Most animals need sleep. Even C. elegans, the nematode worm, has sleep-like states and it is one of the simplest organisms that exists with a nervous system. Sleep comes in many guises, in invertebrates like the cockroach; it can be as simple as folding down your antennae, adopting your favourite posture and decreasing your sensitivity to the world around you. In mammals there are a multitude of strategies, from the famous cat naps that can take up to twenty hours of a feline’s day to the giraffe that can cope with as little as three hours a night. As diverse as the species themselves, sleep it seems comes in all shapes and sizes.
But what about dreaming? Could there be degrees of dreaming as there are degrees of sleep? We know that humans dream but what about the rest of mammals? Their sleep, like ours, consists of REM (Rapid Eye Movement) and non-REM sleep. It is the REM phase in which we dream. This stage is also associated with temporary muscle paralysis. The theory is that we go limp to avoid hurting ourselves from trying to act out our dreams. Non-human mammals experience REM sleep, also with loss of muscle activity in varying degrees. Many a dog or cat owner will tell you their pets twitch and make noises whilst sleeping, but demonstrating scientifically that non-human animals have dream-like experiences has proved difficult.
However, a study published in 2001 found something extraordinary. Researchers trained rats to run around a circular maze whilst measuring their brain activity with electrodes. They found that the rat’s neurons in the Hippocampus (the memory area of the brain) were firing in particular patterns depending on where the rat was in the maze. They were so regular that the researchers could predict the location of the rat through the patterns alone. The scientists continued to measure brain activity whilst the animals slept. Incredibly, during REM sleep, the rat’s neurons began to fire in the exact same patterns as during the maze exercise, suggesting that the animal was reliving the experience in their sleep.
There is an obvious survival advantage to being able to relive memories. It can help consolidate what has been learnt during the event as a long term memory. Dreaming their way through the maze may therefore make it easier for the rats to navigate its twists and turns when awake.
The discovery has thrown open the doors of non-human REM sleep research. The results suggest that other mammals could be dreaming complex sequences of events linked to experiences in everyday life. The reasoning is simple, if a rat can do it, then surely an animal of greater complexity like a chimpanzee can too. Interestingly, the animal that has the largest amount of REM sleep in the world is the Platypus. Which raises the obvious question, what on earth could a platypus be dreaming about?