Delta wave
A delta wave was a brain pattern usually associated with deep sleep or a coma.
 
When Doctor Julian Bashir was attacked by a Lethean named Altovar in 2371, he fell into a coma. He believed himself to be conscious and on Deep Space 9 after the attack; however, he learned that what seemed to be the space station was actually a representation of his mind, since a tricorder he used was only able to pick up delta waves from himself and nothing else. ()

See also 
 Delta-wave inducer

de:Deltawelle
Physiology
Sleep