Thanks to tracking equipment, researchers had previously known that sperm whales spend around 7% of their time completely still at the surface.
But what they did there was a mystery – until a research group in 2008 met a flock of sperm whales “standing” vertically in the water. “It was actually pretty scary,” said one of the biologists, saying that none of the whales reacted when the boat drifted between them. However, it soon became clear to the researchers that the whales were asleep. The mammals up to 20 m long sleep 10-15 minutes at a time, and during this time they neither breathe nor react to the environment. This suggests that the sperm whale may be the only whale that sleeps with both halves of the brain at the same time.