Having a headache is a pain—literally and figuratively. Whether the pain is splitting, pulsating, or downright all-encompassing, headaches can wreak havoc on your life. And it’s even worse when your headaches seem constant as if the pain is nagging you all the damn time. Unfortunately, there is a surprisingly long list of causes of constant headaches. Some reasons behind chronic headaches are not serious, while other causes can signify a deeper health issue is at play.

What causes a headache?
Experts don’t completely understand what’s happening in our skulls when a headache hits, but the most likely explanation is that something causes the blood vessels to swell, subsequently stretching the nerves around them and firing off pain signals.

While the underlying causes are vast, there are three primary types of headaches: migraines, tension, and cluster, Susan Hutchinson, M.D., director of Orange County Migraine & Headache Center, tells SELF. Here’s a quick summary of each type:

Cluster headaches
This type occurs usually on one side of the head, causes one eye to tear, and leaves you feeling extremely agitated. An “attack” may last weeks or months, according to the Mayo Clinic. And as the name implies, they occur in cycles or clusters, and are broken up by periods of remission, the Mayo Clinic says. Each cycle of pain can peak over the span of 5 to ten minutes and can build in severity for three hours before going away, according to the National Institutes of Health Genetic and Rare Disorders Center (GARD). Cluster headache spells last, on average, for 6 to 12 weeks before going into remission.

The cause is unknown, but cluster headaches may occur if something with the body’s biological clock is off, per the Mayo Clinic. Also, cluster headaches generally aren’t triggered by certain factors, like stress or hormonal changes, the way tension headaches and migraines can be. There are exceptions to this, however. Certain medications can trigger cluster headaches, according to the Mayo Clinic. Additionally, alcohol triggers pain spells in over half of those who live with cluster headaches, but it has no effect once the cluster episode has ended, GARD says. They are not very common and seem to run in families, Dr. Hutchinson says.

Migraines
There are actually four distinct stages of a migraine. There is the prodrome stage, which happens a few days before the onset of pain and can include constipation, yawning, food cravings and neck stiffness, according to the Mayo Clinic. There is also the aura stage which lasts for about 20 to 60 minutes. It includes physical sensations like seeing bright spots or flashes of lights, numbness in the face or one side of the body, as well as difficulty speaking and uncontrollable jerking, the Mayo Clinic says. Then there’s the attack stage, which can last from 4 to 72 hours and is typically marked by pain, sensitivity to light, touch, smell and sound, as well as nausea and vomiting. Finally, there’s the post-drome phase that might leave you feeling tired and confused, for the day, according to the Mayo Clinic. It’s important to note that not everyone experiences all four of these phases. Most commonly, migraines cause pain (sometimes so intense that it affects a person’s ability to function) on one side of the head and possibly nausea and/or sensitivity to light, the Mayo Clinic explains.

It’s not totally clear what causes migraines, but it’s possible that they have to do with “changes in the brainstem and its interactions with the trigeminal nerve,” the Mayo Clinic says. (Changes in serotonin levels in the brain may also play a role, but more research is needed to determine how and why.) Experts believe migraines are primarily genetic.

Tension headaches
Anyone can get a tension headache, which is caused by muscle tightness in the head, neck, or scalp, according to MedlinePlus. “Unlike migraines, which we think are genetically predisposed, tension headaches are pretty universal,” Dr. Hutchinson says.