When you get a headache, your brain itself isn’t feeling pain. It has no pain receptors. Instead, a network of nerves surrounding your brain, running through your scalp, and threading through the muscles in your head and neck is picking up signals and interpreting them as pain. About 2 billion people worldwide live with tension-type headaches, and another 1.2 billion experience migraines, making headaches one of the most common human experiences. What’s actually happening inside your head depends on which type you’re dealing with.
Your Brain Can’t Feel Pain
This surprises most people, but the brain has no pain-sensing nerve fibers. The pain you feel during a headache comes from structures around the brain: the membranes covering it (called meninges), the blood vessels running along its surface, and the muscles and nerves in your scalp, face, and neck. These tissues are loaded with pain-sensing nerve fibers, particularly from the trigeminal nerve, which is the main nerve responsible for sensation in your face and head.
The trigeminal nerve branches into three divisions, and its uppermost branch densely covers the meninges with two types of pain fibers. When these fibers are activated, they send pain signals through a relay system to your brain’s pain-processing centers. Your brain then registers the sensation as a headache, even though the brain tissue itself feels nothing.
What Happens During a Tension Headache
Tension headaches, the most common type, feel like a tight band squeezing around your head. The most prominent physical finding in tension headaches is tenderness in the muscles surrounding the skull: the muscles across your forehead, temples, jaw, and the back of your neck. For years, researchers assumed these muscles were simply contracting too hard. The reality is more complicated.
The tenderness likely involves sensitization of the pain receptors in those muscles, meaning the nerve endings become more reactive and start firing at lower thresholds than normal. Essentially, pressure or tension that your body would normally ignore starts registering as painful. In people with frequent tension headaches, researchers have found that this tenderness tends to be worse on the side where the headache is concentrated. In chronic cases, the problem may extend beyond the muscles themselves. The central nervous system can become sensitized too, amplifying pain signals that would normally be filtered out. This central sensitization helps explain why tension headaches can become self-reinforcing: the longer they last, the more your pain system ramps up.
What Happens During a Migraine
Migraines involve a more complex cascade of events, and they can unfold in distinct stages over hours or even days.
The Buildup
Many migraines begin with a prodrome phase, hours to days before the actual headache. You might notice neck stiffness, fatigue, sensitivity to light or sound, or subtle mood changes. About one in four migraine sufferers also experience an aura, which typically develops over 5 to 60 minutes before the pain starts. Auras can include visual disturbances like zigzag lines or blind spots, tingling in the face or hands, or difficulty finding words.
The Pain Phase
During the headache itself, the trigeminal nerve endings around your brain release a signaling molecule called CGRP. This is one of the key chemical events in a migraine. CGRP triggers a chain reaction: it promotes the production of nitric oxide (a compound that widens blood vessels) and sensitizes nearby nerve fibers, making them increasingly responsive to stimulation. The result is a feedback loop where activated nerves release more chemicals, which activate more nerves.
At the same time, serotonin levels drop. Serotonin normally helps regulate pain and keeps blood vessels in a balanced state. When it falls, one of its counterbalancing effects against nitric oxide disappears, leaving the pain-promoting signals unopposed. This is one reason migraine medications that boost serotonin activity can provide relief, and it may also explain a curious observation: some migraine sufferers report that vomiting ends their headache. Vomiting stimulates gut activity, which raises serotonin levels in the blood.
The Aftermath
After the headache resolves, many people experience a postdrome that can last up to 48 hours. Common symptoms include difficulty concentrating, fatigue, and lingering neck stiffness. Some people describe feeling “washed out,” while others report a brief period of euphoria.
Why Light and Sound Hurt
If you’ve ever needed to lie in a dark, quiet room during a headache, there’s a specific reason. During a migraine, light signals from your eyes travel to a relay station deep in the brain called the thalamus, where they converge directly on the same neurons carrying pain signals from the trigeminal nerve. This convergence means that incoming light literally amplifies the activity of pain-processing neurons. The enhanced neural activity then reaches areas of the brain involved in sensory, visual, and auditory perception, which is why sound can feel painful too, not just light.
The Blood Vessel Question
For decades, the standard explanation was that headaches were caused by blood vessels in the brain dilating and pressing on surrounding nerves. This made intuitive sense: the throbbing quality of many headaches seemed to match a pulse in swollen arteries. But recent research has challenged this idea significantly.
Studies measuring blood vessel size during spontaneous migraines found little to no dilation actually occurring. Equally telling, blood vessel dilation doesn’t always produce a headache. Scientists now view the vascular changes as a side effect rather than a cause. The primary driver is the nervous system itself, specifically the activation and sensitization of pain pathways. Blood vessels still play a role in the overall picture, since they’re closely intertwined with the nerve fibers that generate pain signals, but widening arteries alone don’t explain why your head hurts.
Hormonal Triggers
Roughly half of women with migraines notice a connection to their menstrual cycle. The mechanism involves estrogen. In the days before menstruation, estrogen levels drop sharply after being elevated for much of the cycle. This withdrawal appears to lower the threshold for migraine activation by making the trigeminal pain system more excitable. Estrogen normally has a moderating effect on pain signaling within the trigeminovascular system, so when it drops, that protective influence weakens. This is why menstrual migraines tend to strike in a predictable window: the day or two before a period begins through the first few days of bleeding.
Primary Versus Secondary Headaches
Most headaches are “primary,” meaning they are the condition itself rather than a symptom of something else. Tension headaches and migraines fall into this category. “Secondary” headaches are caused by an underlying problem: an infection, a bleed in the brain, a tumor, or elevated pressure inside the skull, among other causes.
Certain patterns can signal that a headache is secondary and needs prompt attention. A sudden, explosive headache that peaks within seconds (often described as the worst headache of your life) is a red flag. Other warning signs include headaches accompanied by fever, confusion, vision changes, weakness on one side of the body, or seizures. A new headache pattern starting after age 65, headaches that worsen with coughing or changing position, or headaches that progressively get worse over days to weeks also warrant evaluation. These features don’t automatically mean something dangerous is happening, but they’re the signs that distinguish routine headaches from those that need imaging or further testing.