Circadian rhythm pt. 2: Beyond human

MOLLY BLOOM: Before we get started with the show, we have a special announcement for our listeners in the Boston area. Brains On is coming to Boston for a live show. And this live show is all about robots. It's on Sunday, April 15. And we're going to be there as part of the Cambridge Science Festival. If you want to learn more about the event and buy tickets, head to branson.org. Hope to see you there.

You're listening to Brains On, where we're serious about being curious. Brains On is supported, in part, by a grant from the National Science Foundation.

A couple episodes ago, we talked about circadian rhythm, the near 24-hour cycle on which our bodies run. Light from the sun or other sources gives us cues to go about our day-- wake up in the morning, eat, take on complex problems, even exercise. As darkness sets in, we get cues to do things like produce more of a hormone called melatonin, which causes us to sleep. And the great conductor of our circadian rhythm, the suprachiasmatic nucleus.

CHOIR: (SINGING) Suprachiasmatic nucleus.

MOLLY BLOOM: You can think of this group of neurons as the conductor of circadian rhythm, directing bodily functions to speed up or slow down.

[MUSIC PLAYING]

Today, we're going to look at another side of circadian rhythm. Light, as it turns out, plays an important role to all sorts of living things.

You're listening to Brains On from American Public Media. I'm Molly Bloom.

ANNIKA RADER: And I'm Annika Rader.

MOLLY BLOOM: Annika cohosted the first part of this special, so it only makes sense that she's back again today, where--

ANNIKA RADER: It's all about the ticks and tocks of circadian clocks.

MOLLY BLOOM: Those clocks aren't just in humans, they're in the cells of animals everywhere.

PHOEBE: My name is Phoebe. And I live in Brooklyn. And my question is-- how do bears know when it's time to hibernate?

MOLLY BLOOM: Great question, Phoebe. And to answer it, 10-year-old Xavier from Edmonton, Canada had a chance to speak with Sara Wilbur. She's a biology graduate student studying hibernation physiology at the University of Alaska Fairbanks.

ANNIKA RADER: And what she has to say is definitely-- what's the opposite of a lullaby?

MOLLY BLOOM: A lullahigh?

ANNIKA RADER: Right. What she had to say about hibernation is definitely a lullahigh.

MOLLY BLOOM: Here's Sara and Xavier. He's super curious and had a lot of questions for her.

XAVIER: I've never actually talked to a scientist.

SARA WILBUR: Really? Oh, well, I'm just a normal person. [CHUCKLES]

XAVIER: Well, you still know a lot more than me.

SARA WILBUR: I've lived a little longer than you have, so I have that on my side.

XAVIER: Why do animals hibernate?

SARA WILBUR: That's a great question. So all sorts of animals hibernate. But I focus mostly on mammalian hibernators, so mammals. But I actually mostly focus on an animal called the Arctic ground squirrel. So animals hibernate in the face of lack of resources, for instance, in the winter, when there isn't things for the hibernator to eat. It saves energy by shutting down for the season, often, in the winter.

XAVIER: Do animals in warm climates hibernate?

SARA WILBUR: Also a really great question. So I think, we often think about hibernation as a strategy that animals use to survive in the face of cold temperatures, but animals in warm temperatures or warm climates also hibernate.

So the formal term for this is aestivation. And that's a summer hibernation, where they would just overheat if they were running around. So they actually undergo this hibernation in the face of really warm temperatures instead of cold temperatures. And then they're running around and being active when the temperature is a little cooler outside.

XAVIER: How long do animals hibernate?

SARA WILBUR: So animals like the species I study, Arctic ground squirrels, will hibernate anywhere from 7:00 to 8:00 months out of the year. Then there are some animals, like, say, a hamster that go through really short hibernation periods. So maybe they'll only hibernate for three days. And then they'll come up and start feeding again. One species, actually, the edible dormouse, hibernates for 11 months out of the year--

XAVIER: Wow.

