Anyone who has had a restless night because the children did not want to sleep or because their partner snored could think of the northern elephant seal as a consolation while making coffee. These animals almost certainly get even less sleep — at least during the seven months they spend at sea each year without ever going on land.
During this whole period, the elephant seals don’t really sleep at all, they only treat themselves to a few “power naps” that never last longer than twenty minutes. In total, they get a maximum of two hours of sleep a day. Scientists recently found that outby putting a kind of cap on 13 elephant seals that recorded the animals’ brain activity, heartbeat and movements, among other things.
The elephant seals cannot afford more sleep. On the one hand, because the large animals – bulls are longer than four meters, females at least three – have to eat constantly to cover their energy needs. On the other hand, because they also have to be careful not to be eaten by a great white shark or an orca themselves. In addition, elephant seals must surface regularly to breathe. There is hardly any time to rest.
Other animals in a similar situation get more sleep because they have mastered the art of shutting down their hemispheres one at a time. Dolphins and fur seals, for example, let one half relax and doze off while the other half stands guard, so to speak, to see if anyone is coming.
Elephant seals can’t do that. The authors of the current study in the science journal Science have found out that the animals literally slip into sleep: First they actively dive, then they stop swimming and continue gliding until they fall into a so-called slow-wave sleep, which is also the case with the people there. Slow brain waves with frequencies between 0.5 and about 3 Hertz can be seen on the encephalogram during this phase. In the case of the elephant seals, this is followed by a sleep phase in which the animals completely give up control, roll over and spiral downwards slowly. One of the animals the scientists observed fell 377 meters in this state before waking up and paddling back up for air. Some also landed on the bottom of the sea and slumbered there for a while.
Even if it sounds like it: This sleeping strategy is not necessarily riskier than that of the animals that watch with half their brains all the time. Because in the depths where elephant seals sleep soundly, there are hardly any enemies left.
Many other animals can only rarely afford to sleep really deeply. Balancing the need to sleep with the need to eat while avoiding being eaten is not easy for most. Horses, for example, usually sleep standing up, ready to flee at any moment. Cows must use their rest periods to chew the cud. And frigatebirds just keep flying when they sleep. In comparison, most people have a reasonably comfortable sleeping situation despite various disruptive factors.