Theta waves were a type of brain wave usually experienced just before or after a period of deep sleep.
In 2154, Phlox determined that Jonathan Archer's theta wave levels were fine while he was comatose. (ENT: "Doctor's Orders")
The Altonian brain teaser responded to neural theta waves. (DS9: "A Man Alone")
In 2367, Doctor Beverly Crusher tried inducing theta waves into the entorhinal cortices of the USS Enterprise-D to help them achieve REM sleep, with no effect. (TNG: "Night Terrors").
Cortical inhibitors were used to control theta wave activity. (VOY: "Remember")
In 2373, B'Elanna Torres' theta wave pattern indicated she was experiencing implanted memories, having received them telepathically from Jora Mirell. (VOY: "Remember")