Sperm whales sleep vertically, motionless, near the surface of the sea.
Why it is true
Researchers found groups of sperm whales drifting head-up or head-down, not reacting to a passing boat. These naps last 10 to 15 minutes and take up only about 7% of the day, making sperm whales among the mammals that sleep the least.
Source: Current Biology (Miller et al., 2008)
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