Almost everything in the body cycles. Blood pressure shifts at different times of the day, body temperature is lower in the morning than in the evening, and insulin and cholesterol levels fluctuate at varying intervals.
“It’s hard to find an aspect of physiology that doesn’t on some level have circadian cycling,” says Amita Sehgal, the John Herr Musser Professor of Neuroscience in the Perelman School of Medicine and an investigator with the Howard Hughes Medical Institute.
Sleep is the most well-known circadian cycle or rhythm. Most human beings sleep at night and are awake and active during the day, a cycle controlled by a rhythmic, biological clock in the brain. Sleep is essential to life, yet the function of sleep is a mystery to medical professionals. No one knows for sure why people sleep, or why sleep needs vary from person to person.
Born in New Delhi, India, Sehgal has been studying circadian rhythm and biological clocks using Drosophila, more commonly known as fruit flies, since she was a postdoctoral researcher at Rockefeller University in the early 1990s. Before enrolling at Rockefeller, she was a Ph.D. student at Cornell University, where she worked on the receptor for a human neuronal growth factor.
Sehgal joined Penn in 1993, and several years later, expanded her work with fruit flies to address the basis of sleep drive, which is largely independent of the body clock. She uses equipment allowing her to examine thousands of flies individually, at the same time. Her lab has made major breakthroughs in the study of sleep and circadian systems, including the discovery of a crucial circadian clock gene and determining how the biological clock responds to light.
The Current sat down with Sehgal, who is also director of the Chronobiology Program at Penn and a member of the Center for Sleep & Circadian Neurobiology, in her office in the state-of-the-art Smilow Center for Translational Research to discuss her interest in science and sleep, fruit flies, sleep mutants, the health hazards of working at night, biological clocks, and circadian rhythm.
Last May, you were elected to the National Academy of Sciences, but in a November interview with The Scientist magazine, you mentioned that you originally wanted to be a lawyer and were even accepted to law school.
I always feel like my true calling in life was to be a lawyer. Early on, I wanted to do English literature, and later on, I wanted to go to law school, but it just wasn’t viewed as a viable career in India at the time. There’s not that much litigation. There’s more now, but even so, not as much in the U.S. It’s just not a litigious society.
How did you become interested in science and sleep?
I think when I started to really get deep into science, which happened in graduate school, I grew to appreciate it. This is what I always tell people when they want to try science out. I feel like a very superficial exposure to science would probably turn you off rather than suck you in, unless you’re a real diehard science type. For most people, they just get intimidated or bored. I think that when you get deeper into science, which is what happened for me in graduate school, you can appreciate it more.
Was there a particular course that made you embrace science?
Molecular biology.
What did you find interesting about it?
For one thing, I worked in a small lab where the faculty member, Moses Chao, was just getting started setting up his lab, so I got to have a lot of direct interaction with him, which was good. He was attempting something very ambitious: He was trying to clone a gene for the receptor for a human neuronal growth factor, and he was using a very novel approach. The idea that this was going to be the first gene of its kind to be cloned was exciting to me. The fact that we were using a novel approach was exciting, and then I think once you get results, it makes a huge difference. The positive reinforcement is incredibly important.
You began working with fruit flies in order to conduct research on circadian rhythm and biological clocks. Can you explain circadian rhythm?
Circadian rhythm is a 24-hour rhythm and it is driven by clocks that are within us—or I should say it’s about 24 hours. Most of the time, the clocks within organisms are not exactly 24 hours. Human clocks tend to be a little long. If you were left to yourself without any day/night in constant darkness or constant dim light, every day you would wake up a little bit later than the day before. You are kept on a 24-hour cycle either by light or by your alarm clock.
What is the biological clock or circadian clock?
The circadian clock is what generates the 24-hour rhythm. There can be other clocks, too, like menstrual cycles.
And these clocks are everywhere in the body?
