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Brain activity of diving seals reveals short sleep cycles at depth

When you imagine a sleeping seal, you might think of a seal sleeping on the beach or bobbing in shallow water. You might have seen videos of harbor seals, floating in the kelp as they sleep, but what about when a northern elephant seal spends 8 months at sea in search of food, traveling over 10,000 km?


Dolphins and fur seals can sleep in half of their brain at a time allowing them to monitor predators while they sleep, but a true seal, like an elephant seal or a harbor seal, sleeps just like us, shutting of their brain completely.


Our new study, recently published in Science, shares just how little seals sleep at sea in the first recordings of sleep for a wild marine mammal. We equipped northern elephant seals with sleep monitors as they dove in the deep Monterey canyon to find that they sleep in short naps deep below the ocean’s surface.

They dive down, start to glide, their brain waves sync up into large slow waves. Then, they transition to REM sleep where they flip upside down and spin a circle, falling like a leaf. Seals sleeping in the open ocean were upside down 100% of REM sleep time, suggesting that just like us, they become paralyzed during REM.


Sleeping at sea not only means they’re vulnerable to predation, but they’re also missing out on foraging opportunities. Seals slept around 10 hours a day on land, but limited sleep to less than 2 hours a day while at sea. 

Using the long-term, 20-year elephant seal dataset curated by the Costa lab at UC Santa Cruz, we developed an algorithm that estimates sleep from time-depth records with 93% accuracy. We analyzed over 3 million dives, identifying over 500,000 sleeping dives for 334 seals on trips lasting up to 295 days.

Across several months at sea, elephant seals averaged only 2 hours of sleep per day, exhibiting unparalleled flexibility in sleep duration and rivaling the record for the least sleep among mammals.

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

Kendall-Bar, JM; Williams, TM; Mukherji, R; Lozano, DA; Pitman, JK; Holser, RR; Keates, T; Beltran, RS; Robinson, PW; Crocker, DE; Adachi, T; Lyamin, OI; Vyssotski, AL; Costa, DP (2023). Brain activity of diving seals reveals short sleep cycles at depth. Science.
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