SARA WILBUR: --which is pretty incredible. Another species that we're very familiar with as a hibernator is bears. So bears also have really long hibernation periods.

XAVIER: They're probably my favorite hibernating animal.

SARA WILBUR: Yeah, they're pretty amazing. And I don't know if you knew this, Xavier, but there's actually a big difference between hibernation in ground squirrels and hibernation in Bears. So ground squirrels, they actually drop their body temperature down to be the same as the outside temperature. So if the outside temperature is five degrees, the squirrel will drop its body temperature to five degrees.

But even more amazing is even though it's below the freezing point of water, it actually isn't frozen solid, like you'd think it would be. So it's actually super cooled. So this means that it's below the freezing point of water, but it isn't in a solid form.

And then the black bear or any kind of bear that hibernates will drop its body temperature just maybe five degrees or so. So it is a little colder, but it actually maintains a really high body temperature while it's hibernating.

XAVIER: That's really cool.

SARA WILBUR: I think so, too. A hibernator, like an Arctic ground squirrel, in the summer has a really strong circadian rhythm. So it exits its burrow at about the same time every day, goes back into its burrow. And what we're finding now is, during hibernation, the circadian rhythm actually shuts off. And what is directing the length of hibernation when the animal comes out again is actually called a circannual cycle, which you can probably guess, is yearly. So it's on a yearly cycle rather than a daily cycle.

With the changing photoperiod throughout the summer, when the squirrel's actually above ground and is exposed to the sun, there might be something during that phase that cues in the circannual cycle. But really, we don't really know what encourages an animal to come out of hibernation or how it knows the exact time to come up. Because if it came up in January, there wouldn't be any food. So how does it know to come up in April when things start to melt and food starts to become available?

XAVIER: Maybe they can feel warmth.

SARA WILBUR: In the case of an Arctic ground squirrel again, its burrow is actually frozen even after the babies are born. So nothing's really happening that we've found so far that's encouraging them to come out of hibernation. It's a mystery, we don't know.

[MUSIC PLAYING]

MOLLY BLOOM: That was biologist Sara Wilbur, speaking with Brains On listener, Xavier, from Edmonton, Canada.

CHILDREN: Brains On.

MOLLY BLOOM: So we just heard all about nocturnal animals and their days at night.

ANNIKA RADER: But it's not just animals living in cycles of light and dark.

MOLLY BLOOM: To learn more, we've booked an interview with-- it just says Plant.

ARABIDOPSIS: Not just any plant, it is me, Arabidopsis, everyone's favorite member of the Brassicaceae family.

ANNIKA RADER: Oh, I didn't see you there. Hi.

ARABIDOPSIS: Hi, new best friends. So excited to meet you. I will talk about anything Want my thoughts on the stock market? Invest in kale. Grooming tips? Try pruning shears instead of hedge clippers. What car do I recommend? Anything but a lemon. Come on, ask away.

MOLLY BLOOM: Actually, we do want to know more about your circadian rhythm.

ARABIDOPSIS: Oh, sure. I can talk about that. I have actually been in scientific studies. In fact, let me call my other best friend, Janet Braam. She is a scientist at Rice University, and wrote a whole paper on me.

[PHONE RINGING]

Hi, Janet Braam.

JANET BRAAM: Hey, Arabidopsis. I haven't heard from you in a long time. How's it going?

ARABIDOPSIS: Great. Great. Hey. So remember when we did that study on my circadian rhythm? Molly and Annika want to know more about it. Tell them about how we, plants, have cycles, too.

JANET BRAAM: Sure, I'd love to help. It turns out that plants clearly know what time of day it is every day. They also know what the season is. So they are, sometimes, much more aware of their environment than we are, I think, sometimes.

ARABIDOPSIS: Yeah. You know how some flowers open in the day and close at night, like clockwork? And sunflowers, they not only turn toward the sun during the day, they turn East at night, just to be ready to soak up rays when the sun rises. How cool is that?

