Brainwaves can be measured with an EEG. That’s how they can diagnose seizures because sharp spikes or disruptions in brainwaves are seen. There are different frequencies of brain waves. When people have been studied meditating depending on their practice level they go into more relaxed brainwave states. I believe Alpha brainwaves are typically seen in meditation, while advanced practitioners exhibit Theta. Delta brainwaves are seen in deep dreamless sleep. Look up some of the studies on it. I think you’ll find it interesting.

Based on what I know about the brain. I think we currently know about as much about the brain as the entirety of space. Also kind of cool how it’s the one organ that studies itself.