The crossword solution for the term 'REALLY DEEP SLEEP' is COMA, which is a four-letter word. COMA is a state of unconsciousness in which a person cannot be awakened and does not respond normally to stimuli. It can be caused by a variety of medical conditions, including brain injury, stroke, drug overdose, and metabolic disturbances. COMA can also be induced medically, as part of a treatment plan for certain medical conditions.