Headaches have dozens of possible causes, but they fall into two broad categories: primary headaches, where the headache itself is the problem, and secondary headaches, where something else in the body is triggering the pain. About 88% of headaches seen in clinical settings are primary, meaning no underlying disease is responsible. The remaining cases stem from identifiable triggers like dehydration, infection, medication overuse, or (rarely) something more serious.
Your brain tissue doesn’t actually have pain receptors. What hurts during a headache are the surrounding structures: blood vessels, nerves, muscles, and the thin membranes (meninges) that wrap around the brain. Different headache types activate these structures through different pathways.
Tension Headaches: Muscle Strain and Stress
Tension headaches are the most common type, and they typically feel like a band of pressure squeezing around your head. The pain is usually mild to moderate, affects both sides, and doesn’t throb.
The cause involves both physical and psychological factors working together. On the physical side, sustained postures (like hunching over a screen for hours) and muscular tension in the neck and scalp create a steady stream of pain signals that sensitize nearby nerves. Over time, this makes those nerves more reactive to stimuli that wouldn’t normally hurt. On the psychological side, emotional stress, poor sleep, and depression can alter how the brain processes pain signals, essentially turning up the volume on discomfort. In people who get frequent tension headaches, this central processing issue may matter more than the muscle tension itself.
Migraines: A Nervous System Chain Reaction
Migraines involve a more complex cascade. The key player is a network of nerves called the trigeminovascular system, which connects blood vessels in the head to the brainstem. When this system becomes activated, nerve endings release a signaling molecule called CGRP, which sets off a chain reaction: blood vessels dilate, surrounding tissues become inflamed, and the nerves themselves grow increasingly sensitive.
That sensitization is what makes migraines so debilitating. Once those nerve pathways are fired up, they amplify signals from neighboring nerves, making even normal stimuli (light, sound, gentle touch on the scalp) painful. The process can also spread deeper into the brainstem, which is why migraines often come with nausea, visual disturbances, and difficulty concentrating. The initial trigger varies from person to person. Hormonal shifts, certain foods, sleep disruption, and weather changes are common culprits, but the underlying vulnerability is neurological.
Cluster Headaches: A Clock Gone Wrong
Cluster headaches are rarer but intensely painful, typically striking on one side of the head around the eye. They arrive in bouts (clusters) that last weeks or months, often at the same time each day, frequently waking people from sleep.
This clockwork timing points directly to the hypothalamus, the brain region that controls your internal body clock. The hypothalamus connects to the autonomic nervous system (which governs involuntary functions like pupil size and tear production) and to the pineal gland, which regulates melatonin. In people with cluster headaches, these circuits malfunction. Studies have found altered levels of melatonin and other circadian biomarkers during cluster periods, along with changes in core circadian gene expression. The result is pain attacks that follow a disturbingly predictable schedule, tied to your body’s own rhythms.
Dehydration and Hunger
When your body loses more fluid than it takes in, the blood becomes more concentrated. This shift in fluid balance can cause the brain to shrink slightly, pulling away from the skull. That traction tugs on blood vessels and the pain-sensitive membranes surrounding the brain, producing a dull, often all-over headache that worsens when you stand up or move quickly. Skipping meals works through a related pathway: drops in blood sugar alter blood vessel tone and can trigger similar pain signals.
These headaches typically resolve within 30 minutes to a few hours of rehydrating or eating, which makes them one of the easiest types to fix once you recognize the pattern.
Caffeine Withdrawal
If you drink coffee or tea regularly, your brain physically adapts. Caffeine works by blocking receptors for a naturally occurring chemical called adenosine, which promotes relaxation and widens blood vessels. With daily caffeine use, your brain grows extra adenosine receptors to compensate. When you suddenly stop or significantly reduce your intake, all those extra receptors are now unblocked and flooded with adenosine. Blood vessels in the brain dilate, cerebral blood flow increases, and the result is a throbbing headache that can start within 12 to 24 hours of your last cup.
Medication Overuse Headaches
Ironically, the very painkillers people use for headaches can start causing them. If you take simple pain relievers on 15 or more days per month, or stronger medications like triptans or opioids on 10 or more days per month, for longer than three months, you can develop medication overuse headache (sometimes called rebound headache). The brain becomes dependent on the medication to regulate pain signals, and each time it wears off, the headache returns, often worse than before. This creates a cycle that only breaks when the overused medication is reduced or stopped, usually under medical guidance because the withdrawal period can be rough.
Neck and Spine Problems
The top three vertebrae in your neck (C1 through C3) share nerve pathways with the head and face. When something goes wrong in this region, whether from arthritis, a disc problem, whiplash, or chronic poor posture, pain signals travel along those shared pathways and are felt as a headache. These cervicogenic headaches usually start at the back of the head or base of the skull and may spread forward. They tend to worsen with certain neck movements or sustained positions and often feel locked to one side.
Other Common Secondary Causes
Beyond the triggers above, headaches can result from sinus infections (where inflammation and congestion create pressure around the forehead and cheeks), viral illnesses like the flu or COVID, alcohol (which causes both direct blood vessel effects and dehydration), hormonal changes during menstruation or pregnancy, and poor sleep. Eye strain from prolonged screen time is another frequent contributor, though it more often causes tension-type pain than anything severe.
Weather changes are a commonly reported trigger, particularly drops in barometric pressure. While many people swear by the connection, the mechanism is less clear than often claimed. True pressure-related sinus pain occurs with large, rapid pressure shifts like diving or flying. The idea that routine weather fluctuations cause sinus inflammation through pressure differentials between your sinuses and the atmosphere is not well supported. That said, weather changes may still trigger migraines through other pathways not fully understood.
Warning Signs of a Serious Cause
The vast majority of headaches are unpleasant but not dangerous. A small percentage, however, signal something that needs urgent attention. Clinicians use a set of red flags to identify these cases:
- Sudden, severe onset: a headache that reaches maximum intensity within seconds (sometimes called a “thunderclap” headache)
- Neurological symptoms: weakness, numbness, confusion, vision loss, difficulty speaking, or decreased consciousness
- Fever with headache and stiff neck: a combination that can indicate meningitis
- New headache after age 65: first-time headaches in older adults warrant investigation
- Progressive worsening: a headache that gets steadily worse over days or weeks without relief
- Headache after head injury: particularly if symptoms are worsening
- Headache triggered by coughing, sneezing, or exertion: can occasionally indicate structural problems
- New headache in someone with cancer or a compromised immune system
Any of these patterns warrants prompt medical evaluation, not because they always indicate something dangerous, but because ruling out serious causes early makes a significant difference in outcomes.