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. Siegel, for example, worked with colleagues to reveal that long-lived, intelligent African elephants also average about two hours a day, vying with elephant seals for the most sleepless mammal.

Jessica M. Kendall-Bar, now a postdoctoral fellow at Scripps Institution of Oceanography, spent hours as a graduate student at the University of California at Santa Cruz observing elephant seals sleeping on the beach at Año Nuevo State Park, south of San Francisco, as part of the study.

She and collaborators collected experimental data on 13 seals wearing sleep caps and combined that with a larger database tracking the movement of 334 free-ranging seals. The work reveals that elephant seals appear to be able to switch between two totally different ways of sleeping without suffering ill effects.

There’s some level of sleep rebound that occurs when the seals return to land, but their short naps in the ocean don’t appear to be causing sleep deprivation.

“It seems like they have two modes of living,” Kendall-Bar said.

The research team found that when seals entered REM sleep, the mode that is associated with dreaming in humans, they lost “postural control” — in other words, their muscles became paralyzed and they flipped belly up, drifting downward in a spiraling corkscrew.

Kendall-Bar noted that elephant seals often grouped their naps together, taking successive dives at dawn after a night of foraging. The time the seals spend in REM sleep increases with each back-to-back dive — sort of analogous to how humans increase the amount of time they spend in REM sleep closer to the morning.

How animals sleep is a question that yields a seemingly endless number of curious answers. Duck-billed platypuses spend up to eight hours in REM sleep, more than any other animal. Dolphins and some other marine mammals sleep with only half their brain at a time — something called “unihemispheric” sleep. And great frigatebirds sleep on the wing for less than an hour per day during six-day foraging flights.

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!”

Kendall-Bar is hoping to use the cap to track the sleep habits of seals whose habitats are threatened by climate change. Identifying where endangered animals like Hawaiian monk seals sleep might help focus conservation efforts.

She is also hoping to use a version of the cap to study human freedivers to see whether there are neurological events that occur when people black out during the dangerous sport.

A third project focuses on adapting the cap to sea lions to measure seizures triggered by domoic acid toxicosis, caused when they ingest neurotoxins created in algal blooms.

This one study can’t elucidate the purpose of sleep, but Siegel argues that the collective picture emerging from the animal kingdom pushes back against the notion that there is something special about human sleep.

“Probably the most important thing that animals can do to evolve is have the right amount of waking. If you don’t need to be awake, it’s highly adaptive to be asleep, because you’re saving energy,” Siegel said.

“The paradox is people have this concept in their mind that the function of sleep is a big mystery, and ask, ‘Why do we need to be asleep at all?’ You can flip this and ask, ‘Why do you have to be awake?’”