How do whales and dolphins sleep?

Chris Smith asked marine biologist Liberty Denman...

Liberty - Yeah, that's a really good question actually. Sleep is a really interesting one because, in the marine world, there's lots of different degrees of sleep, and that's different between different species, almost all of which is quite different how we do it. So obviously as humans, when it's dark outside, we have what we'd call a pineal gland, which releases a hormone called melatonin. Basically means as soon as it gets dark, our body goes to sleep. We obviously tend to do that by shutting our eyes and having a lie down depending on the person. Fish don't have eyelids, so they tend to sleep with their eyes open when they do. And a lot of the research detailing sleep actually is based on behaviour rather than tracking their brain movements in the same way we would for humans. Coral reef fish don't need to swim to breathe because they have this thing called their operculum on their gill and they can flush water over their gills and therefore can basically just have a little rest on the bottom and hide in the coral or wherever else to stay away from predators. Larger fish out in the open obviously have a very different situation. So to stay on fish for a second, those tend to be called obligate ram ventilators, which means they have to be moving through the water to breathe. And that means if they're ever sleeping or resting, they have to be constantly moving. And this is very difficult to understand. So the research is quite thin on the ground, but the assumption is that they tend to just slow down their swimming, face into the current, let the water flow over and just enter a relaxed state.

Chris - You could drown them if you pulled them backwards through the water?

Liberty - In theory, yes. Obviously their gills, it's how they get the oxygen from the water into their system and they're very, very delicate. So yes, going backwards through the water wouldn't move the water across the gills in the way that's required. So that would essentially ruin a fish's day. But that's obviously again a bit different for mammals. The whales and the dolphins as you're talking about, because they obviously breathe air, we have a better understanding of their sleep patterns because we've had them in captivity again. So what happens is these toothed cetaceans, the whales and dolphins, will sleep with one side of their brain at a time, typically shut off from sleeping with the opposite eye awake. And this unihemispheric sleep is assumed to allow for the observation of predators. Things get in the way, just staying aware of the fact they need to breathe.

Philip - Yeah, so they're sleeping under the water, the whales, so they don't have to sleep somewhere where their blowhole can have access to the air?

Liberty - Yeah, so there's different types of sleeping. Dolphins for example will literally be cruising along the surface, and then they can breathe every 30-40 seconds for comfort purposes. But again, for the whales being underwater, they can hold their breath for an incredibly long time and dive to incredible depths, especially the sperm whales because they hunt in the depths and eat giant squid so they can hold their breath for a lot longer and it actually reduces their heart rate and their breathing weight, slows it down. And therefore they can just stay under for longer basically.

Chris - I did read that one of the other things that some of these marine active mammals do, like seals as well, is that they change their dream sleep. When we go to sleep, part of our night is spent dreaming, and part spent in a really deep sleep. And apparently when they go to sea for hunting purposes and feeding, they suppress the amount of time they spend dreaming and they spend a bit more time with one of these phases disabled. And then when they come back ashore they compensate and reset things so that they don't have the same sort of even balance that we do. So they change their way of sleeping when they go to sleep as well as doing what you are saying, turning on and off one side of their brain alone.

Liberty - Yeah. And dreaming is such an interesting thing to think about as well. Obviously here we are talking about whales and fish, and actually species like octopus, cephalopods, we've been tracking dreaming in them and how they change their colour and the texture of their skin as if they're experiencing things, which shows that they're in fact dreaming and they have incredibly extensive intellectual capability. And something that we aren't really appreciating until now, until we're documenting this. So the dreaming side is really fascinating.