ANNIKA RADER: Very cool.

MOLLY BLOOM: But what about you? What's neat about your circadian cycle?

ARABIDOPSIS: Oh, I am all about defense. Tell them, BFF, Janet Braam.

JANET BRAAM: Correct. Right. So what plants do to defend themselves against insects is they make certain chemicals within their leaves or their tissues. And when the insects chew on them, these chemicals get into the insects, the insects get sick, and then they stop eating them. So what we found was that plants make these chemicals at the time of day when the insects are most likely to feed. So in this way, they prepare this defense, just in case the insects attack them that particular day.

ARABIDOPSIS: Take that, bugows.

JANET BRAAM: Yeah, I had a conversation with my son about this. And he said to me something like, well, I know what time of day I'm going to eat my vegetables. That got me thinking. He put himself in the place of an insect, thinking that there's a good time of day to eat these plants, and there's a bad time of day to eat these plants.

And so that made me realize that Arabidopsis, being related to things like cabbage, and broccoli, and cauliflower, that if Arabidopsis were doing this on a daily basis, where it was accumulating these anti-insect chemicals at particular times of day, that maybe, the crops that we eat were similarly accumulating different kinds of chemicals at different times of day. And that could affect the people that eat these vegetables.

ANNIKA RADER: Wait, are you plants trying to make us eat insect poison?

ARABIDOPSIS: No. No way. Veggies love being in salads. For us, it's like performance art. Besides, my insect repellent doesn't hurt humans. In fact, it's awesome for you.

JANET BRAAM: Exactly. What we know is that these same chemicals that make insects sick are actually among the most potent natural anti-cancer chemicals known. So one of the reasons we are told to eat things like cabbage, and broccoli, and cauliflower is because of these particular chemicals.

MOLLY BLOOM: So if plants like you are flooded with these anti-cancer chemicals at certain times of the day, should we be eating you then to get the most out of our meal?

ARABIDOPSIS: Bingo. That is what Janet Braam and I think, based on all of my-- I mean, her research. I fill up with those chemicals in the middle of the day, but then I have less at night. So you are better off eating salad for lunch than you are having us as a midnight snack.

ANNIKA RADER: Who eats salad at midnight anyway?

JANET BRAAM: Right. [CHUCKLES]

ARABIDOPSIS: But get this, Janet Braam and I also learned that even after you pick us from the ground, US, vegetables, still follow our circadian clocks.

JANET BRAAM: What we found is that if we store our harvested vegetables in light dark cycles that they would find out in the natural environment, they do last longer, they seem to stay fresh longer, and some of those nutrients stay at higher levels longer than if you were to store them, for example, in a dark place or a place that had around-the-clock light.

MOLLY BLOOM: So maybe the fridge of the future will have a light and dark mode, to keep plants like you healthy longer.

ARABIDOPSIS: Maybe. Obviously, a good place for more research. Anyway, I hope that answers your questions.

ANNIKA RADER: Oh, yeah. You were a big help. Thanks.

ARABIDOPSIS: And thanks to you, bestest friend, Janet Braam. Talk to you later.

JANET BRAAM: OK. See you later. I have to have you over for dinner sometime.

ARABIDOPSIS: Sure. I'll bring the salad. Bye.

JANET BRAAM: Bye.

ANNIKA RADER: Bye.

MOLLY BLOOM: Bye. That was different.

ANNIKA RADER: Who knew plants were so chatty?

MOLLY BLOOM: Yeah, I'll never see a salad quite the same way again. OK, Annika, I hope your ears are awake, because it's time for the--

[MUSIC PLAYING]

CHILD: (WHISPERING) Mystery Sound.

MOLLY BLOOM: Are you ready to test your ears, Annika?

ANNIKA RADER: Yes.

MOLLY BLOOM: OK. Here it is.

[OWL COOWING]

Any guesses?

ANNIKA RADER: Maybe it's a type of bird or animal, tweeting.

