A coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond to painful stimuli, light, sound, and lacks a normal sleep-wake cycle. A coma is caused by injury to either the cerebral cortex or the reticular activating system. It is a serious medical condition that creates brain irregularity and causes people to experience post-traumatic amnesia. Depending on the length of the coma, patients will suffer a different recovery time.