Photo/IllutrationZebrafish used in sleep research (Provided by the researchers)

Zebrafish sleep just like humans, a team of researchers from Stanford University in California and the National Institute of Genetics in Mishima, Shizuoka Prefecture, has found.

According to their results, published in British science magazine Nature on July 10, zebrafish experience deep sleep and rapid eye movement (REM) sleep that are similar to mammals.

REM sleep has already been found in humans and other mammals along with birds and reptiles but the sleep behavior of fish, with a far longer existence, has remained a mystery.

The researchers monitored the neurological activity of one- to two-week-old zebrafish that had translucent brains during sleep.

Before the experiment began, a type of protein that increases in brightness when its calcium concentration rises was added to the fish, as an indicator of brain neuron activity.

The researchers believe that vertebrates that existed over 450 million years ago also experienced similar sleep characteristics.