MOLLY BLOOM: That's an excellent guess. I'm going to let you ruminate on the answer for just a bit. And we'll be back later in the show to reveal the answer.

[MUSIC PLAYING]

Brains On is powered by you, our listeners. And right now, we're really looking for mystery sounds for an upcoming all Mystery Sound episode. You'd be surprised at the fantastic sounds that are around you every day. Plus, sending in a mystery sound is a surefire way to get on the Brains Honor Roll. If you're not the sound-gathering type, you could also draw a picture inspired by one of our episodes or write to us with whatever you're curious about. That's what Juliette, from Portland, Oregon, did.

JULIETTE: Hi. I'm wondering, why does my dad snore? Thank you.

MOLLY BLOOM: We'll answer that question and hear the latest group of Brains Honor Roll list at the end of today's show. Stick around.

[MUSIC PLAYING]

ANNIKA RADER: You're listening to a special episode of Brains On. I'm Annika Rader.

MOLLY BLOOM: And I'm Molly Bloom. Today is the second part of our look at circadian rhythm. And before the sun sets on this episode, let's check back in on the very important mystery sound.

[OWL COOING]

OK, do you have a new guess?

ANNIKA RADER: I still think it's a bird, but maybe, it could be like a seal or like an otter or something like that.

MOLLY BLOOM: So you think it's an animal? What kind of bird do you think it might be?

ANNIKA RADER: Maybe an owl.

MOLLY BLOOM: Excellent guess. Well, the answer is an Eastern screech owl. So you are 100% correct. Have you heard an owl before?

ANNIKA RADER: I actually haven't heard an owl before, but yesterday, we went on a field trip and saw owls.

MOLLY BLOOM: Well, this is perfect timing then. And did you talk yesterday on your field trip about how owls are nocturnal?

ANNIKA RADER: Well, actually, I did, to my friend, who thought the owls were awake the whole entire day, but for some reason, those owls were awake. They were pretty creepy.

MOLLY BLOOM: [LAUGHS] Well, yes, you are right, owls are nocturnal, which means that it's active when the rest of us are sleeping. Brains On reporter, John Lambert, is here to shed some light on these creatures of the night.

JOHN LAMBERT: Nocturnal animals are not like you and me. We're diurnal, which means we're awake during the day. Instead of getting sleepy when the sun goes down, nocturnal animals are just getting up.

[ROOSTER CROWING]

Biologists think that diurnal and nocturnal clocks pretty much tick the same, but how these animals respond to that clock is flipped. So when the internal clock says that the sun is setting, nocturnal animals wake up instead of going to sleep.

So we know animals can change how they listen to their internal clocks, to be active at night. But why be nocturnal at all? I mean, nighttime is dark, cold, and scary.

[YAWNING]

Some Brains On listeners had similar questions.

ELI FRY: Hi. I'm Eli Fry from Laguna Niguel, California. My question is-- why do nocturnals stay up at night?

JOHN LAMBERT: The answer to this question comes down to survival. Animals need resources, like food and water. And resources can be hard to come by. So animals tend to live in environments where they're experts at finding those resources. For example, camels are experts in arid deserts. And toads are experts in damp ponds. Animals evolve different traits to be able to live in different environments.

A camel wouldn't need to store water if it lived in the swampy home of the toad. And that toad wouldn't find much use for swimming in the sandy desert.

[TOAD CROAKING]

You can use this same idea when it comes to nocturnal and diurnal animals. Day and night are just two different environments. Just like animals adapt to a specific environment, animals can adapt to a specific time of day, too.

If you can manage to get around during the nighttime, you can have access to all kinds of resources without having to compete with all those pesky day-dwelling animals. For example, think about all the mosquitoes and other insects flitting about at night. All this food is unavailable to diurnal animals, but bats can gobble them all up because they're nocturnal.

