Your brain, your heart — in fact, every cell in your body — has its own clock telling you when to be alert and when to pack it in.
Dr. Emily Manoogian, chronobiologist at the Salk Institute for Biological Studies explains how disrupting your circadian rhythms through shift work, long flights, eating at the wrong times, and even staying up too late on weekends can affect your health, mood, and emotional regulation.
Plus how circadian rhythms affect all animals, not just humans.
Phil Stieg: Hello, I’d like to welcome Emily Manoogian, a clinical researcher at the Salk Institute for Biological Studies. A self-described “chronobiologist,” Dr. Manoogian studies the intricate interaction between our body’s biological rhythms and the timing of our daily habits. Much of her recent research focuses on studying how changing when we eat influences our circadian rhythm. Emily, thank you so much for being with us today.
Emily Manoogian: Thank you for having me.
Phil Stieg: So tell us, why should people listen? Why is chronobiology so important to us today?
Emily Manoogian: Great question. I think chronobiology and circadian rhythms as a whole are a huge just really a core component to all life that, unfortunately, most of us have been unaware of for a long time. It’s not just important for humans. It’s important for all life forms on this world because we don’t live in a constant environment. We live in an approximately 24 hour day, and all living organisms have biological rhythms to adjust to that.
Phil Stieg: Chronobiology is the study of circadian rhythms, but there are also other rhythms. I see it’s ultradian and infradian rhythms. Can you define that for us?
Emily Manoogian: Yeah, chronobiology really encompasses all of it. It’s really the timing of biology. Circadian literally just means about a day. So those are daily rhythms, but there are shorter rhythms, and they’re usually factors of twenty four. So they’re usually four, six, or eight hour rhythms, but there can be shorter. And then we have things like lunar rhythms and tidal rhythms that are annual rhythms, where you do see these regular oscillations on much longer timescales as well.
Phil Stieg: I was shocked to see that. You even state that individual cells have their own rhythm.
Emily Manoogian: Absolutely. Every cell that has DNA has a biological clock. So we actually have genes that are transcribed, and translated, and broken down, and go back and feedback on their own transcription with a 24 hour period. So, each individual cell keeps its own clock. If your cell has DNA, it has a clock built into it.
Phil Stieg: So is the circadian rhythm this kind of overarching rhythm that coordinates all the various rhythms? Because I can only imagine entropy in every cell going in its own direction and our bodies flying apart.
Emily Manoogian: Yeah, I think a key part of this is, no biological clock is exactly 24 hours. They’re about 24 hours. And within the body, cells could fall out of phase. You have to have this coordinating signal, and so depending on what rhythm you’re talking about, it can be controlled a different way.
So for instance, most of our behavioral rhythms are really controlled by this master clock in our brain called the suprachiasmatic nucleus which is just a geographical term for where it’s located. But it’s one of the really two areas that we know that has this intricate interaction between these clock cells that then output to many other parts of the brain and the body to control and kind of coordinate everything. It’s frequently known as the conductor of the kind of biological clock orchestra.
Those rhythms really are dictating behavior, hormonal secretion, sleep wake cycles, those types of things. Whereas other things like when you eat, is going to affect your clocks in a little bit different way, and that doesn’t really affect that SCN the same way that light would, for instance. So light has a much bigger influence on your behavioral sleep wake cycle than food would for instance.
Phil Stieg: Specifically related to the brain and our affect. How are circadian rhythms important?
Emily Manoogian: So circadian rhythms are really important in almost every level and some of them are direct and some of them are indirect, right? So circadian rhythms play a huge role in sleep regulation and arousal cues, and attention and alertness. There’s been some research to show they also heavily influence mood or ability to kind of control your behaviors. Like you’re probably going to have a more successful difficult conversation with a spouse at certain times of days than you would at others. There’s even some interesting work on being able to edit papers better in the afternoon than in the morning.
Phil Stieg: I’ll have to remember that. When you’re editing papers do you tend to be kinder in the afternoon or more vicious?
Emily Manoogian: I didn’t say reviewing papers, editing your own work at least, it tends to be best in the late afternoon or evening for most people.
