Rivaling the record for least sleep among mammals, northern elephant seals sleep a mere two hours a day, split into a series of nap-like "sleeping dives" at depths not typically occupied by predators. The findings – which leveraged a new tool to detect sleep at sea – provide insights into the sleeping behaviors of animals who must sleep while avoiding predation. While sleep is a crucial part of life for all mammals, some live in environments where long periods of safe sleep are not possible. As a result, wild animals exhibit a diversity of sleep strategies that balance the need for daily sleep with their ecology and predation risk. Marine mammals in the open ocean alongside predators face unique and challenging conditions. How these creatures meet their sleep requirements remains poorly understood. Using a novel remote monitoring system, Jessica Kendall-Bar and colleagues evaluated the sleeping behaviors of wild northern elephant seals (Mirounga angustirostris), a large seal species known to embark on ocean foraging trips lasting seven months and spanning more than 10,000 kilometers. The system non-invasively recorded the brain activity, heart rate, and three-dimensional spatial movement in seals in both a controlled laboratory environment and in wild free-ranging elephant seals in Monterey Bay, California. These seals took short naps – often lasting fewer than 20 minutes – while diving several hundred meters below the surface, depths below those occupied by their common predators. On these sleeping dives, seals transitioned from an awake downward glide into slow-wave sleep (SWS). During this phase, seals maintained their upright posture, but as they shifted from SWS to rapid eye movement (REM) sleep, sleep paralysis resulted in loss of posture control. Seals in REM sleep turned upside down and drifted downwards in a "sleep spiral." After these short sleep cycles, seals awoke and returned to the surface. The strategy enables the animals to enter full REM sleep but at depths that lower predation risk. Combining this data, Kendall-Bar et al. were able to derive a "biomechanical signature for sleep" that could identify sleeping dives in a 20-year dataset of time-depth records for 334 wild seals. Sleep patterns interpreted from these records reveal that elephant seals average only two hours of sleep per day during the 7 months they spend in the open ocean.
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