A 2008 study shows that sloths in the wild sleep for about nine hours, while sloths in captivity appear to sleep for 15 to 20 hours per day. The study monitored three-toed sloths, so the sleeping patterns of the larger two-toed sloth remain unverified as of 2014.
The length of time a sloth sleeps is likely impacted by its environment. Prior to the 2008 study, researchers presumed that sloths slept most of the day due to their lethargic behavior. In the study, scientists attached small electroencephalogram devices to each sloth's head, which monitored brain wave patterns, giving a clear picture of three-toed-sloth sleep patterns. Other sources report observations of captive sloths sleeping as little as nine hours as well.