"On land, northern elephant seals laze around, sleeping up to 14 hours a day. But when the seals take off on seven-month foraging trips in the ocean, their sleep habits take an extreme turn.

Scientists fastened neoprene caps wired with sensors to the giant marine mammals’ heads during foraging in the open sea, and they discovered the seals take a series of brief nap-dives deep underwater, cumulatively getting only about two hours of sleep
a day.

The study, published in the journal Science, reveals the precise choreography of their bizarre soporific routine. To avoid predators, the seals dive down deep until they start to nod off and glide. As sleep takes over, they flip belly up, twirling
downward before waking up to swim to the surface and catch a breath. Sometimes, they snooze on the ocean floor. The whole cycle takes less than half an hour.

Unlike humans, who would get drowsy and cognitively impaired if they switched to such a radically different sleep schedule, the foraging seals seem to get just enough rest to function with bouts of repeated sleep dives.

The new study shows how evolutionary adaptable — and weird — sleep in the animal kingdom can be. It also underscores a paradox: Sleep is necessary, but it can be truncated drastically across species.

“The basic misunderstanding is more sleep means more intelligence, or a better working brain, and that’s just not true,” said Jerome Siegel, a sleep researcher at UCLA, who was not involved in the research.
...
Research on animal snoozing is delightful, but it also has a serious goal: understanding the purpose of sleep.

“If it were not [necessary], the seals should simply stay awake and on the watch for predators 24/7,” Niels C. Rattenborg, group leader of the Avian Sleep group at the Max Planck Institute for Biological Intelligence, said in an email.

“On the other hand, it demonstrates that the time spent sleeping can be greatly curtailed in response to challenging ecological circumstances. How elephant seals get by on such little sleep is an exciting mystery!”

https://www.washingtonpost.com/science/2023/04/20/elephant-seals-sleep/?itid=hp_health-science_p020_f004

From elsewhere, one theory of sleep goes like this: During the day, brains become very big through absorption of chemicals such that intercellular space becomes so narrow that normal exchange of chemicals and metabolites is difficult. Sleeping is the
process during which brain cells return to their normal sizes.

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