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■ Vertical sleeping in sperm whales was first documented in 2008. A study found that they spend seven per cent of each day still and upright, napping near the surface in stints of 10 to 15 minutes. To get into this unusual position they dive down, then stop swimming and let their heads drift up. Vertical sleeping could make it easier to snatch a breath when they wake up, or it could just occur because their heads are the least dense parts of their bodies. VW