Sleeper (The Mysterious Planet)

A sleeper was an astronaut from the Andromeda constellation. Because of the vast distances from the constellation to Earth, they required suspended animation to make the journey. Hence they were informally called "sleepers".

Three such astronauts found a way into the Matrix, five hundred years before the Sixth Doctor arrived on Ravolox. When they fled to Earth, the Time Lords devastated it with a fire ball, transforming Earth into Ravolox.

The three were able to escape the Time Lord's wrath by building and hiding in an underground bunker. They also created the L3 robot that would later be known as Drathro to maintain the bunker while the three sleepers put themselves in suspended animation. Just prior to the Doctor's arrival, the system began to break down, which Drathro was unable to correct because he was trained in installation and maintenance. As a result the three sleepers died whilst in suspended animation. (DW: The Mysterious Planet)