"Almost all other animals are clearly observed to partake in sleep, whether they are aquatic, aerial, or terrestrial," wrote Aristotle in his work On Sleep and Sleeplessness. But do other animals dream? On that the Greek philosopher also had an opinion. In The History of Animals, he wrote: "It would appear that not only do men dream, but horses also, and dogs, and oxen; aye, and sheep, and goats, and all viviparous quadrupeds; and dogs show their dreaming by barking in their sleep." His research methods may lack sophistication, but Aristotle may not have been too far off the mark.

We certainly can't ask animals if they dream, but we can at least observe the evidence that they might. There are two ways in which scientists have gone about this seemingly impossible task. One is to look at their physical behaviour during the various phases of the sleep cycle. The second is to see whether their sleeping brains work similarly to our own sleeping brains.

The story of how we worked out how to peer into the minds of sleeping animals begins in the 1960s. Back then, scattered reports began to appear in medical journals describing people acting out movements in their dreams. This was curious, because during so-called REM sleep (rapid eye movement), our muscles are usually paralysed.