Dutch psychiatrist Frederik van Eeden coined "lucid dream" in 1913, describing the state in his own detailed dream diaries as one in which "the reintegration of the psychic functions is so complete that the sleeper remembers day-life and his own condition, reaches a state of perfect awareness, and is able to direct his attention." The subjective claim was, for decades, exactly the kind of report psychical research had learned to treat cautiously: vivid, sincerely made, and entirely unverifiable from outside the dreamer's own mind, since sleep researchers had no accepted way to confirm in real time that a sleeping subject was both dreaming and simultaneously aware of dreaming.
Rapid eye movement, REM, sleep had been identified by sleep scientists in the 1950s as the physiological stage most associated with vivid dreaming, but REM's discovery alone did not solve the lucidity problem, since a subject's eyes moving during a normal, non-lucid dream looked identical on a polygraph to a subject's eyes moving during a lucid one. What was missing was a way for a sleeping, dreaming person to deliberately signal to the waking world, in real time, that they had become aware within the dream itself, rather than simply reporting the experience afterward from memory the way every other altered-state claim in this atlas must.