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")