Why do we dream? Why did that capability evolve?
Dr. Garfield: Well, that's the question that has really been unanswerable. Many people have suggested different reasons. From an evolutionary point of view there have been speculations that dreams have evolved to periodically make sure that everything is working in the central nervous system and to be alert, alerted in a way that one could be responsive to danger in an environment. For example, if a person dreams that someone is breaking into their apartment and they see a hand creeping in, and then they suddenly wake up and realize that there is a strange noise, there is something happening that their dreaming mind has picked up first. But it doesn't explain all dreaming.
The current thinking is that dreaming is an important component of memory and that we do know that when people are learning new things, they take a language immersion course, for example, dreaming literally increases. Dreaming increases when we're learning new tasks, and if we're prevented from dreaming after we've learned something new, our memory for it is very poor. Even if we're allowed to sleep, but our dreaming is interrupted. There is a lot of evidence now that suggests dreaming is an important component of short-term memory. So, this is one possibility and perhaps one that's quite important. But, the bottom line is we truly don't know why we dream.