Ever wondered why you've that recurring dream about being chased or ending up back in the classroom at school? Well, a researcher has come forward to solve the mystery.
Finnish researcher Antti Revonsuo reckons it is your link to when wild animals chased our cavemen ancestors and without them we could be wiped out.
According to Revonsuo, people have inherited the dreams - based on a primitive need to survive - because they train their brains to cope with tricky situations today, reports the Sun.
He says: "It's pretty certain that our ancestors did dream because dreaming seems to be biologically programmed into our brain, and the brain that our ancestors had was pretty much identical to ours.
"We know that our ancestors lived in an environment which was full of all sorts of fatal dangers.
"The nature of bad dreams and nightmares is that they contain threatening events and force us to go through those simulated threatening events.
"So, in the waking world, when we encounter similar or different kinds of threatening events then we are more prepared to survive when we have been training for them in our dreams.
"Although we may dread our nightmares, they help us deal with the day ahead. So as a species we should be thankful for these fearsome visions.
"They are a good thing. Without nightmares and bad dreams, there is a good chance humanity wouldn't be here."