Yes. We didn’t know that at the time, but now we do. When I was a postdoc, I looked for new clock genes. At the time, only one circadian clock gene was known, and that was in flies. It was a gene called the ‘period’ gene. That had been discovered more than 20 years earlier. It produces a protein that is part of the clock.
While you were a postdoc, you and a colleague identified a second clock gene, which you called ‘timeless.’ What is the timeless clock gene?
What we did was look for genes that would be required for normal circadian rhythm. There are two ways you can measure rhythms in flies. One is by looking at their emergence from pupae. Flies start life as an egg and then they go through a larval stage, and they form these little cocoons, like butterflies do, called pupae, and then adult flies come out of the pupae. It turns out that the emergence of adult flies from pupae takes place within a few hours of dawn. It’s a process that only takes place once in a single fly because it only comes out of a pupa once, but in a population, it’s a rhythm because you see bursts of flies emerging at successive dawns. It’s the clock within the animal that makes it come out at a certain time, so we used that to look for flies that would come out at the wrong time of day. Anything that happens at a specific time of day is potentially something that is driven by an internal clock, a circadian clock. We looked for flies that came out at the wrong time of day, and we found flies that had this one particular mutation. We called those flies ‘timeless.’ Then later on—this happened after I got here [at Penn], actually—we were able to identify the gene that was mutated.
Do the flies that come out during the wrong time of day have any deficiencies?
Not really, not in the lab. At the time, nothing was detected. In more recent years, people are finding things that are aberrant in animals that don’t have clocks. They find, for instance, that they’re more susceptible to neurodegeneration. The reproductive success may not be as good, but they’re all very subtle effects. There was nothing major, and certainly nothing that popped out at the time. But the thinking is that we didn’t see any difference because in the lab conditions, we take care of our flies and we give them food all the time, so they don’t have to fend for themselves. If they were in the wild and they did have to fend for themselves, the loss of a clock would probably affect them because if they didn’t have a clock, they wouldn’t have any sense of timing, so they wouldn’t know when the food is going to be available. In the lab, they get babied.
How did your research on the circadian clock and circadian rhythm expand to the study of sleep?
Sleep, of course, is a circadian rhythm. Initially, when we were looking at rest activity in the fly, we were just using that as an assay to figure out how the clock works, like what is the clock? How do you make a clock? What are the genes that are part of the clock? The timeless flies, in rest activity, have no rhythm. They had lost rhythm in every assay we could look at. So the questions we were asking with the clock were, how does the timing work? How is that conferred upon processes? Sleep is something that we became interested in later after I had my own lab here, and we were basically asking, when the flies are resting, what is this rest in flies? Is it sleep? And we showed that it is sleep. This was actually worked on in my lab by Joan Hendricks, who’s now dean of the Penn Vet School. She was a professor here at Penn and she worked on sleep apnea in bulldogs. She wanted to learn molecular biology so she came to my lab just to learn molecular biology, to do a sabbatical. During the sabbatical, we decided to—because she came from a sleep background and we were all circadian rhythm—do some experiments to see if the fly rest was a sleep-like state and she showed that it was. In the lab, when we study sleep, we’re not measuring the timing anymore, that’s the circadian part. What we’re interested in is how much the animal sleeps, like how long, and why they sleep. When we look for animals that are sleep mutants, we look not for animals that have messed up timing, we look for animals that have too much sleep or too little sleep, and we ask, what molecules account for that? What is the genetic basis of sleep? How much sleep need does a person or an animal have, and then we ask, why do they sleep?
Why are fruit flies a good model for understanding sleep in humans?
