Sperm Whales Sleep While 'Drifting' Vertically, Scientists Say (VIDEO)

For sperm whales, "drifting off to sleep" isn't just a figure of speech. The video above shows an odd habit in which some sperm whales in the wild slumber vertically for short periods of time, and perform slow rhythmic dives as they literally drift to a complete sleep.

The video, which recently trended on Reddit, depicts a behavior that's just beginning to be understood.

"Many mammals show species-typical sleeping behaviour, such as dogs circling before lying down, lending support to the idea that sperm whales sleep during these drift dives," study author Dr. Patrick Miller, a professor of biology at the University of St. Andrews in Scotland, said in a written statement.

Evidence for this study came about a few years ago when Dr. Miller and his researchers found a pod of snoozing sperm whales at sea, off the coast of Northern Chile. As the whales floated motionless, with their noses at the surface of the water, they seemed unfazed by the approaching boat of researchers--that is, until the boat accidentally bumped into one of them. D'oh.

"Reduced responsiveness that reverses with sufficient stimulation to cause waking is another important criteria that allows us to consider this to be a sleeping behavior," Dr. Miller noted.

The study, which was published in Current Biology in 2008, serves as the first evidence that whales in the wild may sleep with both sides of the brain, Dr. Miller said. His research suggested that such whales are totally asleep for about 7.1 percent of their time at sea, most often between 6 p.m. and midnight.