Headaches happen when pain-sensing nerves around your brain, skull, and face become activated, even though brain tissue itself has no pain receptors. About 40% of the global population, roughly 3.1 billion people, deals with headache disorders. Understanding what’s actually going on inside your head can help you figure out why yours keep coming back and what you can do about them.
Your Brain Doesn’t Feel Pain
This is the part that surprises most people. The brain itself has no pain receptors. What does hurt are the tissues surrounding it: the membranes covering the brain (called the meninges), the blood vessels running along the brain’s surface, and the muscles and nerves in your head, neck, and face. When these structures are irritated, stretched, inflamed, or compressed, they send pain signals through a network of nerves centered on the trigeminal nerve, the main sensory nerve of your face and head.
The trigeminal nerve branches wrap around the blood vessels and membranes covering your brain. When something triggers these nerve fibers, they release signaling molecules that cause nearby blood vessels to widen and surrounding tissue to become inflamed. This creates a feedback loop: inflammation makes the nerves more sensitive, and sensitized nerves release more inflammatory signals. That’s why a mild headache can gradually become a throbbing one, and why your scalp or the area around your eyes can feel tender to the touch even though nothing is wrong with the skin there.
Tension Headaches: Not What You Think
Tension headaches are the most common type, and their name is slightly misleading. For decades, doctors assumed they were caused by tight, oxygen-starved muscles in the head and neck. Research has since shown that muscle activity during a tension headache is normal or only slightly elevated, and muscle oxygen levels stay within a healthy range. The pain is real, but it’s not coming from clenched muscles the way most people imagine.
What actually happens involves sensitization of pain-sensing nerves in the muscles and connective tissue of your scalp, jaw, neck, and shoulders (called pericranial tissues). Researchers have found that when they inject a mix of naturally occurring inflammatory substances into the neck muscles of people who get frequent tension headaches, those people experience significantly more pain than people who don’t get headaches. Their nerve endings have essentially been “turned up,” reacting more strongly to stimulation that wouldn’t bother someone else. Over time, this heightened sensitivity can rewire the pain-processing centers in your brainstem, making headaches easier to trigger and harder to shake. That rewiring can also slightly increase resting muscle tension, which further feeds the cycle.
How Migraines Are Different
Migraines involve a more complex chain of events. One key player is a phenomenon called cortical spreading depression: a slow-moving wave of abnormal electrical activity that sweeps across the surface of the brain. During this wave, brain cells fire intensely, then go quiet. Ions flood in and out of cells in the wrong direction, and the brain’s normal electrical balance collapses temporarily in that region.
If you experience migraine aura (visual distortions, tingling, or difficulty speaking before the pain starts), that’s the wave moving across brain areas responsible for vision, sensation, or language. But the wave also does something else: it triggers the release of inflammatory molecules that reach the membranes surrounding the brain. Those molecules activate the same trigeminal nerve network involved in other headaches, but more aggressively. One molecule in particular, known as CGRP, plays a central role. It’s released by the pain-sensing nerve fibers themselves and causes blood vessels to dilate while ramping up inflammation. CGRP levels rise sharply during migraine attacks, and newer migraine treatments work specifically by blocking it.
The result is intense, often one-sided, throbbing pain that can last hours to days. The nerve sensitization becomes so pronounced that normal stimuli, like light, sound, and even the pulsing of your own blood vessels, register as painful.
Common Triggers That Start the Process
Knowing how headaches work at the nerve level is useful, but most people want to know what sets the whole thing off. The triggers fall into a few broad categories.
Dehydration and missed meals are among the most straightforward. Your brain is sensitive to changes in blood sugar and fluid balance, and both can activate pain pathways directly. Skipping breakfast or not drinking enough water on a hot day is enough to do it for many people.
Poor sleep disrupts the brain’s ability to regulate pain signals. Both too little sleep and irregular sleep schedules are well-established headache triggers, particularly for migraines.
Stress and muscle tension contribute through the sensitization pathway described above. Sustained stress keeps your nervous system in a heightened state, lowering the threshold for headache pain. Interestingly, many people get headaches not during the stressful period but right after it ends, a phenomenon sometimes called a “let-down headache.”
Weather changes are a real trigger, not just an old wives’ tale. When barometric pressure drops, such as before a storm, the pressure difference affects your sinus and nasal cavities, which are essentially air-filled channels. This forces fluid shifts in the tissues and blood vessels around your brain, which can trigger pain. Staying well-hydrated helps buffer this effect.
Caffeine withdrawal is one of the most reliable headache triggers in existence. Caffeine works by blocking receptors for a molecule called adenosine, which naturally promotes blood vessel dilation and drowsiness. When you drink coffee regularly, your body compensates by becoming more sensitive to adenosine. Stop the caffeine suddenly and your system overreacts: blood vessels dilate more than usual, and your overall sensitivity to adenosine spikes. The classic withdrawal headache typically hits 12 to 24 hours after your last cup.
When Painkillers Become the Problem
One of the more counterintuitive ways to get a headache is by treating headaches too often. Medication overuse headache occurs when you take pain relievers on 10 or more days per month (for some medication types, 15 or more days) for longer than three months. The threshold varies depending on the type of painkiller, but the pattern is consistent: frequent use of headache medication gradually makes your nervous system more sensitive to pain rather than less.
The headaches become more frequent, which leads to more medication, which leads to more headaches. The only way to break the cycle is to reduce or stop the overused medication, which usually means a rough week or two of increased headaches before things improve. If you find yourself reaching for painkillers more than two or three days a week on a regular basis, that pattern itself is worth paying attention to.
Headaches Caused by Something Else
Most headaches are “primary,” meaning the headache itself is the condition, not a symptom of something deeper. But sometimes a headache is a warning sign of another medical problem. These are called secondary headaches, and a few features distinguish them from ordinary ones.
A sudden, explosive headache that reaches maximum intensity within seconds (sometimes called a thunderclap headache) is one of the most concerning patterns. It can indicate a burst blood vessel or aneurysm in the brain and needs immediate evaluation. New headaches accompanied by fever, unexplained weight loss, or night sweats suggest a systemic illness may be involved. Neurological symptoms that are new or unusual, like weakness on one side of your body, new numbness, or vision changes that don’t fit your typical headache pattern, are another red flag.
A few other situations raise the index of concern: headaches that begin for the first time after age 50, headaches that are clearly getting worse over weeks or months, and new headaches during or shortly after pregnancy. In someone with a weakened immune system, a new headache pattern also warrants closer attention, since infections that wouldn’t normally reach the brain can become a factor.
None of these red flags automatically mean something dangerous is happening, but they change the odds enough that the headache deserves a closer look rather than another dose of ibuprofen.