Headaches happen when pain-sensitive structures in and around your brain get irritated, stretched, compressed, or inflamed. Your brain tissue itself can’t feel pain, but the network of nerves, blood vessels, and muscles surrounding it certainly can. About 25% of the global population experiences tension-type headaches in any given year, and 14% experience migraines, making headaches one of the most common health complaints on the planet.
How Your Brain Registers Head Pain
Your skull is lined with a thin membrane, and your head and face are wired with a major nerve network called the trigeminal system. When something goes wrong, whether it’s muscle tension, inflamed blood vessels, or chemical shifts in your brain, these nerves pick up the signal and send it to your brainstem, where it registers as pain. The trigeminal nerve is the main pathway for nearly all headache types, which is why head pain can radiate across your forehead, temples, behind your eyes, or down into your jaw.
Different headache types activate this system in different ways. A tension headache involves the muscles and nerves around your scalp and neck. A migraine involves deeper chemical changes inside the brain itself. But the final pain signal travels through the same nerve highway.
Tension Headaches: The Most Common Type
That band-like pressure squeezing both sides of your head is almost always a tension-type headache. For decades, doctors assumed these were caused by muscles in the face, neck, and scalp physically tightening up from stress. That theory has largely been replaced. Research now suggests the real problem is a sensitized pain system: your brain becomes more responsive to pain signals than it should be, and normal sensations from scalp, neck, and shoulder muscles start registering as painful.
This is why tension headaches often come with tenderness in those muscles even though the muscles themselves aren’t damaged or cramping. Stress, fatigue, poor posture, and eye strain don’t directly cause the headache so much as they lower the threshold at which your pain system fires. Think of it like a smoke alarm set too sensitive: normal cooking fumes set it off.
What Happens During a Migraine
Migraines are a different animal entirely. They involve a cascade of chemical events inside the brain, centered on a signaling molecule called CGRP. During a migraine attack, the trigeminal system releases CGRP in large amounts, which dilates blood vessels, triggers inflammation around the brain’s outer lining, and sensitizes nearby nerve endings. This creates the throbbing, one-sided pain that can last hours or days, often paired with nausea, light sensitivity, and sound sensitivity.
CGRP levels are measurably elevated during migraine attacks and drop after successful treatment. This isn’t just a correlation: injecting CGRP into someone who gets migraines can trigger a migraine-like headache, and medications that block CGRP are now some of the most effective migraine treatments available. People who experience migraines appear to have a trigeminal system that’s more reactive to triggers than average, which is why certain foods, hormonal shifts, weather changes, or stress can set off an attack in one person but not another.
Dehydration Shrinks Your Brain (Temporarily)
When you don’t drink enough water, your brain and surrounding tissues physically contract. As the brain shrinks slightly, it pulls away from the skull, tugging on the pain-sensitive nerves attached to the membrane lining the inside of your head. That pulling creates a dull, aching headache that often gets worse when you stand up, bend over, or move quickly. The fix is straightforward: drinking water and giving your body 30 to 60 minutes to rehydrate usually resolves it completely.
Food and Drink Triggers
Certain chemicals found naturally in foods or added during processing can lower your headache threshold. The main culprits include tyramine (found in aged cheeses, cured meats, and fermented foods), sulfites (in wine and dried fruits), MSG (common in processed and restaurant foods), and artificial sweeteners like aspartame. These chemicals don’t cause headaches in everyone, but for people who are already prone to headaches, they create what researchers call a “trigger load.” One glass of red wine on a good day might be fine. That same glass after a night of poor sleep and a stressful workday might be the thing that tips you over into a headache.
Caffeine is a particularly interesting trigger because it works both ways. In small amounts, it can relieve headaches by narrowing dilated blood vessels. But if you drink it regularly and then skip a day, the withdrawal itself causes a headache as those blood vessels rebound and expand.
Sleep Deprivation and Pain Sensitivity
Your brain uses sleep to clear out a chemical called adenosine that builds up during waking hours. Adenosine accumulates the longer you stay awake and is one of the signals that makes you feel sleepy. After extended wakefulness (in one study, 52 hours without sleep), the brain’s adenosine receptor system becomes significantly more active, and this elevated state only returns to normal after a full recovery sleep period.
What this means in practical terms: when you’re sleep-deprived, your brain’s chemical balance is off, and your pain processing system becomes more sensitive. Headaches from poor sleep tend to feel like a heavy, diffuse ache rather than a sharp or throbbing pain, and they often come with difficulty concentrating and irritability. Even modest sleep loss, like getting five hours instead of seven for several nights in a row, can increase headache frequency.
Medication Overuse Headaches
One of the most frustrating causes of recurring headaches is the very medication you take to treat them. When you use pain relievers for headaches more than two or three days per week, your brain adjusts to having that chemical help. When the medication wears off, the pain rebounds and often feels worse than the original headache. This creates a cycle: you take more medication because the headache is worse, which makes the next rebound worse, and so on.
This happens with over-the-counter painkillers, prescription pain medications, and even migraine-specific drugs. Stronger medications like opioids can add actual physical withdrawal symptoms on top of the rebound effect. Breaking the cycle typically requires a period of stopping the overused medication entirely, which means enduring worse headaches for a week or two before the pattern resets.
Secondary Headaches From Other Conditions
Most headaches are “primary,” meaning the headache itself is the problem. But headaches can also be symptoms of something else going on in your body. The International Headache Society recognizes dozens of secondary causes, grouped into broad categories:
- Sinus and structural issues: inflammation in your sinuses, dental problems, jaw tension, or eye strain can all produce headaches localized near the affected area
- Infections: fevers and viral illnesses commonly cause headaches as part of your immune response
- Head or neck injury: even minor impacts can trigger headaches that persist for days or weeks
- Substance use or withdrawal: alcohol, medications, and recreational drugs can all cause headaches directly or when you stop using them
- Blood vessel problems: changes in blood flow to the brain, from high blood pressure to more serious vascular conditions, can produce headaches
The key distinction between a routine headache and one worth urgent attention is the pattern. A headache that feels completely different from anything you’ve experienced before, one that comes on like a thunderclap within seconds, or one paired with fever, stiff neck, confusion, or vision changes is your nervous system signaling that something beyond a typical headache is happening.