A headache is not your brain feeling pain. Your brain tissue has no pain receptors at all. Instead, what you feel during a headache comes from pain-sensitive structures surrounding the brain: blood vessels, membranes called meninges, muscles, and nerves in the head and neck. These structures become irritated, inflamed, or compressed, and they send pain signals through a major nerve network to your brain, which interprets those signals as the ache, throb, or pressure you recognize as a headache. Roughly 35% of the global population experiences a headache disorder in any given year, making it one of the most common health conditions on earth.
The Pain Signaling System in Your Head
The key player in most headaches is the trigeminal nerve, a large nerve that branches across your face, jaw, and the membranes surrounding your brain. When something irritates the tissues this nerve monitors, its fibers fire and relay pain signals to processing centers deeper in your brain. During this activation, nerve endings release signaling molecules that cause nearby blood vessels to widen and leak fluid into surrounding tissue. This creates a local inflammatory response right at the surface of the brain’s protective membranes.
That inflammation does two things. First, it directly activates more pain receptors, amplifying the signal. Second, it makes those nerve endings hypersensitive to normal stimuli they would usually ignore, like the pulsing of blood through arteries or the pressure of bending over. This is why a headache often feels worse when you cough, lean forward, or climb stairs. Peripheral nerve fibers that were previously quiet start responding to ordinary mechanical movement as though it were painful.
Over time, if this signaling continues, the central processing neurons in your brainstem can become sensitized too. Once that happens, even light touch on the scalp or face can feel painful, a phenomenon called allodynia. This central sensitization is one reason headaches can be harder to treat the longer they go on.
What Happens During a Tension Headache
Tension headaches, the most common type, produce a steady, pressing or tightening sensation on both sides of the head. For decades, doctors assumed tense muscles in the scalp, jaw, and neck were squeezing their way to pain. The reality is more nuanced. Studies consistently show that the muscles and soft tissues around the skull are significantly more tender in people with tension headaches than in healthy people, and that tenderness correlates with both the intensity and frequency of the headaches. But muscle activity itself is normal or only slightly elevated, and there’s no evidence of reduced blood flow to the muscles or ongoing inflammation at tender points.
What appears to happen instead is that the nerve endings in these muscles become sensitized. When researchers injected naturally occurring pain-signaling substances into the trapezius muscle of people with frequent tension headaches, those individuals experienced more pain than healthy controls given the same injection. The peripheral nerves in their muscles had essentially turned up their sensitivity dial, reporting normal sensations as painful ones. In people who get tension headaches frequently enough, this peripheral sensitization may eventually drive changes in how the central nervous system processes pain, turning episodic headaches into chronic ones.
What Happens During a Migraine
A migraine is a far more complex neurological event that can unfold in distinct phases over hours or even days. Many people experience a warning phase hours before the pain starts: fatigue, food cravings, mood changes, neck stiffness, or increased yawning. This prodrome phase is thought to originate deep in the brain, though the exact trigger remains debated.
About one in four migraine sufferers then experience an aura, typically lasting 20 to 60 minutes. Aura is caused by a slow wave of abnormal electrical activity that spreads across the surface of the brain. This wave, called cortical spreading depression, causes neurons to fire in sequence and then go temporarily quiet. The result can be visual disturbances (zigzag lines, blind spots, shimmering patches), tingling in the face or hands, or difficulty finding words, depending on which part of the brain the wave crosses.
The headache phase itself involves activation of the trigeminal nerve system described above. Nerve fibers around the brain’s blood vessels release inflammatory signaling molecules that widen blood vessels, cause fluid leakage, and trigger mast cells (immune cells that release histamine and other irritants). This cascade creates the throbbing, often one-sided pain characteristic of migraine. The throbbing quality has long been linked to pulsation in arteries, though some research has questioned whether the timing of the throb always matches the arterial pulse. What is clear is that the trigeminovascular system becomes deeply sensitized, making the head exquisitely sensitive to movement, light, sound, and sometimes smell.
After the pain subsides, many people enter a postdrome phase, sometimes called a “migraine hangover,” marked by fatigue, difficulty concentrating, and lingering sensitivity. A full migraine cycle, from first warning signs to full recovery, can last anywhere from a few hours to several days.
Cluster Headaches and the Brain’s Clock
Cluster headaches are rarer but among the most painful conditions known. They produce excruciating, stabbing pain on one side of the head, usually around or behind the eye, lasting 15 minutes to three hours. They arrive in clusters: multiple attacks per day for weeks or months, followed by long pain-free periods.
Brain imaging studies have revealed something striking about cluster headaches. During an attack, a specific region deep in the brain, the hypothalamus, lights up on the same side as the pain. This activation appears only during the pain state and is not seen in cluster headache patients between bouts. The hypothalamus houses the body’s master biological clock, which governs sleep-wake cycles, hormone release, and body temperature. Its involvement explains the clockwork regularity of cluster attacks, which tend to strike at the same time each day, often during sleep.
Patients with cluster headache show measurable disruptions in their circadian biology: lower testosterone levels during cluster periods, blunted melatonin peaks at night, and irregular responses to hormones regulated by the hypothalamus. The intense pain itself is carried by the trigeminal nerve, while the hallmark autonomic symptoms, a watery eye, drooping eyelid, nasal congestion, and facial sweating on the affected side, result from activation of parasympathetic nerve pathways running alongside the facial nerve.
Common Triggers and Why They Cause Pain
Dehydration is one of the most straightforward headache triggers. When your body loses fluid, the brain can shrink slightly, pulling away from the meninges that line the inside of the skull. This traction stimulates pain receptors embedded in those membranes. Rehydrating usually reverses the process, which is why a dehydration headache often resolves within an hour or two of drinking water.
Hormonal fluctuations explain why headaches are significantly more common in women, particularly around menstruation. When estrogen levels drop, as they do just before a period, pain receptors on trigeminal nerve cells become more excitable. Research in animals has shown that higher estrogen levels increase the production of specific pain-sensing proteins on these nerve cells, essentially priming them to fire more easily. When estrogen then drops, the sensitized system is left exposed, and pain thresholds fall. This is why menstrual migraines tend to hit in a predictable window around the start of a period.
Other well-established triggers include irregular sleep, alcohol (particularly red wine and beer), strong smells, bright or flickering lights, skipped meals, and stress or the letdown period after stress. These triggers don’t cause damage. They activate or lower the threshold of a pain-signaling system that is already primed in people who are headache-prone.
How Pain Medication Can Backfire
One of the more counterintuitive things that happens with headaches is that the medications used to treat them can, with frequent use, make them worse. This is called medication overuse headache. Taking acute pain relief, whether over-the-counter painkillers or prescription migraine drugs, more than two or three days per week can gradually sensitize the central nervous system. The brain adjusts to the regular presence of pain-blocking chemicals by lowering its own pain threshold. The result is more frequent headaches, which prompt more medication, which drives the cycle further.
There may also be a behavioral component. People with medication overuse headache show a higher prevalence of substance dependence patterns, suggesting the cycle involves learned habits around reaching for medication at the first hint of discomfort. Breaking the cycle typically requires gradually reducing the overused medication, a process that can temporarily worsen headaches before they improve.