During meditation, theta waves were most abundant in the frontal and middle parts of the brain.

“These types of waves likely originate from a relaxed attention that monitors our inner experiences. Here lies a significant difference between meditation and relaxing without any specific technique,” emphasizes Lagopoulos.

“Previous studies have shown that theta waves indicate deep relaxation and occur more frequently in highly experienced meditation practitioners. The source is probably frontal parts of the brain, which are associated with monitoring of other mental processes.”

“When we measure mental calm, these regions signal to lower parts of the brain, inducing the physical relaxation response that occurs during meditation.”

Science Daily: Brain Waves and Meditation

(via Chris23)