How Your Circadian Rhythm Tunes Your Health

How Your Circadian Rhythm Tunes Your Health

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 a 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, the 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. 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, the 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 sunscreen. 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 surroundings 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. 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 anymore. And that can, over the 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 intersting 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 academia. 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 a healthy diet should be very healthy, whereas the mouse on the high-fat diet becomes obese. It is known. But what is interesting is if we look at what time of the day these mice ate their food, the mice on the high-fat diet, for some reason, they are addicted to that food, they eat both during day and night. But the mouse on the normal diet eats only during the daytime.

So what happens, the mouse that is eating the diet only in its natural nighttime, they have a very good clock, their genes turn on all the time and turn off at the right time of the day. Whereas the mice on the high-fat diet, the clock is messed up, so the genes don't turn on at the right time, or they don't turn off when they're not required. So that made us think whether it's the food, or the timing, or both, that actually made the mouse on the high-fat diet fat. So we did another simple experiment. Now we took mice that are identical and gave them two choices. One: they can eat whenever they want. And the other ones were trained or disciplined, to eat the same number of calories on the same high-fat diet every day, only at nighttime.

And as the weeks went by, both mice eating the same number of calories, same high-fat diet, actually [came out] very differently. So mice which were not disciplined, they actually weighed around 50 grams. But the mice who had a little bit of discipline on when to eat, they weighed 28 percent less, most of their body weight loss was due to reduced fat, and if we looked into their liver, they're loaded wih fatty liver. Whereas that entire disease was prevented by just timed eating. That is not the only thing that happened. Actually, these mice had much better blood sugar, blood cholesterol, and they were actually much more athletic. For example, they burned more energy, they had more endurance, and they could do complex motor control tasks. So essentially, what we showed was it is the time, and by training, mice to be disciplined, we can actually prevent many diseases in mice.

Well, the bottom line is, when you eat may be as important as what you eat. But hopefully, all this research that we're doing will one day translate into humans. But let's imagine what will happen if everything that we did in mice translates into humans. Imagine a 13-year-old student going to school weighing 100 pounds. He has two choices: either to eat and live as many other students do. Stay up all night, eat at random times. Or have a little discipline to eat only during the daytime, the way we're designed for hundreds and thousands of years to do, and have a long night's sleep at night. So now fast-forward when this person becomes 30 years old, starts a family. This is what might happen. The person on the top might weigh around 200 pounds and on the bottom might weigh around 150 pounds. And what is more important is, we know obesity predisposes us to a variety of diseases. And each of these diseases stays with us for life.

And the cost of treating and the cost of lost income from each of these diseases, almost a quarter of a million dollars. So now, we can think that next time we go to take a bite, we can look at our watch, see what time it is. Or, in the nighttime, we have to remember to switch off that light and take a good night's sleep. Thank you..

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