What happens in the brain during a cluster headache?since theres other symptoms like droopy eyelid, runny nose, something must be going on in there!

Short answer...: It's not well understood. Cluster headaches are thought to be vascular in origin (dilation of blood vessel in the head putting pressure on nearby nerves, like migraines) but that's really only the beginning. Up in the skull, it's all very complex, interconnected and cross-wired, and we're still trying to figure it all out!

See below: The exact cause of cluster headaches is unknown, but abnormalities in the hypothalamus likely play a role. Cluster attacks usually occur with clocklike regularity during a 24-hour day, and the cycle of cluster periods often follows the seasons of the year. Unlike migraine and tension headache, cluster headache generally isn't associated with triggers, such as foods, hormonal changes or stress.