But it’s interesting. I mean if you disrupt these rhythms you disrupt many different pathways. You’re disrupting sleep which as we know is an extremely important factor to mood and health. So it’s very integrated. But when you do disrupt these patterns you have moodiness, you have fatigue and headaches and all these kind of things where you’re making maybe not great decisions.
But beyond that, I mean circadian disruption has also been linked to every effective disorder like major depression, bipolar. So we know that it’s really important component to be able to kind of keep the system working properly.
Phil Stieg: I imagine everybody is familiar with disruption of their circadian rhythm through jet lag. But also I was a little bit curious about social jet lag. What is that?
Emily Manoogian: Yeah, so let’s start with real jet lag first. I think it makes it a little easier. So our biological clocks can only shift pretty much up to an hour in a day, and it’s easier to delay than it is to advance. So if you fly three hours, or ten hours, or twelve hour time zone difference, your body is completely out of sync.
Certain organs are able to shift faster than others, so your vital organs can actually shift a little quicker than some of the others. But it causes this phase misalignment between your rhythms. Your body’s also fighting its environment because it’s not used to this cue. So it’s a very hard thing for it to do.
And I think an important thing to keep in mind on why these shifts are so hard is because the circadian system is anticipatory. It’s there to prepare your body for whatever it’s going to need to be able to do. And when you do these really huge shifts, you no longer have that happening anymore. Your cortisol peaks are misaligned, your heart rate is misaligned. When you’re tired is misaligned, everything is off. And then even physically within your body, it can take a week to two weeks, depending on how far you’re traveling, for all those clocks to really coordinate again. Which is why even if you get on a sleep wake cycle, you still might have some nausea, your stomach might be a little off, you’re not having regular cycles. Like things like that can still be disruptive.
Social jet lag is a similar phenomenon where you don’t physically change locations. You’re still in the same time zone, but due to behavioral reasons, you’ve kind of functionally changed the time zone that your body is in. And I think the most relatable example of this is just a weekday to weekend where maybe you have to wake up early for work. You go to bed a little bit earlier during the weekdays, and Friday comes along suddenly. You stay up much later, maybe binge watching a show or going out with friends.
You’re getting a lot of bright light exposure late into the evening, probably consuming calories as well, giving a lot of cues to your body that you should be awake, that light is available later in the day, then you sleep in quite late the next day. Now you’re not having your light cues until much later in the day. And so even though you’re still in the same environment, the cues that you’re giving your body really are as if you had transported yourself. And then Monday comes around and you have to force yourself back. And so this can also be an extremely disruptive cue to the circadian system.
Phil Stieg: In reading about that made me think about firemen, policemen, ICU nurses, nurses that work night shifts and day shifts. What’s the immediate functional effect? And are there any long term effects? Meaning you don’t live as long or there’s other diseases that occur because you’re doing this to your body?
Emily Manoogian: Yes. Unfortunately, shift work is a known health concern and, as you mentioned, some of the most vital components of our society, doctors, firefighters, police, journalists, are all doing shift work. In fact, if you look within the U.S. about 20% of the population is doing some sort of shift work. So it’s probably a more widespread problem than we realize. And unfortunately it is known to come with both short-term and long-term consequences.
Short term is frequently associated with moodiness, fatigue, headache, nausea type feelings. I think we’ve all had an all nighter jet lag where we felt that you’re not quite right. I think that’s a good example of the feeling of circadian disruption.
Long term consequences do seem to be more severe. In fact, the World Health Organization does list shift work as a carcinogen because we do know that rates of cancer go up. We also see increased rates of many cardiometabolic diseases such as diabetes. And it’s something that we’re still working to fully understand. We know that these really short-term effects that have been kind of isolated in a lab where you can see compromised glucose regulation, you can see compromised cortisol timing
Phil Stieg: Which is the stress hormone.
Emily Manoogian: Exactly, yeah. And I mean that’s going to be a big arousal cue. Like you get a little cortisol peak when you wake up in the morning to help your body wake up and start the day, right. But when you misalign all these cues, they can now hit at a very inopportune time.
