*The following blog post is an excerpt from the book Twenty-Two Fantastical Facts about Dolphins*
Almost all animals that live in the ocean are able to breathe water - extracting the life-giving oxygen that's swishing around in seawater with their gills. Crustaceans, fish, clams, amphibians - they all have gills, and don't need (and generally prefer to avoid) air. But marine mammals - which include dolphins, whales, seals, manatees, sea lions, otters and a handful of others - still need access to air in order to get their oxygen. The ancestors of all marine mammals were air-breathing land-animals, and during their millions-of-years-long return to the ocean, none of them bothered to evolve gills or other anatomical features that would allow them to stay submerged in water 100% of the time.
Thanks to the "whatever, that's good enough" attitude of evolution, dolphins are forced to stick their blowholes out of the water every few minutes in order to fill their lungs with oxygen-rich air. Some species can hold their breath longer than others, but most need to be at the surface a few dozen times every hour to breathe.
But if dolphins need to swim to the surface to breathe at regular intervals, how then do they ever get any sleep? Some species - like pilot whales - engage in behavior called logging where they float on the surface of the ocean with their blowholes above the waves. This seems a pretty logical way for a dolphin to sleep since it's able to keep its airway exposed at all times. But the thing is, most species of dolphin don't sleep at the surface like this.
In fact, dolphins don't really sleep at all.
When humans fall asleep, our conscious minds shut down, leaving the more primitive parts of the brain found in our brainstem - like the medulla oblongata - to take care of unconscious processes like breathing or making the heart beat. But the medulla oblongata in dolphins does not control their breathing. Dolphins must think about every breath they take using the parts of the brain that - in humans - is mostly shut off when we sleep. This means that dolphins need to remain conscious at all times or they simply stop breathing. In fact, if a dolphin ever does lose consciousness - like if you give it a general anesthetic - it will suffocate pretty quickly.
Luckily, dolphins have evolved a workaround. All animal brains must enter periods of sleep or restfulness in order to survive. Nobody is really sure why, but if an animal is denied sleep for long enough, it will die. Dolphins have found a way to get the rest they need by sleeping one half of their brain at a time. Dolphin brains, like all mammal brains, are divided into two hemispheres. Dolphins are able to shut down just one hemisphere at a time and enter into something called unihemispheric slow-wave sleep. This allows for one half of their brain to go offline while the other stays awake, so dolphins can continue swimming, watch for predators, and return to the surface to breathe with the awake part of their brain.
Dolphins in this half-asleep mode usually swim lazily near the surface. Each half of the brain will sleep for a couple of hours before waking up and letting the other half get some shut-eye. And it really is shut-eye in most cases - you can usually tell which half of a dolphin's brain is asleep by looking at their eyes: the one connected to the sleeping half is shut while the other one is open. Dolphins often sleep like this at night time, and typically for about 8 hours per day - not unlike most humans. Well, except for the half-closed-eye thing.
Scientists have tested dolphins to see just how awake they really are when sleeping one half of their brain. In one experiment, dolphins were asked to touch a paddle every time they heard a tone. The tone was played at random every few minutes. Researchers kept producing the tones all day and all night for days at a time without the dolphins messing up, or even displaying signs of being tired. The experiment ended after 5 days, but probably could have been kept up forever. The dolphins, you see, weren't tired because they were able to sleep one half of their brain as needed, with the other half wide-awake and able to concentrate on the (really boring sounding) paddle task.
When it comes to staying awake and maintaining constant vigilance, it's dolphin mothers that are the true insomniacs. Newborn dolphins hitch a ride with mom by swimming right next to them, getting sucked into the wake that their mother's body creates while she swims through the water. If a dolphin calf positions itself in just the right spot next to mom, it will be propelled through the water without having to swim very hard, which helps it conserve energy. In order to make sure their newborns stay afloat and don't get too tuckered out, dolphin mothers need to keep swimming at all times in order to create this protective swim-bubble for their calves. Spend too much time logging near the surface, and their little ones might sink. So dolphin moms just keep swimming without a break for days/weeks/months until their calf is strong enough to handle the world on its own. They likely engage in unihemispheric slow-wave sleep at some point, but to the casual observer it looks like they don't sleep at all.
Based on my own experiences raising a human infant, this "total lack of sleep for months at a time" thing sounds pretty familiar...
Want to read more zany dolphin science trivia? Order a copy of the book Twenty-Two Fantastical Facts about Dolphins