Whales Snooze Half a Brain at a Time

Jan. 29, 2008 -- Sperm whales literally drift to sleep, but it's a snooze like no other, according to a recent study that found whales perform slow, rhythmic dives as they slumber.

Because these drift dives keep the whales in constant motion as they rest, scientists now think the seafaring mammals sleep with one side of their brain at a time. The two sides alternate until both are rested.

Sperm whales might even break a world record for least amount of sleep needed by a mammal.

"If the only sleep sperm whales get is during these drift dives, it would be less than any mammal studied so far," lead author Patrick Miller told Discovery News.

Miller, a senior research fellow at the University of St. Andrews Gatty Marine Research Institute, and his colleagues affixed suction cups with data-logging tags onto 59 sperm whales at various open-water locations worldwide. The tags allowed the scientists to monitor the whales' movements 24/7.

The researchers, whose study was recently published in Current Biology, noticed the whales performed the mesmerizing drift dives 7.1 percent of the time, usually between 6 p.m. and midnight.

The scientists observed two types of drift dives. The first, head-up drift dives, happen when a whale's rear end slowly sinks into the water from a horizontal posture.

During the second type, head-down, the whale descends slowly with its head directed toward the ocean floor. It travels downward about one or two body lengths in depth before flipping back upward toward the water's surface. The researchers think the whale's internal buoyancy causes this natural upward motion, similar to how a sinking apple eventually bobs back to the surface.