Sperm whales sleeping

Until just a few years ago, it was thought that sperm whales, like other cetaceans, only allowed one side of their brain to rest at a time, "keeping one eye open," as it were, in order to do "important things that require physical activity, such as coming to the surface to breathe or avoid predators," explains Nature's Matt Kaplan. 

"They never fully let their guard down." 

But in 2008, a team of researchers off the coast of northern Chile happened upon a pod of vertically bobbing sperm whales that seemed completely oblivious to its presence.

Not a single whale responded to the team's boat until one of them was accidentally nudged, at which point it awoke and fled, along with the rest of the group.

The team's findings suggest that, unlike other cetaceans, sperm whales appear to enter short, but periodic, bouts of sleep throughout the day — an observation that Kaplan says could hint that sperm-whales are actually "the least sleep-dependent mammals known."