Scientists reveal why cats always sleep on their left side

Many cats seem to prefer to rest on their left side, according to a recent study by an international team of researchers from the University of Bari Aldo Moro (Italy), Ruhr University Bochum, Hamburg Medical School, and collaborators from Germany, Canada, Switzerland, and Türkiye. The team reviewed hundreds of YouTube videos of cats sleeping on their sides. They suggest that this preference for sleeping on their sides may have evolved as a survival mechanism, improving their ability to hunt or flee quickly after waking up.

Sleep is a vulnerable time for all animals, and cats tend to sleep for 12 to 16 hours a day in elevated positions to minimize the risk of being threatened from below. Curious about whether cats show a preference for sleeping on one side, Dr. Sevim Isparta from the Department of Animal Physiology and Behaviour Research in Bari and Professor Onur Güntürkün from the Biological Psychology Group in Bochum initiated an investigation.

' Behavioral asymmetry can be advantageous because both hemispheres of the brain are specialized for different tasks ,' explains Professor Güntürkün.

The researchers conducted a detailed analysis of 408 publicly available YouTube videos showing cats lying on their sides with their entire bodies visible for at least ten seconds. To ensure accuracy, they only looked at unedited footage, excluding any content that had been edited or flipped. The analysis found that about two-thirds of the cats were sleeping on their left sides.

In general, cats that sleep on their left side will perceive their surroundings when they wake up using their left visual field (left eye), which is processed in the right hemisphere of the brain. This is the hemisphere that specializes in spatial perception, processing threats, and coordinating rapid escape movements. If a cat sleeps on its left shoulder and wakes up, visual information about predators or prey will go straight to the right hemisphere, which is best processed.

' Therefore, sleeping on the left side may be a survival strategy for cats ,' the researchers concluded.