Northern elephant seals sleep underwater for just two hours per day on their months-long foraging journeys.
Science correspondent, BBC News
US researchers tracked the animals, recording their brain activity as the seals swam for thousands of kilometres. They followed eight wild mammals on their foraging trips, which lasted about seven months and spanned more than 6,200 miles. They recorded the animals' brain activity, heart rate, movement and body position.
It revealed that, at depths of more than 984ft, the seals would fall asleep and descend in what the scientists describe as "sleep spirals" for about 20 minutes at a time. "They look like falling leaves," said Ms Mukherji.
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