There's an old saying that elephants never forget. You can now say they almost never sleep. Scientists on Wednesday said that a first-of-its-kind study tracking the sleep behavior of wild elephants found the world's largest land mammal sleeps two hours per day on average, and some days not at all, and does so mostly standing up. <br/><br/>This represented the shortest-known sleep time of any mammal. Previous research showed captive elephants got four to six hours sleep daily.<br/>'<br/>A research professor in the School of Anatomical Sciences at the University of the Witwatersrand in South Africa led the study has been published in the journal PLOS ONE.