Why do we dream? You asked Google – here's the answer | David Shariatmadari

Every day, millions of people ask Google some of life's most difficult questions. In this series, our writers answer some of the most common queries

Maybe we should ask the duck-billed platypus.

Back in the 1950s, scientists working on humans identified a state marked by increased brain activation, accelerated breathing and heart rate, and muscular paralysis. But perhaps the most remarkable feature was a flickering of the eyes beneath closed eyelids - because all these physiological changes took place while the subjects were fast asleep.