A migraine is not just a bad headache. It’s a cascade of electrical, chemical, and vascular events that unfolds across your brain over hours or even days, involving regions responsible for pain processing, sensory filtering, and basic functions like sleep and appetite. The process typically moves through distinct phases, each driven by different brain activity, starting well before the pain hits and lingering after it fades.
The Early Warning: Your Brain Activates Hours Before Pain
Many people with migraine notice subtle changes 2 to 48 hours before the headache begins: fatigue, food cravings, mood shifts, neck stiffness, or difficulty concentrating. These aren’t just coincidences. Brain imaging studies show that during this “prodrome” phase, several deep brain structures become unusually active, most notably a region called the hypothalamus, which regulates sleep, hunger, mood, and body temperature. That explains why the early warning signs feel so physical and widespread.
Along with the hypothalamus, blood flow increases in areas involved in emotional processing, memory, and pain modulation. The brainstem, which acts as a gatekeeper for pain signals traveling up to the rest of the brain, also ramps up activity. This early activation suggests that migraine doesn’t start with pain at all. It starts with your brain’s regulatory systems falling out of balance, lowering the threshold for what will eventually trigger the painful phase.
The Electrical Wave Behind Aura
About one in three people with migraine experience aura: shimmering lines in their vision, blind spots, tingling in their hands or face, or even temporary difficulty speaking. What causes this is a phenomenon called cortical spreading depression, a slow-moving wave of intense electrical activity that sweeps across the surface of the brain. Neurons fire in a burst of excitation, then go silent as the wave passes over them.
This wave typically begins in the visual processing area at the back of the brain. As it rolls forward, it briefly increases blood flow in the affected area, followed by a longer period of reduced blood flow and suppressed activity. Brain imaging has captured this pattern in real time: a leading edge of activation trailed by inhibition, creeping across the cortex. When the wave moves through visual areas, you see the zigzag lines or blind spots. When it reaches areas that process touch or language, you get tingling or word-finding trouble. The symptoms map directly onto whichever brain region the wave is passing through.
Even people who don’t experience aura may have a milder version of this wave occurring in brain areas that don’t produce obvious sensory symptoms.
How the Pain Signal Gets Triggered
The throbbing pain of migraine originates from a network of nerves that wraps around the blood vessels in your brain’s protective membranes. This network, called the trigeminovascular system, is the brain’s primary pain-sensing apparatus for structures inside your skull. When it becomes activated, nerve endings release a powerful signaling molecule called CGRP, which is abundant in these nerve fibers.
CGRP sets off a chain reaction. It causes blood vessels in the membranes surrounding the brain to dilate and become inflamed, a process called neurogenic inflammation. It also triggers the release of nitric oxide, another vasodilator, and sensitizes the nerve endings so they respond to stimuli that wouldn’t normally register as painful. Pressure changes from your own pulse, for instance, start generating pain signals, which is why migraine pain often throbs in rhythm with your heartbeat.
The signaling doesn’t stop at the nerve endings. CGRP released within the nerve cluster itself interacts with neighboring neurons and surrounding support cells, creating a feedback loop that amplifies and sustains the pain. This peripheral sensitization can then drive changes deeper in the brain, making second-order pain neurons in the brainstem more excitable. Once that central sensitization sets in, even touching your scalp or moving your head can become painful.
Why Serotonin Matters
People with migraine consistently show lower levels of serotonin, a brain chemical that, among many other roles, helps keep blood vessels from dilating excessively and keeps pain signaling in check. When serotonin drops, it leaves vasodilators like nitric oxide and CGRP unopposed. Blood vessels expand, pain pathways activate more easily, and the whole system tips toward a migraine.
This is why the most widely used migraine-specific medications, triptans, work by mimicking serotonin. They bind to serotonin receptors on the trigeminal nerve endings and blood vessels, which constricts dilated vessels and, critically, suppresses the release of CGRP. When CGRP levels drop back to normal, the inflammatory cascade calms down. Newer medications take a more direct approach, blocking CGRP itself or the receptors it binds to, rather than working through serotonin. In clinical trials, these newer treatments provided complete pain freedom within two hours for about 19 to 21 percent of patients, compared to roughly 11 to 14 percent on placebo. For prevention, they reduced monthly migraine days by about 4 days compared to about 2.5 on placebo.
Why Light and Sound Become Unbearable
During a migraine, ordinary light can feel blinding and normal conversation can feel like noise. This isn’t psychological. It’s a measurable change in how your brain’s relay center, the thalamus, processes sensory information. The thalamus normally acts as a filter, deciding which signals deserve your attention and which get dampened. During a migraine, neurons in the posterior thalamus become hyperactive.
Research has revealed something striking about these thalamic neurons: they respond to both pain signals from the brain’s membranes and light signals from the eyes. The same individual neurons process both types of input. This convergence of pain and visual pathways in one location explains why light doesn’t just bother your eyes during a migraine but actually makes the headache worse. These neurons also project to a wide range of brain areas involved in vision, pain processing, and attention, which is why the sensory overload during a migraine feels so total and inescapable. Brain imaging in humans confirms that people with light sensitivity show significantly more thalamic activity than those without.
The Genetic Wiring That Makes It Possible
Not everyone’s brain is equally vulnerable to this cascade. Migraine runs in families, and genetic research has identified specific mutations in genes that control how ions flow in and out of neurons. These ion channels are essentially the electrical switches that determine how easily a neuron fires. In people with certain mutations, neurons are set closer to their firing threshold, making the brain more electrically excitable at baseline.
This heightened excitability means it takes less provocation (whether from stress, hormonal shifts, sleep disruption, or other triggers) to set off the cortical spreading depression wave or to activate the trigeminovascular system. It also helps explain why migraine brains react differently to sensory stimulation even between attacks. The predisposition isn’t just about having headaches. It’s about having a nervous system that processes signals with less of a buffer zone.
Blood Vessels: Cause or Consequence?
For decades, migraine was considered a vascular disorder, caused by blood vessels in the brain first constricting and then painfully dilating. The current understanding is more nuanced. The trigeminovascular hypothesis treats blood vessel dilation as a consequence of nerve activation and neurogenic inflammation, not the original trigger. Neurons activate first, release CGRP and other molecules, and blood vessels dilate in response.
However, emerging evidence suggests the relationship runs in both directions. Neurosurgical observations confirm that directly stimulating or stretching arteries inside the skull can produce migraine-like pain. Every known molecular migraine trigger potently dilates intracranial blood vessels. And treatments that constrict blood vessels or block vasodilatory chemicals effectively stop attacks. The most current model treats intracranial blood vessel dilation as both a cause and an effect within the migraine cascade, with vascular changes and nerve activation feeding into each other in a loop rather than following a simple one-way sequence.
The Migraine Hangover
After the pain fades, many people feel drained, foggy, and off-balance for hours or even a day or two. This postdrome phase is not just psychological exhaustion. Imaging studies from the 1980s onward show that abnormal blood flow patterns in the brain outlast the headache itself. Increased blood flow to certain regions, particularly in the cortex, persists after the pain has resolved. The visual cortex, in particular, continues to show altered activity compared to baseline. Even successful treatment with medication doesn’t fully normalize these blood flow changes right away. Brain perfusion in cortical and brainstem regions can remain disrupted after the pain is gone, which likely accounts for the cognitive sluggishness, difficulty concentrating, and general sense of vulnerability that characterizes the hours after a migraine.