconsistent sleep. Without that natural dampening, the pain pathways become more active.
The Migraine Cascade
Migraines affect about 24% of adults worldwide and involve a more complex chain of events than tension headaches. The process centers on a signaling molecule called CGRP, which is abundant in the trigeminal nerve system.
When trigeminal nerve endings in the membranes around the brain become activated, they release CGRP from their tips. This sets off a cascade: CGRP causes blood vessels in the membranes to widen, triggers the production of nitric oxide (another vessel-widening chemical), and promotes inflammation. That inflammation further irritates the nerve endings, which release even more CGRP, creating a self-reinforcing cycle. Inside the nerve cluster itself, CGRP spills over to neighboring nerve cells and supporting cells, prompting them to produce additional inflammatory molecules. This is why migraines can escalate and persist for hours or days.
Serotonin, a chemical messenger better known for its role in mood, also plays a part. It normally helps keep blood vessels slightly constricted. When serotonin levels drop, blood vessels lose that constricting influence, leaving nitric oxide’s vessel-widening effect unopposed. This imbalance is one reason migraines are associated with fluctuations in brain chemistry rather than a single static problem.
Migraine Aura
About one in four people with migraines experience aura: visual disturbances like zigzag lines, blind spots, or shimmering patches that typically precede the pain phase by 20 to 60 minutes. These are caused by a phenomenon called cortical spreading depression, a slow-moving wave of intense electrical activity that rolls across the surface of the brain. This wave nearly completely shuts down the affected brain cells for about a minute, then silences their electrical activity for several more minutes as it passes. As the wave moves across the visual processing area, it creates the characteristic visual symptoms. Critically, this electrical wave also activates the trigeminal pain pathways, which is why aura often leads directly into the headache phase.
Cluster Headaches and the Body Clock
Cluster headaches are rarer but far more intense, producing severe, stabbing pain around one eye that lasts 15 minutes to three hours. They arrive in clusters: daily attacks for weeks or months, followed by long remission periods. This cyclical pattern is the key to understanding them.
The hypothalamus, a small structure deep in the brain that regulates your internal clock, sleep cycles, and hormone release, is strongly implicated. PET scans taken during active cluster attacks show activation in a specific region of the hypothalamus. Structural studies have also found anatomic abnormalities in that same region. People with cluster headaches show decreased melatonin levels and, in some cases, a complete loss of normal circadian rhythm. Genetic research has identified variations in genes linked to circadian rhythm and the sleep-wake cycle in cluster headache patients.
The attacks tend to strike at the same time each day, most often at night, and recur in seasonal patterns. All of this points to the hypothalamus misfiring and triggering the trigeminal nerve and the autonomic nervous system (which controls involuntary functions like tearing, nasal congestion, and pupil size on the affected side).
Secondary Headaches: When Something Else Is Wrong
All the headache types described above are primary headaches, meaning the headache itself is the condition. Secondary headaches are symptoms of another problem, and they work through different mechanisms.
Severely elevated blood pressure can overwhelm the brain’s ability to regulate its own blood flow. Normally, blood vessels in the brain automatically constrict or dilate to keep blood flow steady regardless of what your blood pressure is doing. When blood pressure spikes beyond the range this system can handle, the excess pressure forces fluid across blood vessel walls and into brain tissue, causing swelling. This swelling activates pain-sensitive nerve endings in the blood vessels and membranes.
Other secondary headache triggers work through similar principles of mechanical or chemical irritation. Sinus infections inflame tissue near branches of the trigeminal nerve. Dehydration reduces the fluid cushion around the brain, allowing it to tug on pain-sensitive membranes. Caffeine withdrawal dilates blood vessels that had adapted to caffeine’s constricting effect. In each case, the final pathway is the same: something irritates a pain-sensitive structure in or around the skull, and the trigeminal system carries that signal to conscious awareness.
Why Pain Lingers and Worsens
One reason headaches can be so stubborn once they start is a process called sensitization. When pain signals fire repeatedly, the neurons carrying those signals become progressively easier to activate. Early in a headache, only intense stimulation registers as painful. After sustained signaling, even normal sensations like light touch on the scalp, bright light, or routine head movement can amplify the pain. This is why treating a headache early tends to be more effective than waiting: once sensitization sets in, the pain system is harder to quiet down.
Overuse of pain medication can produce a similar effect over time. Frequent use of painkillers causes the brain’s pain-modulating systems to adapt, and when the medication wears off, the baseline pain threshold drops lower than it was before. This creates a rebound headache cycle where the treatment itself becomes a contributing cause, affecting an estimated 4% of the global population.