Headaches are caused by activation of pain-signaling nerves in and around the brain, not by the brain tissue itself, which has no pain receptors. The specific trigger varies: muscle tension, blood vessel changes, nerve inflammation, hormonal shifts, dehydration, and dozens of other factors can all set off the process. Understanding which mechanism is at work helps explain why headaches feel so different from one another and why they respond to different treatments.
Primary vs. Secondary Headaches
Headaches fall into two broad categories. Primary headaches, including migraines, tension-type headaches, and cluster headaches, are the condition itself. Nothing else is wrong; the pain system is misfiring or overreacting on its own. Secondary headaches are symptoms of something else: a sinus infection, a head injury, dehydration, medication overuse, or rarely, a more serious condition like bleeding in the brain.
A headache counts as secondary when it appears in close timing with another disorder known to cause head pain, and when it worsens or improves in step with that disorder. This distinction matters because treating a secondary headache means treating the underlying cause, not just the pain.
How Tension Headaches Develop
Tension-type headaches are the most common variety, and they start at the periphery. Sustained tightness in the muscles of the scalp, jaw, neck, and upper shoulders sends a steady stream of pain signals through thin nerve fibers to the brain. Over time, this constant input sensitizes the nervous system, meaning the brain starts interpreting even mild pressure as painful. Trigger points in the head and neck muscles, small knots of contracted tissue, can reproduce the dull, band-like pain pattern people associate with tension headaches.
Pressure sensitivity in the scalp and neck muscles increases proportionally with headache frequency and severity. In people who get tension headaches often, even nerve fibers that normally respond only to light touch begin carrying pain signals. The brain’s own pain-dampening system also weakens. Normally, the brain can dial down incoming pain signals through an internal filtering process, but in frequent tension headache sufferers, this filter doesn’t work as well, which helps explain why occasional tension headaches can become chronic.
What Happens During a Migraine
Migraines involve a deeper malfunction in the brain’s pain-processing network. The key player is the trigeminal nerve, the largest nerve in the head, which connects blood vessels in the membranes surrounding the brain to pain-processing centers deeper in the brainstem. When a migraine is triggered, nerve endings around these blood vessels release a signaling molecule called CGRP in large amounts.
CGRP sets off a cascade: blood vessels dilate, surrounding tissue becomes inflamed, and the trigeminal nerve becomes increasingly sensitive. That sensitization doesn’t stay local. It spreads inward, amplifying pain signals at every relay point between the nerve and the brain. Once central sensitization takes hold, normal sensory input like light, sound, and movement gets processed as painful, which is why migraines often come with sensitivity to light and noise. When this cycle of sensitization shifts from being driven by the initial trigger to sustaining itself independently, episodic migraines can progress toward chronic ones.
Why Cluster Headaches Follow a Clock
Cluster headaches are rarer but extraordinarily painful, often described as the worst pain a person can experience. They’re driven by the hypothalamus, the brain’s internal clock. PET scans show the hypothalamus lighting up during cluster attacks, and structural differences have been found in this region in cluster headache patients.
The hypothalamic connection explains the distinctive timing of these headaches. Attacks strike at the same time of day (often at night), follow seasonal patterns, and cycle through active periods and remissions, all hallmarks of a process governed by the body’s circadian rhythm. During an attack, the trigeminal nerve activates a reflex arc that triggers the autonomic nervous system on one side of the face, causing the tearing eye, nasal congestion, and facial flushing that accompany the pain.
Common Headache Triggers
Dehydration
When your body loses more fluid than it takes in, the resulting concentration shift in your blood can cause the brain to shrink slightly. This pulls on the membranes and blood vessels surrounding the brain, structures packed with pain receptors. The traction on these membranes is what produces the headache. Stretching of veins that drain blood from the skull may also contribute. Rehydrating typically resolves this type of headache, though it can take time.
Food and Drink
Certain compounds in food act on the nervous system and blood vessels in ways that can trigger headaches, particularly migraines. Tyramine, found in aged cheeses and fermented foods, promotes the release of nitric oxide, which dilates blood vessels. Histamine, present in cheese, yogurt, nuts, and red wine, can cause similar vascular changes. Flavonoids in chocolate, citrus fruits, coffee, tea, and red wine interfere with enzymes that help regulate neurotransmitter levels. Not everyone is sensitive to these compounds. For those who are, keeping a food diary is the most reliable way to identify personal triggers.
Hormonal Changes
Estrogen plays a direct role in modulating pain within the trigeminal system. The sharp drop in estrogen that occurs in the days before menstruation is a well-established migraine trigger, a pattern first documented in the early 1970s and confirmed repeatedly since. This estrogen withdrawal hypothesis explains why menstrual migraines cluster in a predictable two-day window around the start of a period and why they often improve during pregnancy (when estrogen stays high) and after menopause.
Weather and Pressure Changes
Drops in barometric pressure, the kind that accompany incoming storms, can trigger migraines. The mechanism is straightforward: changes in atmospheric pressure alter the external force on your body, which affects blood vessel diameter. When vessels in and around the brain dilate in response to falling pressure, the resulting change in blood flow can activate pain pathways in susceptible people.
How Painkillers Can Cause Headaches
One of the more counterintuitive causes of headaches is the very medication used to treat them. Medication overuse headache develops when someone with an existing headache condition takes pain relievers too frequently. The International Headache Society defines overuse as 10 or more days per month for some medications, or 15 or more days per month for others, sustained over at least three months.
The result is a headache that occurs on 15 or more days each month, essentially a near-daily headache that paradoxically worsens the more medication you take. The nervous system adapts to the frequent presence of painkillers, and when they wear off, pain sensitivity rebounds. Breaking the cycle requires reducing or stopping the overused medication, which often means a temporary period of worse headaches before improvement.
Warning Signs of a Serious Cause
Most headaches are not dangerous, but certain features signal that a headache may be caused by something that needs urgent attention. Clinicians use a checklist of red flags to screen for secondary causes:
- Sudden, explosive onset: A headache that reaches maximum intensity within seconds (thunderclap headache) can indicate bleeding around the brain.
- Neurological symptoms: Weakness, confusion, vision loss, difficulty speaking, or decreased consciousness alongside a headache.
- Fever with headache: May suggest infection, including meningitis.
- New headache pattern after age 50: First-ever headaches appearing later in life warrant investigation.
- Positional component: Headaches that dramatically change with standing or lying down can indicate pressure problems inside the skull.
- Progressive worsening: A headache that steadily intensifies over days or weeks without responding to treatment.
- Headache after head trauma: Even if symptoms seem mild initially.
Any of these patterns represent a meaningful departure from typical primary headaches and point toward an underlying cause that imaging or other testing can identify.