It turns out—and this was one of the things that was encouraging to us when we embarked upon the sleep work—that the way you make a clock in the fly is the same as how you make a clock in humans, to the point now where there are human circadian disorders that are caused by mutations in genes that were first found in flies. The disorder that has been most studied is called advanced sleep phase syndrome where the timing is totally off. People go to bed at like 7 p.m. and wake up at 3 a.m., and it runs in families. Some of my colleagues in the circadian field have been studying these families, and they have found these genes that cause this effect, and those genes are the ones that were initially found in flies. We know that the circadian genes and the circadian mechanism are the same in flies and humans. What we’re hoping for is that what we find out about sleep in flies—like what makes the animal sleepy or why the animal needs to sleep—will be the same in humans. So far, that seems quite encouraging. The genes we’re finding in flies are the same as the ones that are popping up in studies of mice and humans.
I saw an interview you did with Neil deGrasse Tyson for PBS’ ‘NOVA’ where you said, like humans, fruit flies are active during the day and sleep at night, and some of them have an afternoon siesta.
Yes, especially males.
You also mentioned that fruit flies sleep up to 12 hours per day.
Yes, the males sleep more than females. If you deprive them of sleep, they don’t do very well. And when they’re young, they sleep a lot, like human infants do.
What effect does light have on the circadian clock? Does light tell people when to wake up?
Light has an arousing effect, but the clock is also telling you to wake up. Even if the light didn’t come on, you would wake up probably close to the time that you normally do, maybe a little bit later.
What about people who work at night?
They’re compromised. Hugely. Because they’re going against everything. Shift workers have huge health problems.
What do you mean by compromised? What are the health repercussions of working at night and sleeping during the day?
They are at higher risk for some cancers, for cardiovascular dysfunction, metabolic dysfunction.
For third-shift workers, even after, say, working at night for 15 years, their body won’t adjust?
They never completely do and it’s not just day/night, it’s also the social cues, which your clocks will synchronize to. They are reversed. The other thing is that they tend to also be sleep deprived because they never get as much sleep during the day as they would have gotten at night if they had been on a normal shift.
Is there something that could disrupt a person’s circadian rhythm, like a disease or ailment?
In modern society, we are constantly disrupting our rhythms, even if we’re not shift workers, because we expose ourselves to light all the time. Ideally, what you want is bright light during the day, like sunlight, and completely dark at night. But most of us don’t get that because we’re not outdoors enough to get the bright light, and indoors, even late at night, we’re looking at computer screens, we have lights on, and that’s disruptive for the clock. The clocks are not working as well within us if they’re not getting total light and dark.
People are now talking about all these screens you should get for your electronic gadgets that block blue light because blue light is what’s really going to reset your clock the most. Mis-timed feeding will mess up your circadian system. When you eat at the wrong time, that also is bad for your clocks. Not for the clock in the brain; the clock in the brain is pretty resistant to the time of feeding, but the clock in the liver is sensitive. If you eat at the wrong time of day, your liver clock is going to adjust to the time of feeding; the brain clock isn’t. So it’s like within your body, you have two clocks on different time zones.
But it is also the case that in many diseases, circadian rhythms are affected. Some of those are metabolic, like diabetes, and some of them are neurological, like some of the neurodegenerative disorders, and some of them are psychiatric, like schizophrenia and bipolar disorder. As to how much of a contribution that circadian disruption makes to the disease is still an open question.
What about people who are blind? Do their biological clocks adjust to not seeing light?
With blind people, it depends upon the amount of degeneration in the eye. Within the eye, there are cells that allow you to see visually, that create an image. Those are the rods and cones. For the circadian clock, there is another type of cell in the eye that is important, and that’s the retinal ganglion cell, and that actually can sense light. Basically, your circadian clock in your brain can see light using any one of these cells: rods, cones, and the retinal ganglion cells. Vision can only occur with rods and cones, but the circadian clock can use all three. When a person is blind, usually they’re defined as blind because they cannot form an image. They can’t see, so they don’t have functional rods and cones. If they still have intact retinal ganglion cells, then they can synchronize their clocks. But sometimes when people are blind, the eye degenerates completely. In that case, they would not be able to synchronize their clocks. The short answer is that whether or not a blind person can synchronize to light depends upon the degeneration in the eye, and it will be different from person to person. For the ones that have so much degeneration in the eye that they don’t have any of these cells and they can’t synchronize to light, they might use social cues to synchronize their clocks.
