Elephant seals sleep underwater in 10-minute naps while out at sea, gently floating down to depths of almost 400m – sometimes coming to rest on the seabed.

Researchers discovered the remarkable behaviour after fitting the animals with neoprene caps similar to those used by humans to measure brain activity. The results showed they also fall into paralytic REM sleep – the time when humans have their most vivid dreams.

The study of wild northern elephant seals showed they only needed an average of two hours’ sleep per day while out foraging at sea, a figure rivalling their land-dwelling namesakes African elephants, which have also been found to catch very little shut-eye.

For elephant seals, their sleep pattern varies dramatically depending on the season.

‘Elephant seals are unusual in that they switch between getting a lot of sleep when they’re on land, over 10 hours a day, and two hours or less when they’re at sea,’ said lead author Jessica Kendall-Bar, of the Scripps Institution of Oceanography. ‘This flexibility in sleep duration is unparalleled among mammals and rivals the mammalian record for the least sleep.’

While at sea – for up to eight months at a time – elephant seals are most at risk of predation by sharks and killer whales at the surface, so only spend one or two minutes breathing between dives. Each dive lasts up to 30 minutes, and may include a nap.

While sleeping, the seals turn on their backs and sink in a spiral motion down to depths of up to 377m.

‘They’re able to hold their breath for a long time, so they can go into a deep slumber on these dives deep below the surface where it’s safe,’ said Kendall-Bar, who completed the research while at UC Santa Cruz.

‘By learning more about where, when, and how animals sleep at sea, we can improve the management and protection of their critical resting habitats.’

In addition to the electroencephalogram monitors in the cap, the seals carried time-depth recorders and accelerometers to better understand their movements and behaviours out at sea and identify ‘sleepscapes’. The team tracked 334 wild northern elephants seals off the north-west Pacific coast of the US.

Previous research tracked the seals on regular long dives, but had not been able to ascertain when or where the marine mammals were sleeping.

Other species of seal sleep at sea in a vertical position known as ‘bottling’, with just their head above the water.

Co-author and professor of ecology and evolutionary biology at UCSC Terrie Williams said: ‘Normally, we’re concerned about protecting the areas where animals go to feed, but perhaps the places where they sleep are as important as any other critical habitat.’

The study was published in the journal Science.

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