In fact, you even see this on a very small scale when you have daylight savings time and you’re doing these shifts, even that one hour shift can be enough to trigger heart attacks for people that are really on edge. And it’s not even likely due to the cortisol peak. In fact, even arterial stiffness and plaque formation has circadian rhythms and that is actually thought to be related to this increased risk of heart attacks.
Phil Stieg: So since you say 20% of society is a shift worker, I don’t mean to terrify them with all this information. If I took what you said to heart, I might be thinking about a different occupation. Are there ways to compensate, cope with or minimize the effect?
Emily Manoogian: Absolutely. And I don’t want to say everyone, all shift workers are doomed, that’s not the case. But they definitely have a unique challenge. So one of the things that we actually did with firefighters that were on 24 hours shifts, and they can work these 24 hours shifts up to four days in a row.
We did a study with them because we can’t control when they get their light, we can’t control when they sleep. They have to get up and help people in the middle of the night, but we could control when they ate. And as I said, one of the things we know is compromised in shift work is glucose regulation and likely leading to increased rates of pre-diabetes or diabetes, which is heavily tied into many cardiovascular diseases.
And so we were able to actually just restrict when they ate to the day. The participants in San Diego were able to stick to that ten hour eating window. They had reported health benefits. So we saw in all participants, even though many of them were healthy, we saw a decrease in VLDL size, which is related to cardiovascular events and forming plaque. So it was really exciting to see that go down. And in participants that did have elevated HBA 1C, we saw that significantly decrease, we saw their diastolic blood pressure significantly decrease. And this is all compared to controls that were actually even put on a Mediterranean diet, which was thought to be kind of the best thing you could do at the time. So I think there’s some hope there in some of the things we can do. And I think that’s just one example.
Phil Stieg: So to the extent that we can, we can try to regulate their rhythm. Obviously, eating isn’t easily controllable. I would think as long as they’re not working, you could pull the blinds and make a dark room and try to regulate the light dark rhythm. What about blue light, computers, and TVs?
Emily Manoogian: Yes. Okay, so taking a step back. I think the three things that are going to be the biggest cues that we know of to affect your circadian rhythms are light, food, and exercise. And I’d say the fourth one, really, that isn’t well studied is social interactions. But we know that that’s going to affect your behavior as well.
So light is absolutely another place where we can take a step. Blue light is the strongest wavelength of light to talk to that part of your brain, the SCN, to tell it what time of day it is. And we’re actually the least sensitive to it in the morning when we expect it, and we’re the most sensitive to it at night when our body’s ready to go to bed.
So using blue light as an arousal cue to help you stay awake potentially in place of food, is an arousal cue or making sure you really dim it down when you do need to go to sleep and it might actually be light outside. And trying to restrict that light, I think, can be very helpful. I think most people have noticed now a lot of their phones or computers or tablets now automatically have some of these functions that might remove blue light at certain times of day, which can be really helpful.
There are a lot of things you can do to try to make sure you’re getting tons of bright light in your morning, whatever time that may be, and really dimming down light for an hour or two before you go to bed. And then exercise, I think, trying to be active during your day and moving around. And for night shift workers, this isn’t a problem, especially like nurses or doctors. You’re on your feet all night, you’re being plenty active. But still, for people on a non-shift worker, if you’re moving around, making sure you are getting a lot of physical activity can also be helpful. And so really using those cues as much to your advantage as you can, can be quite helpful.
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Narrator: Most animals follow a strict day/night cycle that dictates when they’re active and when they sleep. But what happens when animals find that their normal routines are unexpectedly overshadowed by the disappearance of the sun?
Total solar eclipses happen when the moon passes between the sun and Earth and blocks the sun completely. During this time, known as totality, it doesn’t get completely dark—more like twilight than true nighttime. But it’s just dark enough to confuse animals that set their internal clocks by the sun.
This false dusk can awaken nocturnal fish and start mosquitoes swarming. Meanwhile, formerly busy bees cease their buzzing. Bats may emerge and begin to hunt, as orb-weaving spiders call it a day by breaking down their webs, and birds head to their roosting spots where they sing evening songs.
