Headaches happen when pain-sensitive structures in and around your brain become irritated, stretched, or inflamed. Your brain tissue itself can’t feel pain, but the membranes surrounding it, the blood vessels running through it, and the nerves connecting it to your face and neck all can. Roughly 40% of the global population experiences a headache disorder in any given year, with tension-type headaches affecting about 25% of people and migraines affecting 14%.
How Your Brain Registers Head Pain
The key player in nearly all headaches is a network called the trigeminovascular system. This is a set of nerve fibers from the trigeminal nerve, which is the main sensory nerve of your face, that wraps around the blood vessels in your brain’s protective lining (the dura mater). When something irritates these nerve fibers, they fire pain signals down into your brainstem and upper spinal cord, which your brain interprets as a headache.
What makes this system especially powerful is that it doesn’t just send pain signals. When activated, those nerve endings also release chemical messengers directly into the surrounding blood vessel walls. These chemicals cause the vessels to widen and leak fluid into nearby tissue, creating inflammation. That inflammation then irritates more nerve fibers, which release more chemicals, creating a feedback loop that can sustain and worsen the headache over hours.
Tension Headaches: The Most Common Type
Tension-type headaches feel like a band of pressure tightening around your head. They account for about one in four people worldwide and are far more common than migraines, though they tend to get less attention because they’re usually less severe.
The pain in tension headaches is closely tied to tenderness in the muscles around your skull, including those in your forehead, temples, jaw, and the back of your head. In one study comparing headache types, every single person with chronic tension headaches had tenderness in at least one of these muscle groups, compared to about half of people without headaches. This muscle tenderness appears early in the development of tension headaches, not as a result of having them for a long time. That’s why the things that tighten these muscles, like stress, poor posture, clenching your jaw, or staring at a screen for hours, are such reliable triggers.
Over time, if tension headaches become frequent enough, your central nervous system can start amplifying pain signals on its own, making you more sensitive to stimuli that wouldn’t normally hurt. This shift from a peripheral muscle problem to a central nervous system problem is what distinguishes occasional tension headaches from the chronic form that occurs 15 or more days per month.
Migraines: A Different Process Entirely
Migraines aren’t just bad headaches. They involve a distinct cascade of neurological events that tension headaches don’t share. The throbbing, often one-sided pain is typically accompanied by nausea, light sensitivity, or sound sensitivity, and attacks last anywhere from 4 to 72 hours.
One of the key events in many migraines is something called cortical spreading depression: a slow wave of electrical activity that rolls across the surface of the brain. This wave causes a temporary surge of activity followed by a period of suppressed activity, and it’s what produces the visual disturbances (flashing lights, blind spots) that some people experience as an “aura” before the pain starts. As this wave passes, it triggers the trigeminovascular system, prompting the release of inflammatory chemicals and the dilation of blood vessels in the brain’s lining.
Several chemical shifts happen during a migraine attack. Levels of a molecule called CGRP rise sharply. CGRP is a potent vasodilator that widens cerebral blood vessels and drives inflammation, and it has become such a well-understood target that an entire class of newer migraine treatments works by blocking it. Serotonin levels also fluctuate during attacks, and nitric oxide, another powerful vasodilator, gets released in the early phase. Meanwhile, levels of the brain’s main excitatory chemical, glutamate, run higher in migraine sufferers even between attacks, which may help explain why their brains are more reactive to triggers in the first place.
Common Triggers That Start the Process
Understanding the mechanism is useful, but most people want to know what sets it off. The triggers that activate pain-sensitive structures in and around the brain fall into several categories.
Dehydration
When you lose enough fluid, your brain physically shrinks slightly and pulls away from the skull. This tugging puts pressure on the pain-sensitive nerves surrounding your brain, producing a headache that typically affects the entire head and worsens when you stand up or move around. Rehydrating usually resolves it within one to three hours.
Caffeine Withdrawal
Caffeine works by blocking adenosine receptors in the brain. Adenosine normally dilates blood vessels, so caffeine’s blocking effect constricts them and reduces blood flow. If you regularly consume caffeine and then stop, those receptors are suddenly unblocked, blood vessels rebound and widen, and cerebral blood flow increases. This vascular shift triggers head pain, typically within 12 to 24 hours of your last dose. Even moderate habitual intake, just a couple of cups of coffee a day, is enough to cause withdrawal headaches.
Weather Changes
Drops in barometric pressure affect the air-filled cavities in your sinuses and nasal passages. The pressure difference forces fluid into surrounding tissues, disrupts fluid balance in and around the brain, and can trigger headaches, particularly in people who are already prone to migraines.
Food and Alcohol
Certain foods contain compounds that interact with blood vessels. Aged cheeses, cured meats, fermented foods, and some wines contain tyramine and other vasoactive substances that can trigger headaches in susceptible people. Alcohol itself is a vasodilator and also promotes dehydration, giving it two separate pathways to headache.
Stress and Sleep
Stress causes sustained contraction of head and neck muscles and also shifts levels of brain chemicals like serotonin. Interestingly, headaches often strike not during the most stressful period but during the “letdown” afterward, like a weekend after a hard week. Too little sleep, too much sleep, and irregular sleep schedules are all reliable headache triggers, likely because they disrupt the brain’s chemical balance.
When Headaches Come From Overusing Pain Medication
One of the most common and least recognized causes of frequent headaches is the very medication people take to treat them. Medication-overuse headache affects about 2% of the population and develops when you use acute headache treatments too often: 10 or more days per month for combination painkillers or specific migraine drugs, or 15 or more days per month for simple painkillers, sustained over at least three months.
The result is a cruel cycle. The headaches become more frequent, you take more medication, and the medication itself starts perpetuating the headaches. The pain typically becomes a near-daily, dull, persistent ache that’s different from the original headache pattern. Breaking this cycle usually means gradually reducing the overused medication, which often makes headaches temporarily worse before they improve.
Headache Warning Signs That Need Urgent Attention
The vast majority of headaches are uncomfortable but harmless. A small number, however, signal something serious. The features that distinguish dangerous headaches from routine ones are well established.
- Sudden, explosive onset. A headache that reaches maximum intensity within seconds, sometimes called a “thunderclap headache,” can indicate bleeding in or around the brain and needs emergency evaluation.
- Neurological changes. New weakness in an arm or leg, numbness, vision changes, or difficulty speaking alongside a headache suggest the brain itself is being affected.
- Fever or systemic illness. A headache with high fever, neck stiffness, night sweats, or weight loss may point to infection or another systemic condition.
- New headaches after age 50. A person who has never had significant headaches and develops a new pattern after 50 is more likely to have an underlying cause.
- Progressively worsening pattern. A headache that steadily becomes more severe or more frequent over weeks, rather than coming and going in a stable pattern, warrants investigation.
- Position-dependent pain. Headaches that change dramatically when you stand up, lie down, or strain (coughing, bearing down) can indicate a pressure problem inside the skull.
These red flags don’t mean every headache with one of these features is dangerous, but they’re the situations where imaging or further testing can rule out conditions like aneurysms, tumors, infections, or blood clots that require treatment beyond pain relief.