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A 2008 article in Nature sets the scene: a whale-behavior research team led by Luke Rendell at the University of St. Andrew's was drifting along the coast of northern Chile when all of a sudden, they were in the middle of a very unusual sight. A pod of massive sperm whales sat bobbing in the water, each animal perfectly vertical with its nose pointed toward the surface and its tail toward the bottom. Every single one floated motionless, not reacting to the boat's invasion. "It was actually pretty scary," Rendell told Nature. Not because it looked like something from a horror movie (it did), but because the only thing worse than finding yourself in the middle of several 100,000-pound animals is being in a situation where you could startle them at any moment.

Instead of starting the engine and risk waking them all up at once, Rendell decided to go out the way he came in — drifting, with the sails up to help push the boat along. At the last moment, however, the boat nudged one of the whales. "We had no idea how they would react; each of the animals probably weighed up to twice as much as our boat, and could have sunk us. If they had decided to take action collectively — sperm whales do engage in communal defence [against] killer whales — then we could have been in real trouble," Rendell said. They did wake to a frenzy of activity, but eventually the gentle giants just swam away to find a more peaceful place to engage in their bizarre sleeping habits.

Beyond the adrenaline rush, this encounter told the researchers something they didn't know before: sperm whales don't sleep like other cetaceans. Dolphins, for instance, engage in what's known as unihemispheric slow-wave sleep, which means they only shut off one hemisphere of their brains at a time (and leave one eye open). That way, they can still swim, come up for air, and avoid predators. Scientists had assumed sperm whales slept that way too until 2008, when Rendell's team published their findings in the journal Cell. They appear to sleep fully, but only for 10–15 minutes at a time.

The team's discovery actually provided an explanation to something Patrick Miller, another author on the Cell paper, had been wondering. He had been using data-logging suction cups to monitor 59 whales, and noticed that seven percent of their time was spent drifting in shallow water. The encounter with the sleeping whales finally explained why. If Miller's data is correct and sperm whales only sleep for seven percent of their day, that makes them the least sleep-dependent mammals there are — even less than the giraffe, which sleeps for only 8 percent of its day.

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