African elephants -- the largest land animal -- sleep for just an average of two hours a day and regularly survive for nearly two days without sleep, a study has found.

Previous studies of sleep in captive elephants have shown that they sleep for four to six hours per day.

However, "in their natural habitat, wild, free-ranging elephants sleep only for two hours per day, the least amount of sleep of any mammal studied to date, but this appears to be related to their large body size," said Paul Manger from the University of Witwatersrand in South Africa.

For the study, published in the journal PLOS ONE, the team monitored two free-roaming African elephant matriarchs in Chobe National Park in Botswana for 35 days. The elephants were implanted with an actiwatch, in the trunk to track sleep accurately and a collar with a gyroscope to track sleeping position.

The researchers found that the elephants slept an average of two hours a day -- the shortest known sleep time of any land mammal.

On several days, the elephants were found to survive without sleep for up to 46 hours.

They travelled long distances of around 30 km during these periods, possibly due to disturbances such as lions or poachers and were found to sleep lying down only every few nights.

"In addition, it appears that elephants only go into REM (rapid eye movement), or dreaming, sleep every three to four days, which makes elephant sleep unique," Manger added.