Seals can sleep underwater because they have the ability to hold their breath for long periods of time, thanks to their physiology adapted to diving, including a large lung capacity and the ability to slow down their heart rate.
Seals have special nostrils that close automatically when they dive, creating a perfect seal that prevents water from entering. To sleep better underwater without having to constantly surface for air, their flexible ribcage allows them to withstand pressure without getting too tired. Their blood also contains much more hemoglobin than ours, which is ideal for storing more oxygen during prolonged underwater naps. Additionally, they have a good reserve of oxygen in their muscles in the form of myoglobin, which facilitates long and calm dives. Finally, to top it all off, their heart rate naturally enters energy-saving mode, slowing down during apnea to consume less oxygen while sleeping.
Seals, when they dive, hold their breath by significantly reducing their heart rate: this is called bradycardia. The heart beats more slowly to conserve oxygen. The latter is primarily stored in the muscles, thanks to a protein called myoglobin, which captures and retains oxygen more efficiently than in other animals. Moreover, when they are underwater, seals limit blood circulation to certain vital organs (brain and heart, in particular), thereby optimizing the use of available oxygen to the maximum. All of this prevents them from experiencing a lack of air and allows them to stay underwater for long periods without difficulty—a significant boost for their underwater sleep!
Thanks to onboard cameras and tracking devices, scientists have been able to catch seals during their underwater naps. What they see: the seal descends slowly, sometimes in a vertical or horizontal position, and remains calmly drifting at depth. At this moment, the seal enters a kind of semi-sleep, where certain areas of its brain stay awake while others sleep. This is known as unihemispheric sleep, which is also typical in dolphins. After a few minutes of quiet rest, the animal gradually rises to the surface, without panicking or rushing, to take a quick breath of air before starting to rest again. These direct observations show that seals have developed unique techniques to manage their breathing, sleep, and buoyancy at the same time.
Researchers have observed that seals alternate between periods of deep sleep and light sleep, especially when they are underwater. During these underwater resting phases, their heart rate decreases significantly, a biological trick called bradycardia that preserves their oxygen reserves. Studies have also shown that their brain manages to maintain certain sensory monitoring functions even during deep sleep: in other words, they remain somewhat vigilant to detect potential threats. Using sensors attached to these animals, scientists have found that they can stay underwater sleeping for up to twenty minutes at a time before needing to surface for air. Overall, these findings indicate that seals effectively combine rest periods with energy-saving mechanisms, allowing them to recover while remaining submerged for extended periods.
Seals are not the only marine mammals that sleep underwater, but each has its own personal method. Dolphins and whales, for example, often practice "unihemispheric" sleep: basically, they put only one half of their brain on pause while the other stays awake to come up for air at the surface. Seals, on the other hand, can sleep completely with both sides of their brain underwater, thanks to their much superior abilities of prolonged apnea compared to many other marine mammals. Unlike seals, sea otters sleep floating on their backs at the surface so they don't miss a breath or drift too far. Seals are better at conserving oxygen, which allows them to have these effective underwater naps without having to come up frequently.
Some seals can stay underwater for up to 30 minutes while they sleep. They carefully control their oxygen consumption while sleeping to avoid prematurely depleting their reserves.
The seal's lungs almost completely empty of air before diving: this prevents the formation of bubbles in the blood during rapid ascent and facilitates sleeping underwater at great depths.
Unlike humans, seals primarily store their oxygen in their muscles thanks to myoglobin, a protein that is abundant in them. This adaptation allows them to perform longer breath-holds underwater.
During underwater sleep, some seals adopt a unihemispheric sleep state, where only half of their brain rests while the other half remains alert. This allows them to stay vigilant and manage their breathing underwater!
No, immediately after birth, young seals generally remain on land or on the ice, close to their mother. It is only after several days or weeks of growth that they become capable of sleeping and resting underwater safely, thanks to the gradual development of their physical and respiratory abilities.
Sure! Here’s the translation: Yes, in addition to seals, various marine mammals such as whales, dolphins, and sea lions also have the ability to sleep underwater. However, their sleeping patterns can vary: some practice unihemispheric sleep (one half of the brain sleeps while the other remains awake), while seals often enter a state of complete apnea by fully sleeping underwater.
Seals can generally sleep underwater for about 15 to 30 minutes, sometimes longer. This depends on their species, age, previous activity level, and physical condition. They must then come to the surface periodically to breathe before diving back down to continue their rest.
Yes, most of the time, seals close their eyes when they sleep underwater to enter a deep sleep. However, they also go through phases of lighter sleep during which their eyes may remain partially open, allowing them to maintain some awareness of their environment and potential predators.
Seals have adapted anatomical and physiological mechanisms that protect them against the risks of drowning. They automatically surface regularly to breathe, and their bodies effectively limit oxygen consumption during sleep by reducing their heart rate and slowing their metabolism.
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