How Your Circadian Rhythm Tunes Your Health: Satchin Panda at TEDxYouth@SanDiego 2013

Hello. So when I was a student back in India going to school, we lived in a small house. I’d play with my sisters every afternoon when my mother used to cook dinner. One day, in the middle of our play, my sister paused and said, “It must be 5:00 now.” I wondered. I said, “How did you figure that out?” because there was no clock around, she was not wearing one.

She smiled and said, “Look there’s a frog that hops into our front yard every evening at 5:00.” I was amazed. I said, “Wow, this little frog has a clock and can come to our house every afternoon at 5:00?” Little did I know that my sister’s sharp eye on the frog clock would change my career.

So several years later I studied clocks, and now we know that every plant, every animal, even we humans, we have clocks. And it’s becoming clear how these clocks, we call it “circadian rhythms” – the 24-hour clock – have profound effect on our health and physiology. When we are born, our little babies, they actually don’t have a fully functional clock. So they’re not wired properly, so at random times of the day they would go to sleep, they would cry, wake up, eat a little bit and go back to sleep.

So imagine if we all did not have clocks, just like babies, then half of us would be crying and half would be sleeping. But luckily, by three to six months of age, babies have clocks, so they can stay awake all day and then can sleep throughout the night.

And now we know that these circadian clocks, present in different parts of our body, turn on and off thousands of genes at different times of the day. And by doing so, they actually tune our physiology, metabolism and mood to the right time of the day. For example, last night around 2:00 in the morning, many of us were in our deepest sleep, and then the clock prepared us to wake up by warming up our body a little bit, by pacing our heart a little bit more, and as soon as we woke up, opened our eyes and started our day, our melatonin level that makes us sleep went down. The stress hormones, the cortisols, began to rise. And the digestive juice and all the hormones that help us digest food, they began to flow.

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And after breakfast, when we stepped into the offices or school, our brain was actually at its peak performance to solve complex math and to make critical decisions. And then in the afternoon, the clocks in muscles fine-tuned our muscle tone and then improved our motor control, and that’s the best time to hit the gym. And in the evening, as the sun goes down, when the clock prepares us to sleep, our core body temperature falls down, the melatonin begins to rise, stress hormone goes down, and we go to sleep. This happens almost every day in a very regular fashion.

But what is interesting is we have to reset the clock, otherwise, my clock will be tied to India, and I’ll be sleeping right now. And by learning what things in our environment tune our clock, we’ll have better control of our circadian rhythm and on our health. We knew that light resets the clock, because if I put a healthy individual, a normal person, or even a blind person who doesn’t have any rod and cone, in a room and shined bright light for a couple of hours in the middle of the night, that will screw up the clock and will mess up the sleep-wake cycle.

So for a long time we knew that there is a light sensor that is in our eye that resets our clock, but we didn’t have any idea what it is. So after almost a century, finally, we found a clue from these little frogs. When you put a frog in bright sunlight, then some frogs will change their skin color as if they’ve got a sun screen.

And we found there is a light sensor in the frog clock – frog skin – that’s also present in human eyes. There are nearly 5,000 of these blue-light sensors that sense blue light in our surrounding and send that information to different parts of the brain. So what happens in the morning as we open our eyes and start our day, this melanopsin, blue-light sensors, senses light and tunes our internal clock to the local time. And then, throughout the day, it senses light, makes us more alert, makes us happier, suppresses sleep. In the evening as the sun goes down, the light sensor is off, and then we can go to sleep.

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So what will happen, now imagine if we spend our whole day sitting in a dimly lit classroom, or a long winter with cloudy skies and this light sensor is not active any more. And that can, over long term, drive us towards depression. And the flip side is, later at night, as we gaze into those rectangular pieces of glowing objects, also known as television screens, computers and phones, that activates this light sensor and messes up sleep. But what is interesting is, light is not the only thing that resets our clock. When it comes to the rest of our body, it’s the time we eat that’s actually more important.

So in modern day, we stay up late into the night and eat again at nighttime, then the question is whether this frequent eating, both during day and night, messes with our clock and physiology and metabolism so that we become fat. So to find a clue, we looked into an experiment done in many labs, both in industry and academic. And that is, you take twin mouse brothers who are identical, and then give them food – one gets a healthy diet, and the other one gets a high-fat diet. You come back after a few weeks, then the mouse on the healthy diet should be very healthy, whereas the mouse on the high-fat diet becomes obese. It is known.

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