Humans spend about one third of their lives sleeping. But some mammals (哺乳动物), like the northern elephant seal, survive with much less sleep.
Researchers in a new study described the unusual sleep pattern of these ocean animals. They found that when these mammals go to feed on trips that can last seven months, they sleep just two hours each day. Those two hours of sleep are made up of short moments of rest lasting only 10 minutes each as they dive deep to avoid predators (捕食者). The only other mammals known to get so little sleep are African elephants.
The researchers placed a head covered with sensors on the heads of the seals that they studied. The sensors recorded sleep signals created by the seals' brains and heart rate. The sensors also recorded the animals' location and depth beneath the sea.
The researchers studied female seals because they go out on long open-ocean trips while males feed in coastal waters.
During dives lasting about 30 minutes, the seals went into a deep sleep called slow-wave sleep while keeping a controlled downward movement. When they then experienced rapid-eye-movement (REM) sleep, the seals fell into a turning pattern. Then, at the deepest point of their sleeping dive—up to 377 meters deep—they wake up and swim back to the surface.
Terrie Williams is a scientist at the University of California at Santa Cruz who helped write the study. She said, "It is remarkable that a wild animal will fall into deep, paralytic REM sleep when there are predators on the hunt." She added that the seals solve this problem by going into deep sleep in the deep parts of the oceans where predators usually do not hunt them.
Williams said the brain's ability to control awakening the sleeping seals at a depth before they drown is also a discovery about how mammalian brains work. She commented on this saying that it shows the survival control of the seals' brains.