Another mode of survival, don't get eaten. Some animals are nocturnal to avoid diurnal predators. This is what scientists think happened with our early mammalian ancestors. Mammals came onto the scene during the reign of the dinosaurs, more than 200 million years ago. While the sun was out, pterodactyls, as big as a giraffe, ruled the skies.

And dinosaurs, like T-rex, were experts at stomping and chomping on land, making it a dangerous world for little creatures. But our furry little ancestors could avoid all this trouble by laying low during the day and waking up at night, to safely search for food while the dinos dozed.

This idea is known as the nocturnal bottleneck hypothesis, that our mammalian ancestors survived during the Mesozoic, because they were nocturnal. They only became diurnal when dinosaurs went extinct.

Since all mammals around today descended from these early nocturnal mammals, some of their traits have stayed with us. In fact, 70% of mammals are still nocturnal, and just about all, except for monkeys and apes like us, have eyes that still look a lot like nocturnal eyes, even if they're used during the day.

Speaking of eyes, Monroe from San Mateo, California, wrote in with this question-- how do nocturnal animals see in the dark? That's a great question, because moonless nights can be 100 million times darker than daylight.

To answer this question, first, we need to talk about how eyes work in the first place. Light enters our eye through our pupils and hits photoreceptor cells. These photoreceptors convert this light into electrical impulses, which travel to the brain to make a mental image. If you want to get the nitty gritty details, check out our episode on cats. Meow.

At night, animals need to make the most of the little light that's out there. One strategy, have big eyes. The bigger the eye, the more light it can catch. Take owl eyes, if owls were as big as humans, their eyes would be the size of softballs, except, their eyes aren't round like ours, they're shaped more like tubes, so they'd be tubular softballs. This shape helps animals like the owl maximize depth perception at night, so they can spot a mouse scurrying in the snow 100 yards away.

OWL: Who thought I'd see you there, mousey? You can't hide.

JOHN LAMBERT: Whoa. That owl came out of nowhere. The downside of these tubular eyes is that they can't look from side to side, hence, a super flexible neck that can rotate 270 degrees around.

OWL: No mouse can escape my gaze.

[OWL HOOTING]

JOHN LAMBERT: Jeez, Louise. Take it easy, Mr. Owl. Some animals just have lots of little eyes pooled together. These are called compound eyes and are really common in insects. The nocturnal dung beetle uses its compound eyes to find piles of dung, using only the light from the stars. Oh, look, here's one now.

DUNG BEETLE: Got to find dung, roll dung into a ball, find dung ball hideout, enjoy delicious, delectable [MUNCHING] dung ball.

JOHN LAMBERT: Whenever light hits one of these little eyes, a bunch of the surrounding eyes get activated too. This means that they can get a rough image with less light. So dung beetles don't see a bunch of individual stars like we do, but bigger blobs of star, instead. They use this stellar map to find their way to the safest spot to, well--

DUNG BEETLE: [MUNCHING] Dung. [MUNCHING]

JOHN LAMBERT: Side note, never accept a dinner invitation from a dung beetle. Some animals slow things down so they can see at night. Remember our toad friend?

[TOAD CROAKING]

It has slow eyes that collect light for a couple of seconds before telling the toads brain what they see. This literally allows more light to be shed on the scene, painting a brighter picture, despite the darkness. That's a pretty amazing system there, Mr. Toad.

TOAD: The only downside is, it's harder for me to keep track of fast-moving things. But that's OK, life at my speed is pretty chill.

JOHN LAMBERT: So as long as an animal has the right tools, nighttime can be the right time.

[MUSIC PLAYING]

MOLLY BLOOM: Brains On reporter, John Lambert.

ANNIKA RADER: Quite illuminating.

[MUSIC PLAYING]

Here at Brains On, we know you're curious and have lots of questions.

MOLLY BLOOM: Questions even the smartest people you know might not have the answers to.

ANNIKA RADER: Is there a plant that you think we should interview?

MOLLY BLOOM: How about a question only an astronaut can answer?

ANNIKA RADER: Or maybe there's a mystery sound you want us to guess.