If the eclipse lasts long enough, birds may fall silent and prepare for sleep. But when the light returns, they switch to the songs they sing at dawn, probably wondering why they don’t feel rested.
For some animals, an eclipse is downright stressful. In 2001, after a total eclipse in Zimbabwe, hippos seemed agitated for the rest of the day, and impala were more skittish than usual. During 2017’s Great American Eclipse, which spanned much of North America, many animals in a South Carolina zoo behaved strangely. A male gorilla charged the glass in its enclosure. Baboons appeared “highly vigilant” and ran around as a group.
A 1984 eclipse stirred an even more unusual response in captive chimpanzees at an Atlanta research center. As the sky darkened, chimps climbed to high spots in their enclosure, positioning their bodies toward the sun and turning their faces upward. One youngster even stood and gestured in the direction of the sun. Scientists reported that none of the chimps acted this way during a normal sunset.
Researchers suspect that animals with complex brains recognize that something weird is happening during a solar eclipse. Intelligent animals are therefore more likely to be upset and behave abnormally when that unexpected shadow falls—even if they aren’t typically afraid of the dark.
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Phil Stieg: Everybody wants to make a big deal out of daylight saving time. It’s one hour twice a year. Is it that big of a deal that we have to go through all of this conversation?
Emily Manoogian: I think that’s the very surface level of you have these two shifts and yes, those can be quite harmful. You do see an increased rate of car accidents and heart attacks and strokes.
Phil Stieg: Around that time period?
Emily Manoogian: Yes. Look at it. It’s pretty dramatic. Just that day following. It’s pretty severe. The real problem, though, is when you have a misalignment for long periods of time and depending on where you’re located, this could be more or less severe for you.
If you have a lot of light access, then this may not affect you the same way. But again, light is a very important cue to tell your body that it’s time to be awake. Daylight savings time, although it gives you an hour later in the afternoon, which can be quite enjoyable, it frequently leads to having to wake up in the dark. That is a serious problem. Now, that would be the most common in the winter, which is why we shift back then. They’ve tried permanent daylight saving time before. It does not work and people do not like waking up and going to school and starting their day in the dark. Our bodies are not meant for it. It’s Horrible.
Phil Stieg: Couldn’t agree more.
Emily Manoogian: It’s pretty clear, actually, there’s more of a consensus on this than most things. The scientific sleep and circadian community is all for permanent standard time, not permanent daylight saving time.
This past shift, having to wake up in the dark, it’s a lot harder. And my husband had mentioned to me, he’s like, I can do an hour shift. Like I could go to Arizona or something and I’m fine. Why is this so much harder? And I was like, because it’s dark. When you’re waking up, the light didn’t shift with you. And that’s very hard. Especially when we get into children having to go to school in the dark and adolescents that are already trying to shift back. It would kind of undo any of that good work we’ve done by delaying school start times a little bit. So keeping a permanent daylight saving time can lead to a lot of problems.
Phil Stieg: You mentioned it a little bit earlier. I’d like you to explain what you mean by time restricted eating and how you achieve that. If you told me I could only eat at 8:00 a.m. and 5:00 p.m. and not graze and snack, it’s a tough call.
Emily Manoogian: Oh, no, it’s not that restrictive.
Phil Stieg: Okay, thank you.
Emily Manoogian: So time restricted eating is really just the idea of aligning your food intake to your active phase and to keep it pretty consistent. Because as I said before, the circadian system is anticipatory. So it really needs to be able to anticipate these food cues and prepare when insulin is going to be secreted or suppressed and have enzymes ready to digest food. So time restricted eating is really not very restrictive. In fact, there are forms of it that are.
But time restricted eating does not require any caloric restriction. It only requires that you contain your food, depending on your definition, somewhere between eight and ten hours a day. There have been studies that have gone down to four hours. They do not show any additional benefits. I don’t think there’s any need for that.
