"How does a dolphin (or other sea mammal) sleep without drowning?
(Part of “Joe’s Answer Bag Week”)
How? Very carefully.
Aquatic mammals have a unique challenge when it comes to sleeping. They have a voluntary respiratory system (as opposed to our mostly involuntary one), meaning that they have to come to the surface, actively open the flap of skin covering their blowhole, and then take a breath. But they also need to sleep. How do they do it?
Observations of dolphins have shown biologists that they sleep either floating still at the surface (called “logging”, a deeper sleep) or by swimming very slowly, usually with another dolphin around. You might be wondering “But Joe, how can they swim if they are asleep?”, to which I say “Nice to see you’re paying attention …”
Dolphin sleep is not like human sleep. I couldn’t find any examples of them reaching anything like our deep REM sleep. Instead they seem to do something that resembles napping. They are able to “shut off” one half of their brain and the opposing eye, with the other half staying awake to watch for danger and to control the voluntary breathing. Infant dolphins get pushed along in their mothers’ slipstream while resting, since their lack of buoyancy means that they have to swim or sink. And all whales have the ability to tolerate much more carbon dioxide in their blood than we can, allowing this sleep/breathe trade-off to work.
So while dolphins spend about as much time per day as we do in a sleep-like state, they space it out throughout the day, and can be more active at night when there’s lots of squiddy snacks to be had. Every animal needs rest, especially one with the brain energy demands of a dolphin. They’ve just evolved a very unique and useful way to get that rest.