different mammals sleep different amounts. some, such as bats, sleep 18–20 hours per day, while others, including giraffes, sleep only 3–4 hours per day. there can be big differences even between closely related species. there can also be differences between laboratory and field studies: for example, researchers in 1983 reported that captive sloths slept nearly 16 hours a day, but in 2008, when miniature neurophysiological recorders were developed that could be affixed to wild animals, sloths in nature were found to sleep only 9.6 hours a day.