Headaches affect roughly 40% of the global population, about 3.1 billion people, and they stem from a wide range of causes. Some arise on their own from how your brain processes pain signals. Others are triggered by something specific, like dehydration, illness, or a medication you’re taking too often. Understanding what’s behind the pain helps you figure out whether it’s routine or something worth investigating further.
Primary vs. Secondary Headaches
Every headache falls into one of two broad categories. Primary headaches are the condition itself. Nothing else is wrong; your nervous system is generating pain through its own internal mechanisms. Tension-type headaches, migraines, and cluster headaches all fall here. They account for the vast majority of headaches people experience.
Secondary headaches are caused by an underlying problem: an infection, a head injury, a blood vessel issue, high blood pressure, or even a tumor. The headache is a symptom, not the main event. Doctors distinguish between the two by looking at timing. If a new headache appears alongside another condition known to cause head pain, and the two worsen or improve together, the headache is considered secondary to that condition.
What Happens in Your Brain During a Migraine
Migraines involve a cascade of events in the nervous system. The pain originates when nerve fibers that wrap around blood vessels in the brain’s protective membranes become activated. These fibers belong to the trigeminal nerve, the main sensory nerve of the face and head, and they release a signaling molecule called CGRP. That release triggers inflammation around the blood vessels: they dilate, nearby tissue swells, and immune cells in the area become activated.
In migraines with aura (the visual disturbances or tingling some people experience before the pain), a slow wave of electrical activity sweeps across the brain’s surface. As it passes, it causes cells to release a burst of chemicals, including potassium, glutamate, and nitric oxide. These chemicals reach the surface of the brain and activate pain-sensing nerve endings there, which sets the full headache in motion. This is why migraine pain often feels pulsing or throbbing and can come with sensitivity to light, sound, or nausea. The brain’s pain-processing system has essentially become temporarily overactivated.
Why Tension-Type Headaches Feel Different
Tension-type headaches are the most common form, reported by more than 70% of some populations. They typically produce a steady, pressing sensation on both sides of the head rather than the one-sided throbbing of a migraine. The most consistent physical finding is tenderness in the muscles around the skull, particularly in the temples, forehead, and back of the head.
What’s interesting is that this muscle tenderness may not actually come from a problem in the muscles themselves. Researchers believe it reflects changes in how the nervous system processes pain signals. Peripheral nerve endings in the muscles may become sensitized, meaning they fire more easily, or the brain’s own pain-filtering systems may not be dampening signals the way they should. Over time, this sensitization can become self-reinforcing: frequent headaches make the pain system more reactive, which makes headaches more frequent. That cycle helps explain why episodic tension headaches sometimes become chronic.
Dehydration and Brain Volume
Dehydration is one of the most common and easily fixable headache triggers. When your body loses fluid, brain tissue actually shrinks slightly. MRI studies have shown that even mild dehydration (a serum osmolality increase of less than 1%) reduces brain tissue fluid by about 1.6%, with measurable decreases in overall brain volume, white matter, and cortical thickness. The brain sits inside a rigid skull, and these volume changes are thought to pull on the pain-sensitive membranes surrounding the brain.
The good news: rehydration reverses the process. The same imaging studies showed cortical thickening returning after fluid intake. For most people, a dehydration headache resolves within an hour or two of drinking water, though more severe dehydration takes longer.
Weather, Stress, and Other Common Triggers
Many people blame sinus pressure from weather changes for their headaches, but the evidence tells a different story. Research has found no support for the idea that routine barometric pressure shifts cause sinus inflammation. The facial pain and pressure people feel during weather changes are more likely migraine-related. This matters because treating these headaches as sinus problems (with decongestants, for example) misses the actual cause.
Other well-established triggers include poor or irregular sleep, skipped meals, alcohol (especially red wine), strong smells, bright or flickering lights, and hormonal fluctuations, particularly drops in estrogen around menstruation. Stress is perhaps the most universally reported trigger, though it often works indirectly by disrupting sleep, causing muscle tension, or changing eating patterns.
Medication Overuse Headache
One of the more frustrating causes of chronic headaches is the very medication you take to treat them. Medication overuse headache develops when pain relievers are used too frequently over a period of three months or more. The thresholds depend on the type of medication. For common over-the-counter options like ibuprofen or acetaminophen, using them on 15 or more days per month crosses the line. For triptans (a migraine-specific medication), opioids, or combination painkillers, the threshold is lower: 10 or more days per month.
The result is a headache that occurs on 15 or more days each month, often present when you wake up. The pain drives you to take more medication, which perpetuates the cycle. Breaking out of it typically requires gradually reducing or stopping the overused medication, which can temporarily worsen headaches before they improve. This is one reason frequent headaches deserve a closer look at what you’re taking and how often.
Medical Conditions That Cause Headaches
Secondary headaches can stem from a long list of underlying conditions. Some of the more common ones include sinus infections, head or neck injuries, and viral illnesses (headache during and after COVID infection, for instance, is widely recognized). Others are rarer but more serious.
Vascular causes include stroke, bleeding in or around the brain, and tears in the arteries of the neck (cervical artery dissection). Cerebral venous sinus thrombosis, a blood clot in the brain’s drainage system, is a recognized cause particularly in women using estrogen-containing contraceptives or who are pregnant or postpartum. A sudden, severe spike in blood pressure (systolic above 180 or diastolic above 120) can also produce headache, typically bilateral and pulsating.
Other secondary causes include idiopathic intracranial hypertension (a buildup of spinal fluid pressure, more common in young women with obesity), brain tumors (especially fast-growing ones or those located near the base of the skull), and inflammatory conditions affecting blood vessels in the head, such as giant cell arteritis.
When a Headache Signals Something Serious
Most headaches are not dangerous, but certain features raise concern. Neurologists use a checklist sometimes called SNOOP4 to identify red flags:
- Systemic signs: Fever, night sweats, unexplained weight loss, or a weakened immune system accompanying the headache.
- Neurological symptoms: New weakness in an arm or leg, numbness, vision changes, or confusion. Primary headaches don’t typically cause these.
- Onset is sudden: A headache that reaches maximum intensity within seconds, sometimes called a thunderclap headache, can indicate a ruptured aneurysm or other vascular emergency.
- Older age: A new headache pattern starting after age 50 is more likely to have a secondary cause, since most primary headache disorders begin earlier in life.
- Progression: A headache that steadily worsens over days or weeks, becoming more severe or more frequent, rather than the fluctuating pattern typical of primary headaches.
Any of these features, especially a sudden-onset severe headache, warrants prompt medical evaluation. The vast majority of headaches turn out to be benign, but these warning signs exist because the exceptions matter.