Headaches have dozens of possible causes, but nearly 98% fall into three categories: tension headaches, migraines, and cluster headaches. These are called primary headaches, meaning the headache itself is the problem rather than a symptom of something else. The remaining small percentage are secondary headaches, caused by an underlying condition like an infection, injury, or vascular problem. Understanding which type you’re dealing with points you toward the right fix.
Tension Headaches: The Most Common Type
Tension headaches feel like a tight band of pressure around your head, and they’re the headache most people experience. The pain comes from a combination of muscle tightness in the head and neck and heightened sensitivity in the brain’s pain-processing pathways. Stress is the leading trigger. Poor posture is a close second, particularly the forward-head position that comes from hours at a computer or phone screen. When your neck stays flexed, your shoulders compensate by rounding forward, creating muscle imbalances that tighten the muscles at the base of your skull and pull on pain-sensitive tissue.
Disturbed sleep is another major trigger, and the connection runs deeper than just feeling run down. Sleep loss increases fatigue and sympathetic nervous system activity (your body’s fight-or-flight response), which can directly worsen or trigger head pain. Poor sleep also reduces levels of a brain chemical called orexin that normally helps suppress pain signaling. When orexin drops, the brain’s pain-filtering system becomes less effective, and headaches follow.
What Causes Migraines
Migraines involve a different mechanism than tension headaches. The trigeminal nerve, which is the main sensory nerve of your face and head, becomes activated and releases a signaling molecule called CGRP from its nerve endings. This kicks off a cascade: blood vessels in the membranes surrounding the brain dilate, surrounding tissue becomes inflamed, and the nerve fibers themselves become increasingly sensitized. Once that sensitization takes hold, it can spread to other parts of the nervous system, which is why migraines often come with light sensitivity, nausea, and pain that worsens with movement.
CGRP doesn’t just act at the nerve endings. It also gets released inside clusters of nerve cells near the base of the skull, where it interacts with neighboring neurons and support cells to amplify and sustain the pain signal. This self-reinforcing loop helps explain why migraines can last hours or even days once they start. Newer migraine treatments work specifically by blocking CGRP activity, which confirms how central this molecule is to the process.
Hormonal Triggers
For women who get migraines around their period, the trigger is a sharp drop in estrogen levels. Estrogen normally has a protective, pain-dampening effect on the trigeminal nerve system. During the menstrual cycle, estrogen peaks the day before ovulation, dips, rises again during the mid-luteal phase, then drops steeply right before menstruation begins. That final steep decline removes the protective effect and leaves the trigeminal system more reactive. Studies show that pain sensitivity across the body is measurably higher during low-estrogen phases. In one experiment, the same painful stimulus applied to women’s foreheads produced larger areas of pain and redness during menstruation compared to the luteal phase, pointing to increased trigeminal sensitization when estrogen is low.
Cluster Headaches and the Brain’s Clock
Cluster headaches are rarer but far more intense, producing severe, stabbing pain around one eye that lasts 15 minutes to three hours. They’re strongly linked to the hypothalamus, the brain region that regulates your circadian rhythm. The attacks tend to strike at the same time each day (often at night), follow seasonal patterns, and come in clusters lasting weeks or months before remitting. Brain scans taken during attacks show activation in the lower part of the hypothalamus, and structural imaging studies have found physical abnormalities in that same region.
Patients with cluster headaches also show decreased melatonin levels and disrupted circadian rhythms. Genetic research has identified possible links to genes involved in the sleep-wake cycle and circadian timing. Deep brain stimulation targeting the hypothalamus has proven effective for patients who don’t respond to medication, further confirming the region’s role.
Dehydration, Caffeine, and Other Physical Triggers
Dehydration causes headaches through a mechanical process. When your body loses too much fluid, the brain’s surrounding environment becomes more concentrated, and brain tissue can pull slightly away from the skull. This puts traction on the pain-sensitive membranes and blood vessels that line the inside of the skull, producing a dull, aching headache that typically worsens when you stand up or move around. Drinking water usually resolves it within an hour or two.
Caffeine withdrawal is another common and often misunderstood trigger. Caffeine narrows blood vessels in the brain and blocks a chemical called adenosine that promotes relaxation and blood flow. When you suddenly cut back, adenosine activity rebounds, blood vessels in the brain dilate, and cerebral blood flow increases. The result is a throbbing headache that typically starts 12 to 24 hours after your last cup of coffee, peaks between 20 and 51 hours, and can persist for two to nine days. You don’t have to quit cold turkey to trigger it. Even a noticeable reduction in your usual intake can be enough.
Skipped meals can trigger headaches through blood sugar drops, and alcohol causes headaches both through dehydration and through byproducts your body creates while metabolizing it.
Medication Overuse Headaches
Ironically, taking pain relievers too often can cause more headaches. If you’re using over-the-counter pain medications on 15 or more days per month for simple analgesics like ibuprofen or acetaminophen, or 10 or more days per month for stronger medications like combination painkillers or triptans, and this continues for three months or longer, the medication itself can start generating daily or near-daily headaches. The brain adapts to the frequent presence of pain relief and becomes more sensitive to pain when the medication wears off, creating a cycle of increasing use and worsening headaches.
Breaking the cycle requires reducing the medication, which usually makes headaches temporarily worse before they improve. This is one of the most common reasons people with occasional headaches transition to chronic daily headaches.
“Sinus Headaches” Are Usually Migraines
If you frequently get headaches you’d describe as sinus pressure, there’s a good chance they’re actually migraines. Studies consistently find that about 80% of people who self-diagnose or are diagnosed with sinus headaches actually meet the clinical criteria for migraine. The confusion makes sense: migraines can cause facial pressure, nasal congestion, and watery eyes, all of which feel like a sinus problem. But true sinus headaches almost always accompany an active sinus infection with thick, discolored nasal discharge and sometimes fever. If your “sinus headaches” come and go without an infection, responding to migraine-specific treatment is worth exploring.
Morning Headaches and Sleep Apnea
Waking up with a headache regularly, particularly a dull, pressing headache on both sides of the head, can signal obstructive sleep apnea. During apnea episodes, breathing repeatedly stops and restarts, which can reduce blood oxygen levels and raise carbon dioxide in the blood. The increased carbon dioxide causes blood vessels in the brain to dilate, and the repeated sleep disruptions prevent restorative rest. The exact mechanism is still debated. Some research ties morning headaches to time spent at low oxygen levels during the night, while other studies find the headaches correlate more with the sleep fragmentation itself than with oxygen measurements. Either way, if morning headaches are a pattern for you, especially combined with loud snoring or daytime fatigue, sleep apnea is worth investigating.
Warning Signs of a Dangerous Headache
Secondary headaches, those caused by an underlying problem, are uncommon but can be serious. Clinicians use a set of red flags to distinguish worrisome headaches from benign ones:
- Thunderclap onset: a headache that reaches maximum intensity within seconds, which can indicate bleeding in the brain
- Neurological symptoms: confusion, weakness on one side, vision changes, difficulty speaking, or loss of consciousness
- Fever and systemic illness: headache with high fever, stiff neck, or rash may suggest meningitis or another infection
- New headache after age 65: first-time headaches later in life have a higher chance of being secondary
- Pattern change: a headache that feels fundamentally different from your usual headaches, or one that progressively worsens over days to weeks
- Positional component: headaches that change dramatically when you stand up or lie down
- Triggered by exertion: headaches brought on by coughing, sneezing, or exercise
Any of these features, especially in combination, warrants prompt medical evaluation. A sudden, severe headache that feels like “the worst headache of your life” should be treated as an emergency.