By Helen Briggs
Science reporter, BBC News
The three-toed sloth is the slowest mammal in the world
The sloth's popular image as a lazy creature that sleeps for most of the day has been called into question.
Rather than snoozing for more than 16 hours a day, as observed in captivity, sloths in the wild doze for less than 10 hours, research suggests.
Scientists caught sloths living in the rainforest of Panama and fitted them with a device that monitors sleep.
The findings, published in a Royal Society journal, may help shed light on human sleep disorders, they say.
Lead researcher Niels Rattenborg, of the Max Planck Institute for Ornithology in Starnberg, Germany, said the study demonstrated for the first time that it was possible to record sleep in a wild animal.
"The real exciting finding was that they only slept 9.6 hours a day, which is much less than what people popularly believed and less than had been observed in a previous study of sloths in captivity," he told BBC News.
"So they still may be sloth-like in terms of their speed of movement but in terms of their sleep they don't seem to sleep an inordinate amount of time."
The work, published in the Royal Society journal Biology Letters, attempts to find traits that predict whether an animal sleeps more or less than another species. This might provide clues to the function of sleep, said Dr Rattenborg.
He added: "I think this finding is really going to open the door to a whole new age of sleep research on animals sleeping in their natural habitat."
Proof of principle
Animals vary in the amount of sleep they need. Pythons, for example, sleep for 18 hours a day, while giraffes survive on just two hours.
They spend much of their time in trees
To investigate sleeping patterns in wild sloths, the scientists, from Germany, Switzerland and the US, developed a small machine capable of monitoring brain patterns associated with sleep.
They caught three female brown-throated three-toed sloths living in rainforest near the Smithsonian Tropical Research Institute on Barro Colorado Island, Panama.
The animals were fitted with the data recorder and then released.
When re-captured several days later, measurements showed that they slept for an average of 9.6 hours a day, compared with a sleep time of 16 hours a day reported in sloths kept in captivity.
Dr Neil Stanley, an expert in sleep disorders at the Norfolk and Norwich University Hospital, UK, said animals tended to sleep much more in captivity, where they have all their needs met.
"It's intuitive that animals would sleep less in the wild than in captivity - this technology gives us the opportunity to prove that's true," he said.
Despite many years of research into the function of sleep, there are still many unresolved questions.
It is known that sleep plays an important role in maintaining normal mental functions, but the precise mechanisms are unclear.