Wild African elephants sleep an average of just two hours per day—less than any other mammal—a new study shows.

Researchers from Wits University and the University of California, Los Angeles tracked two matriarch elephants in Chobe National Park in Botswana for 35 days, and found that the pair survives moreso on “power naps” than longer durations of sleep.

An activity data logger device was implanted under the elephants’ trunks to record when the animals fell asleep. The team considered the elephants sleeping if their trunks remained still for at least five minutes straight.

“We reasoned that measuring the activity of the trunk, the most mobile and active appendage of the elephant, would be crucial, making the reasonable assumption that if the trunk is still for five minutes or more, the elephant is likely to be asleep,” said Paul Manger, from the School of Anatomical Sciences at Wits University.

The team also put a GPS tracking collar and gyroscope on the elephants to determine both sleep position and location.

The average amount of shut-eye for the two elephants was just two hours per day. Matriarch 1 had an average of four sleep episodes per day, while Matriarch 2 had five separate periods of sleep.

However, one surprising observation was that the pair only went into REM sleep every three to four days when they were lying down, not on their feet.

Elephants are known as the animals that “never forget,” but the researchers question whether the link between REM sleep and memory consolidation is valid, since the matriarchs only seem to go into REM sleep every few days. Most mammals go into REM sleep on a daily basis.

On several occasions, the matriarchs would even go a couple days without any sleep.

“We observed on five occasions that the elephants went without sleep for up to 46 hours and traversed around 30 km in 10 hours, possibly due to disturbances such as potential predation or poaching events, or a bull elephant in musth,” wrote the authors.

Despite these instances of traveling long distances on no sleep, the elephants did not make up for it the following day by getting additional sleep time.

Previous studies have shown that elephants can sleep up to six hours a day, either standing or lying down. But the current study finds fault in these observations because they were reported in a captive setting where food and water are provided to the animals, and they have no risk of predators.

“The results of this study reveal a far richer story of elephant rest and sleep than would be predicted from captive studies, and indicate numerous areas for fruitful future research to deepen our understanding of these iconic mammals and their sleep,” added the authors.

Another interesting finding was that environment factors, like temperature and humidity, were associated with when the elephants fell asleep and woke up, not sunrise and sunset times.

“This finding is the first that indicates that sleep in wild animals is likely not to be related to sunrise and sunset, but that other environmental factors are more crucial to the timing of sleep,” said Manger.

The researchers plan to conduct additional studies to learn more about how elephants survive on such little sleep, and also track male elephants to see if the matriarchs’ sleep patterns are an accurate representation of the entire herd. The mystery of REM sleep and memories is also of interest to the researchers for follow-up studies.

The results were published in PLOS ONE.