Phil Stieg: What do you mean by contain it between eight and ten hours? That means you can only eat during an eight to ten hour window during the day?
Emily Manoogian: Yes.
Phil Stieg: Eight to ten hours apart?
Emily Manoogian: Correct. Not apart, within. So it’s a window. Think of it as an eating window.
Phil Stieg: So, I can eat from 8:00 a.m. to 8:00 p.m.
Maybe 8:00 a.m. till 6:00 p.m. would be better. But there have been some studies in humans that have shown twelve hours doesn’t really have an additional benefit.
If you think about a 24 hour day, hopefully you’re in bed or at least resting for eight hours. Say don’t eat for at least an hour after you wake up. Don’t eat for three hours before you go to bed. That’s now 12 hours you have left. Okay, just cut off two more hours. I think you should probably be eating within the first few hours after you wake up. But listening to your body is always probably a good thing. And making sure you have food available when you’re ready for it. And I realize there’s some challenges to that. Some jobs you can eat at your desk and you have a lot of flexibility and some you really don’t. And so that gets a little bit more complicated.
So all of this work started because if you put mice on a high fat diet, they actually change when they eat. They start eating more throughout the day when they would normally be resting. And so it became this thing of like, oh, are they gaining weight because of when they’re eating or what they’re eating? And that’s really how time restricted eating was born. And then they found if you restrict that same high fat or high fat, high sugar diet to just anywhere between an eight to twelve hour period, the mice don’t gain weight. If you switch them from eating whenever they want to having a shorter eating window, they lose the weight. They don’t develop fatty liver disease. All these really dramatic findings.
Now, in humans, we don’t see as big of a weight change. We do see some weight benefits. We see usually somewhere between a 3% to 5% weight loss, and this might be due to a change in calories. In mice, we can control the exact amount of calories they’re eating, and they’ll eat the exact same amount, and they’re still not gaining the weight. Whereas in humans, when you go on a time restricted eating schedule, you might eat a few less calories because you’re not having that late night snack or that extra thing in the morning. So it is possible that’s part of the weight loss factor there.
I don’t see time restricted eating as a weight loss mechanism. I see it as an overall health benefit as part of a healthy lifestyle. Because even without a change in weight, we still see big improvements in glucose regulation and cardiovascular health. And I think that’s where we really see the power of it.
Phil Stieg: I just have to state up front, we’ve cured multiple diseases in rats and mice. We still haven’t fixed it in humans. Even though we have similar gene pools, it’s a very different disease process than those animals.
So I see that you’re contributing to disruption of our circadian rhythm by starting My Circadian Clock, an app. So somebody’s going to sit up at night playing with your app?
Emily Manoogian: No. You wouldn’t play with it at night? No. The “My Circadian Clock” app is really meant to be able to understand when people eat, because historically, nutritionists don’t ask when or if they do. It’s just like breakfast, lunch, dinner, snack, but not really the timing of it or how it relates to your sleep. And so, our lab had created an app to really get an understanding of that and to do it fairly quickly.
So, it’s not entering calories, it’s just take a quick picture or name your food when you consumed it and it will track when you’re eating it. It’s very quick to do. If you’re just logging, you’re probably spending less than five minutes a day and that would be logging all foods, sleep and exercise. You can also log health measures if you want to check changes as maybe you set a new goal. But it’s freely available for both iPhone and Android. It’s all research based.
There’s no commercial interaction with it and it’s a way for you to be able to understand when you eat, how that relates to your sleep and your exercise, and be able to kind of give you a tool to change that if you’re interested.
Phil Stieg: So tell me about how, we have a young athletic crowd out there. They’re running. They go running at four in the afternoon. The endorphins kick in. I find that I love running at four to five. The endorphins,
Emily Manoogian: That’s a perfect time to exercise.
Phil Stieg: But I don’t get tired then. I’m not ready to go to bed at 10:00 p.m. when I should go to bed. How does athletic activity affect the circadian rhythm?
Emily Manoogian: Yeah, it’s another way that you’re telling your body what time it is. Generally late afternoon is a great time to be working out. You actually have your highest muscle capacity. You’re going to perform your best in the late afternoon.
