We form and store huge numbers of experiences in the head every day, and sleep seems to be the way the brain copes with it all.

Scientists say sleep strengthens the memory and helps the brain sort out the masses of information we soak up each day.

"Sleep helps us extract rules from our experiences. It's like knowing the difference between dogs and cats, even if it's hard to explain."

Lead researcher Bob Stickgold, professor of psychiatry at the Harvard Medical School, said: "We're not just stabilising memories during sleep. We're extracting the meaning."

/ananova.com/