There's an old saying that elephants never forget. You also can say they almost never sleep.

A study tracking the sleep behaviour of wild elephants found the mammals sleep for just two hours a day on average, and some days not at all, and they do so mostly standing up.

Elephants sometimes go for up to 46 hours without sleep while walking distances of about 130 kilometres. Credit:AP

This represents the shortest-known sleep time of any mammal. Previous research showed captive elephants got four to six hours of sleep daily.

"Sleep needs to be studied in an animal's natural environment if we are truly to understand it," said Paul Manger, a research professor in the School of Anatomical Sciences at the University of the Witwatersrand in South Africa, who led the study published in the journal PLOS ONE.