How do people 'wake up' from comas?

A person in a coma can sometimes wake up months, years or even decades after they fell unconscious, seemingly out of nowhere. One of the longest cases is of Munira Abdulla, a woman who fell into a coma after a car accident in 1991 and awoke an astonishing 27 years later.

But what makes someone wake up from a coma — a prolonged state of deep unconsciousness?

"The short answer is that we don't really know," said Martin Monti, a psychology professor at UCLA who studies comas. "This is why you still don't have a lot of interventions to help people recover," Monti told Live Science.

For a coma to occur, there must be an insult to the brain — such as an injury, iNFLaMMAtion or infection. Before a person can wake up, their brain needs to recover by either regrowing damaged neurons or expanding on other brain networks to take over the job of the injured brain region.

But this physical recovery of brain networks isn't enough on its own because comas also slow down the brain's activity. "Everything becomes a little bit more silent," Monti said. Thus, in this state, brain networks don't communicate as efficiently as they usually do. The brain may need some kind of jump start to get back up to speed and lead someone to wake up.

"Everybody believes it, and it's very, very reasonable," Monti said about the jump-start theory. "But we don't have good data on it."

Related: Why do our brains have folds?