Your lab is studying longstanding questions about sleep regulation and function. Is sleep required for more than just rest?
It’s not just physical exhaustion because the physical exhaustion requires rest. You could be sitting at your table all day long looking at your computer and at the end of the day, you need to sleep. It wasn’t physical fatigue. It could be mental fatigue, but then what is that mental fatigue? Or what does that mental fatigue trigger in your brain that makes you sleepy? Why couldn’t you just lie down and rest your muscles?
Am I correct that your lab is currently studying the role of clocks in non-brain tissues?
We have looked at the liver. We looked at the testes also at one point. And the thymus.
You are also addressing the relevance of the circadian sleep systems to other aspects of physiology, particularly metabolism and healthy aging. How are circadian systems important in aging?
It turns out that with age, sleep is affected, circadian rhythms are affected, so with respect to sleep/wake, your sleep gets fragmented. You don’t sleep as well at night, you sleep more during the day, so the rhythm of sleep is affected, and the ability to maintain long episodes of sleep is affected. We’ve also looked at what happens to actual clocks within the body, using the fly as a model, and we find that in the brain, the clocks are still OK with age, but in the body, they actually tend to be weaker.
You are the director of the Chronobiology Program at Penn, which you created in 2013 to bring together researchers at the University working in diverse areas impacted by circadian clocks. Can you talk about the program?
We work on circadian rhythms and we are studying mostly functions of the brain and how clocks control them, like rest activity cycles and sleep/wake cycles. But circadian clocks are found in all body tissues. There are people here who are studying clocks in different realms. Mitch Lazar’s lab [the Willard and Rhoda Ware Professor in Diabetes and Metabolic Diseases], for instance, studies circadian control of metabolic function. Garret FitzGerald’s lab [the Robert L. McNeil, Jr., Professor in Translational Medicine and Therapeutics] is interested in circadian control of cardiovascular function. There are people who are looking at immune responses. There are many people around here who study sleep. And then there are people around here who are not thinking about circadian regulation yet, but they are working on a process that probably is rhythmic, and we’d like to encourage those people to actually come into the group and take the circadian component into account. For a scientist or even a clinician, things are going to be very different based on the time of day, and most people are not really paying attention to that.
You have two daughters. Do you talk to them about the importance of sleep?
All the time. I used to always tell them, ‘The night before an exam, the most important thing is to sleep.’ That, I think, has sunk in. It took a long time, but I think that they have recognized that it is important to get a good night’s sleep. My older daughter, I tease her about being a circadian mutant. I was talking about advanced sleep phase syndrome where people go to bed really early and wake up really early; there’s another disorder that’s the opposite, which is delayed sleep phase syndrome, where people are up really late, and she falls into that category. I think that is part of why she couldn’t sleep when she was a baby. Advanced sleep phase and delayed sleep phase are the extreme ends. They are not considered within the normal distribution. But even within a normal distribution, there are people who are morning people and who are evening people, and the thinking is that it’s because of small polymorphisms in their circadian clock genes.
Is there an ideal time that people should go to sleep?
I wouldn’t want to put a perfect time on it recognizing that this business of morning people and evening people is a real thing, and there is a biological basis for it. I think telling somebody who is an evening person that they need to be asleep by 10 o’clock is just not going to work. There are individual variations that are probably genetically driven and you can’t really change that. In the circadian field, we call them owls and larks. The larks are the morning people and the owls are the evening people.
How many hours of sleep do you get a night?
About eight.
In The Scientist interview, you said sleep has always been important to you, even as a child.
It’s always been important to me to get a good night’s sleep, even when I was a kid. It wasn’t my parents who had to tell me to go to bed. I was putting myself to bed and then panicking if I didn’t fall asleep right away because I wasn’t going to get enough sleep.
