Author of From Fatigued to Fantastic, Jacob Teitelbaum, M.D. defines conscious sleep as “the ability to be aware of the self, but not of our body or surroundings, during different stages of sleep.” Western scientific studies have focused predominantly on this state during REM sleep, and how a person can tap into their consciousness and experience lucid dreaming, Teitelbaum tells mbg.
However, conscious sleep is possible in non-REM sleep as well. In fact, in Eastern meditation traditions, conscious sleep is taught as a way to maintain self-awareness, but without being aware of the body or environment, during deep non-dream sleep, Teitelbaum explains over email.
According to a review published in the journal Progress in Brain Research, the concept of conscious sleep was highlighted by Elmer and Alyce Green of the Menninger Foundation in Topeka, Kansas. The couple teamed up with the Indian master of yoga meditation, Swāmī Rāma at the time, to further explore how a person could find themselves in their deepest, non-REM sleep, but still, have a sharp awareness of their surroundings.
From the yogi's perspective, conscious sleep was (and still is) considered to be a form of deep meditation that teaches those who practice how to sustain their meditative state, regardless of what’s happening in the world around them.