MOLLY BLOOM: Email us at hello@brainson.org to help you track down answers, and maybe, just maybe, find some new questions along the way.

[MUSIC PLAYING]

Plants and animals react to cycles of the sun, just like us. Animals hibernate to conserve energy when resources like food are scarce. And plants use circadian rhythms to follow the sun and even help protect them from predators.

If you have more questions about sleep, like, why do we even do it? Or what's going on in our brains when we doze? Check out the latest two episodes from our pals at the podcast, But Why? They'll explain how sleep helps us grow, and why we sometimes do that weird twitch thing just before we drift off. Listen to But Why? wherever you get your podcasts. That's it for this episode of Brains On.

ANNIKA RADER: Brains On is produced by Marc Sanchez, Sanden Totten, and Molly Bloom.

MOLLY BLOOM: We had production help this week from John Lambert, Emily Allen, Lauren Dee, and Emily Bright. Our engineers were Michael Osborne, Corey Chapelle, Johnny Vince Evans, Stell Klein, and Mark Schultz. Many thanks to Eric Ringham, Gwen Holtmann, Lindsey Henning, Tracy Mumford, Dr. Alon Avidan, Rowena Orr, Stacy Maison, Karina Rader, Nathan Rader, Carl Reichert, and Tara Whitfield.

ANNIKA RADER: You can find more episodes of Brains On at brainson.org or wherever you get your podcasts.

MOLLY BLOOM: We're also on Instagram and Twitter, we're @Brains_On. And we're on Facebook too.

ANNIKA RADER: If you have any questions you'd like to hear answered on Brains On, email it to us anytime.

MOLLY BLOOM: The email address is hello@brainson.org.

ANNIKA RADER: Your curiosity fuels our show. Before we go, it's time for the Moment of Um.

[VOCALIZATIONS]

JULIETTE: I'm wondering, why does my dad snore? Thank you.

WEILI GRAY: Well, first of all, to answer, why we snore when we are asleep, I should start with what happens when we're awake. My name is Dr. Weili Gray. And I am a sleep medicine physician, who works in Newport, Vermont.

So why do we snore? So when we're awake, our muscles are nice and firm, and we don't make any sounds when we're breathing, unless we're sick. But usually, we don't make any sounds. And the air moves to the nose and the mouth, and it goes down to the throat, and it goes all the way down to the windpipe, and back to the lungs, and then back up.

However, when we fall asleep, all the muscles in our body relax, including the muscles in your throat and in your neck. So once those muscles relax, they get very floppy, and they expand. And now, there's not as much room for the air to move in and out. And when the air is moving, it gets turbulent, and it makes vibrations. And so that's what the snoring sound is from.

So snoring in itself is not bad, but the problem occurs when the muscles get so floppy that they essentially block the airway so that air can't move in and out. And then we have something called sleep apnea, which essentially means that the person has stopped breathing in their sleep.

The brain, actually, is pretty smart. When it senses that the airway is blocked and the person has stopped breathing in their sleep, it gets into a panic mode. Eventually, it will wake the person up. And this may only be for 10 seconds or a very short period of time, and the person may not remember it. But see, once we're awake, we go back to that normal breathing again-- the muscles firm up, and now, the person is able to breathe again. They will go back to sleep, because we all get very sleepy at night, and we all have a need for sleep.

[MUSIC PLAYING]

WOMAN 1: Um.

WOMAN 2: Um.

WOMAN 3: Um.

MOLLY BLOOM: I'm wide awake and ready to recite this list of names. It's time for the Brains Honor Roll, the amazing listeners who keep this show fueled with their ideas, questions, and mystery sounds. Here they are.

[MUSIC PLAYING]

[LISTING HONOR ROLL]

MAN: Brains On Live.

MOLLY BLOOM: Brains On is supported, in part, by a grant from the National Science Foundation. We'll be back soon with more answers to your questions.

ANNIKA RADER: Thanks for listening.