The term 'EXTREMELY DEEP SLEEP' can be summarized by the crossword solution of COMA. COMA is a state of unconsciousness in which a person cannot be awakened, fails to respond normally to pain or light, lacks a normal wake-sleep cycle, and does not initiate voluntary actions. It is caused by injury to the brain, such as a traumatic head injury, stroke, or drug overdose. COMA can also be caused by other medical conditions, such as diabetes, liver failure, or an infection.