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How our body’s circadian clocks affect our health beyond sleep

How our body’s circadian clocks affect our health beyond sleep


We spend 90 percent of our time indoors, so lighting is important

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Rhythm Clock

“Everything cannot happen at once,” says Satchin Panda, a circadian biologist at the Salk Institute for Biological Studies. This is true when it comes to experiences in the world, and it is also true when it comes to our bodies.

We cannot fall asleep and wake up at the same time, nor sleep and do heavy-duty digesting. The body needs time to rest and repair, and so it works on a set clock called the circadian rhythm. The circadian rhythm has gotten more notice recently — after all, last year’s Nobel Prize in Medicine went to three scientists who discovered this day-night cycle. But popular knowledge focuses on how the circadian rhythm affects how we sleep, ignoring many of the other ways this cycle runs our lives.

The Verge spoke to Panda, author of The Circadian Code (out today from Penguin Random House) about how and why the circadian rhythm developed, why it can be harmful to eat late at night, and what this research means for the homes of the future.

This interview has been lightly edited and condensed for clarity.

Most people have a hazy idea of what the circadian rhythm is — something to do with our natural body schedule and when we wake up. What’s the scientist’s definition?

The problem is, there’s not a very clear definition. It’s the 24-hour rhythm. The bottom line is that almost every hormone, every brain chemical, every digestive enzyme and so on is pre-programmed to peak at a certain time of the day and then tap out at another time of the day. It’s an in-built schedule for different programs to do different things at the optimal time, and these timing mechanisms are the circadian rhythm.

The circadian rhythms are controlled by what we call circadian clocks, which are present in every organ and every cell, and these clocks tell our brain when to sleep, tell our gut when to digest our food optimally, tell our heart to pump more blood and slow down at another time.

Photo: Chris Keeney Photography

Why is this necessary?

This is because everything cannot happen at once. We cannot sleep and digest our food at the same time, we cannot try to focus on a task and at the same time fall asleep. For our organs to function well, they need time to rest and repair, so that’s why we flip flop through different states.

Do most people have a pretty similar circadian rhythm? Why do we hear so much about night owls versus early birds?

Most of us are programmed to go bed from about 9PM to 11PM and wake up around sunrise time. Very, very few people deviate — so they’ll go to bed from 7 to 9PM and wake up around 2 or 3AM, but these people are extremely rare. It might be a genetic mutation. Think about our ancestors who are hunter-gatherers, maybe it’s beneficial for some people to go to bed early and wake up early to guard the village.

But we are also coming to realize that some of the variation with night owls might because they’re more sensitive to bright lights, and might be due to habits. People who are used to having an evening coffee and then have a lot of bright light in the evening and more likely to sleep very late. And since they do it on a daily basis, they think it’s natural to them.

In the book, I talk about the example of my dear friend who is also a circadian biologist. He took his lab on a camping trip. Almost everybody in the lab thought they were night owls because they all go to bed after midnight. But when he took them camping and they had very little access to bright light, all of them became early birds. They went to bed three to four hours earlier and woke up very fresh, early in the morning. So some of the variance is definitely due to external habits.

We think of circadian rhythm as mostly affecting sleep. What are some other systems that it affects?

Our intestines, our stomach, and our gut have circadian rhythms, too. Late at night, just like our brain goes to sleep, our stomach and gut start to shut down. Our intestines and gut don’t move food down the digestive tract, so if you eat late at night, the food just sits there. At the same time, the stomach has a buildup and starts to produce acid.

During the daytime, the circadian rhythm in our mouths actually produces saliva that neutralizes that acid. But in the evening, even our mouths shut down, which is why we don’t salivate much in our sleep. When you have a lot of acid, food isn’t going down, and acid is coming to our mouths, we get acid reflux. It’s a very simple thing, but just eating early in alignment with the gut rhythm can help.

Another idea is in terms of exercise. Exercise has many benefits for the circadian clock and sleep cycle. So for example, taking a short walk in the morning has as huge impact on synchronizing the brain clock and improving arousal and alertness.

What does all research into circadian rhythms mean for how we live? Should we be doing more with artificial lighting to improve our health?

In modern societies, we spend close to 90 percent of our time indoors, so it’s usually not the outdoor summer or winter days that determine our mood. I was surprised myself. When we tracked people in San Diego by putting a light sensor on their wrist, we found that nearly half the people in sunny San Diego were spending their days in dimly lit office rooms — so they were essentially living in winter in Boston, from a light perspective.

That really opened my eyes and made me more conscious about light. Now, if I’m driving, unless the sun is directly hitting me, I don’t wear sunglasses because I want to get that half-hour of sunlight. At the same time, I avoid brightly lit grocery stores at night.

What does this mean for indoor lighting?

For the first time in human history, we actually have nearly complete control on how much light we have at different times of the day, so we can program lighting in our indoor environments in ways that simulate midsummer or spring days.

I’m hopeful that in 10 or 15 years, for example, we won’t have light switches on our houses anymore. We’ll have programmable switches. We tell it what time we want to wake up, and the smart home will program the right amount of light and time. Maybe it can even sense that we’re getting depressed and try to determine the lighting. The technology will probably be there soon, it’s just more expensive, but there is demand.