In fact, sports teams use this to their advantage. Sometimes they’ll actually move games. So if someone’s traveling, it’s really inopportune for them. And it’s not just a mental thing, it’s not just a fatigue thing. Your muscles actually function differently at different times of day. So you physically are more capable.
Phil Stieg: That’s their cell based biological clock.
Emily Manoogian: Yeah, cell based, biological, and network based. Your skeletal muscle functions very differently at different times of day. And not just skeletal, but if you look at the heart function and blood pressure, all these things are changing.
Exercise plays a big role. It tells your body it’s time to be awake. It’s another arousing cue. And so exercise is great, but exercising just before bed can be a little disruptive. It’s probably going to delay your rhythms a bit. Exercising first thing in the morning can help to potentially wake up. But there’s been some very interesting work that’s come out on how exercise affects glucose regulation. And for individuals with diabetes exercising first thing in the morning could actually be harmful. It’s another arousal cue and it’s another cue to your system to say this is the time that we’re going to be awake. So, if you do it when you think you should be sleeping, it’s another challenge to the system.
Phil Stieg: So give me some practical strategies that I, and everybody listening, can use to maximize number one, their circadian rhythm. But for this show, most importantly, their brain function.
Emily Manoogian: Yeah, well, so one, your brain isn’t going to work as well as far as cognitive abilities late at night, or super early when your body’s trying to rest. So try to have difficult conversations or tasks during your normal wake hours. It’s pretty simple when you think about it.
Try to make it so your schedule is consistent. So you’re not having huge shifts in when you wake up and go to sleep on weekdays or weekends or any other type of subset of the days.
Let your body wake up before you eat, usually at least about an hour. Get a ton of light in the morning. I think try to eat a majority of your calories in the first half of your day. Get your exercise in during that active phase.
Have a dinner that’s a little less super carbie, like a little lower on the carbs, good protein, good fiber, not some huge binge meal. And then let your body calm down for at least three hours before you go to bed. And when I say that, I mean don’t do intense exercise, don’t keep a ton of bright lights on and stop eating.
Phil Stieg: You don’t agree with people that practice the art of daily fasting where they don’t eat and then they have one big, huge meal at the end of the day?
Emily Manoogian: No, I understand that there’s a fasting period of time restricted eating, but that’s a very modest period. It really only requires you to not eat for a few hours of your day. You’re still eating over a majority of the day that you’re awake. I think these intensive fasting things do have some benefits. I don’t think they should be done regularly and I think they completely miss the circadian benefit.
I think eating late in the day had some of these obvious problems of, you are probably not going to be as active. You go to bed, your glucose levels stay high. Late night eating leads to prolonged glucose elevation throughout the night. So I think that’s one of the problems that really compromises your sleep. It compromises your glucose regulation. When you’re having these huge meals, it can be very disruptive.
Phil Stieg: So, my last question – where are we going? What’s next in circadian rhythms and peripheral rhythms?
Emily Manoogian: Yeah, so I think one is awareness. You can’t improve on something that you don’t know about. And so talks like these are great to help spread the word and have people maybe think about, when do I eat, when do I wake up, how do I feel when I do those things? I think that’s a very good first step.
Where the research is going, we’re just starting to get into how to optimize circadian health. There’s been a little bit done in light, there’s been a lot in food recently, and now I think the field is going more towards exercise as well.
And we’re also trying to move outside of not only cardio-metabolic health, but what about other disorders and how does this really affect the body? How does it affect the body in different stages of life? But I think a lot of this is intuitive. If you go back and look at historical context, many cultures have figured out a lot of these things that we’re kind of rediscovering and we’re just now understanding the biological basis of.
Phil Stieg: Dr. Emily Manoogian, thank you so much for highlighting the importance of circadian rhythms in our lives, and how we need to be aware of regulating our sleep, diet, exposure to sunlight, and physical activity. I particularly appreciate learning about social jet lag and the importance that it has in our well-being, and our relationships with everybody around us.
Emily Manoogian: Thank you so much for having me.