The man called Bazdilot Cordelion made a conscious decision not to dream.
He employed self hypnosis to put his body into a light sleep and his brain into a deep sleep for mere minutes at a time. It was a measure that enabled him to remain active for long periods, as well as to move immediately as he woke should an enemy appear.
Short-term sleep utilizing the deconstruction of consciousness was a simple spell, widely used even by amateurs. Although, as the deconstructing of one’s consciousness was something like temporary suicide, only a limited number of mages made frequent use of it.
Bazdilot heard that there were magecraft-using mercenaries who employed a number of additional sleep spells for various purposes. He, however, fundamentally disliked dreams, and therefore refused to allow himself REM sleep.
That was what made Bazdilot suspicious.
From a certain moment onward, he was conscious that he was “dreaming.”

Fate/Strange Fake: Volume 4, Chapter 10 ("Day 2")