How to optimize your circadian rhythm
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You're listening to Life Kit from NPR. Hey, everybody. It's Marielle. You guys, I've been trying so hard to make my bedroom pitch black when I sleep, but I am fighting an uphill battle. I have blackout shades and curtains over them, but the street light on my block shines directly into my window, and the light somehow sneaks around the edges of the barriers I've created. Also, light gets in through my living room windows and then creeps under my bedroom door.

Yeah, okay, I'm being a little over the top here, but once my eyes adjust at night, it's not actually that dark in my room.

A lot of us are in this situation. In modern society, we are bombarded with light in the evenings, and research shows that can impact our health. When we get light and how much of it we get has an influence on all sorts of important functions in our bodies. NPR health correspondent Will Stone reported on some recent findings on this. And after the break, he'll shed some light on how we should think about all of this.

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All right, Will, so tell us more. What about this research grabbed your attention? This research was a pretty massive undertaking. The researchers recruited about 90,000 people in the UK, and they had them wear activity devices on their wrists. Those were also equipped with these light sensors, which meant you could track how much light each person was getting around the clock. And this was done as part of what's known as the UK Biobank. It's a big biomedical database that tracks information on health and lifestyle.

which meant that researchers could actually follow all of these people, tens of thousands of people, over about eight years and see who died and from what causes. And they ended up with something like 13 million hours of light data that they could use to look for associations. So these would be associations between light and your chance of dying? Yes.

That's right. Yikes. And what they did was they'd look at the group of people who had the brightest evenings and see how their risk of dying compared with people who had pretty dark evenings.

And what they found was pretty striking. I'll let Daniel Windred explain. It was his study. Windred is a postdoctoral researcher at Flinders University in Australia. We found that people exposed to the brightest nights had a 21 to 34 percent higher risk of premature mortality than

Whereas people exposed to bright days had 17 to 34% lower risk of premature death. So basically what they saw was a dose response relationship, meaning, you know, as your night got brighter, the higher risk of dying in the coming years, and then the brighter your days were, the less your risk of dying was. And Winred says, you know, that risk added up if you were falling short on both ends. We estimated that people with both

bright days and dark nights could be living up to five years longer than people with bright nights and dark days. Okay, five years. I feel like that freaks me out a little bit. It makes me think I should be doing something differently. How concerned are

should we be? Well, yeah, it's a great question. I mean, the first thing to say is up front, this is just one study. It does not prove causation. It shows a very convincing relationship correlation between light exposure and the risk of death. I will say that they were able to control for a lot of factors that could also explain these findings like income, physical activity, underlying health conditions. And

Even when they controlled for those things, they did see this clear story, which was basically, you know, you want bright days and you want dark nights. I spoke to a researcher named Dr. Charles Seisler about this study. I wanted to get his take because he's done research in this area for a long time. Seisler's at the Brigham and Women's Hospital and Harvard Medical School.

This is a very exciting milestone. Thinking back to nearly 40 years ago, when we first discovered that light was the most powerful synchronizer of human circadian rhythms, that was incredibly controversial and disputed to come full circle to the recognition that not only is light the most powerful synchronizer of our internal circadian clock, but keeping our clock

in sync with the 24-hour day is critical for our health and wellness and longevity. It's interesting that what freaks me out is exciting to researchers, but what do you take from this?

I think the key point here is that we have these circadian rhythms, and I'm sure folks have heard about this before. These are the patterns in our biology that fluctuate approximately over the 24-hour cycle. They have enormous influence on our bodies, on our hormones, our blood pressure, metabolism,

And this circadian machinery is really all throughout our body. It's in our organs. It's in our cells. But the central timekeeper, the kind of orchestrator, if you will, is in the brain. And this region in the brain responds very powerfully to light. And so obviously for millennia, humans evolved with the natural dark light cycle. But this has changed in a big way in our modern society.

In the last one and a half centuries, we've completely changed the temporal environment to which we're exposed as a species. And we are flooding the nighttime with light that was never possible before. And we are shielding ourselves from light during the daytime. At this point, lots of research, including these carefully controlled lab experiments,

show that inappropriately timed light can be hazardous. It's linked to mental illness, cancer, especially metabolic and cardiovascular-driven diseases like diabetes, heart disease, and obesity.

All right. So what can we actually do with these findings? It can feel overwhelming when the environment isn't working in your favor, when there are streetlights on every corner or maybe you have a porch light outside or, you know, you're just inside all day working in an office and you're not getting so much daylight. Yeah, I think this is a really important point. We have limited control over our physical environment and

And, you know, our lighting environment, we can't necessarily spend all day hanging out in the sun or turn out the lights promptly in the evening. But I do think there are some helpful kind of takeaways from this research and things to keep in mind that most of us can apply to our lives.

The first thing is simply make sure you're getting outdoors during the day and getting the light in that context. Because the light outdoors is stronger? Yeah, exactly. If you are in a place with tons of windows, you might be getting plenty of light during the day, but if you're in a place where there's a lot of light, you might be getting a lot of light during the day.

For many of us, you know, if we're in a typical indoor environment, a typical office environment, it's not a lot of light and it's easy to kind of overestimate how much you're getting. Light can be measured in units known as lux. And you can even get an app that measures this for you. If you're curious, I have one pulled up right now. I'm sitting here in my indoor office space, which actually has quite a few windows and it's like 150 lux.

And that is not very bright, as Windred explains. There's a massive difference in the light intensity between an indoor environment, which might be, let's say, 200 or 300 lux, and an outdoor environment, which might be 50,000 lux.

And this is what I heard from all the researchers I spoke to, that there's kind of orders of magnitude difference between indoor and outdoor environments typically. And so as long as you can prioritize daylight getting outside, that will help. Our brain is particularly sensitive to light in the morning. So if you can get outside, then it will help you be more alert. But even if you can't,

Late afternoon is fine. When you get home from work is fine. And Seisler says, actually, there's some evidence that being exposed to these kind of transitions in light, whether that's dawn or dusk, can be particularly important. The bottom line is, you know, don't overthink it. Whenever you can get outside, he says, try to accumulate about 45 minutes at least over the course of the day. Even if it's cloudy, you will still be getting a lot of light. All right. And what about light at night?

This is obviously a big struggle for many of us. I know that for me, there are streetlights that are pretty near my bedroom window. And I feel like it can be hard to completely black that light out, even with blackout curtains. Definitely. I have the same issue where I live.

I mean, the bottom line is the darker your night, the better you will be. In the study, people who had the lowest chance of dying in the coming years were exposed to what Windred described as barely any light between about midnight and 6 a.m., so quite dark.

On the other hand, the people who had bright light during the middle of the night, and specifically they found kind of between 2.30 and 3 a.m., that was associated with the highest risk of mortality. And Woodrid says this is actually when your circadian system is the most sensitive to the light. So, you know, if anything, at the bare minimum, make sure you're not getting bombarded with light in the dead of night. Okay. So...

It sounds like if you get up to go to the bathroom in the middle of the night, try not to turn any big lights on, for instance. Don't look at your phone. Definitely don't look at your phone. And actually, Sizler says they have these kind of special...

Night lights that are kind of a muted orange-red kind of shade, and they turn on when you walk by them so you don't trip over and hurt yourself in the middle of the night. And he recommended looking into those kinds of things if you want to keep your house really, really dark. I've tried eye masks too, but I feel like they always fall off by the morning. I am not someone who can sleep with an eye mask. I think just the discomfort of having something around my face outweighs any benefits. Yeah.

But okay, so is there anything, if you are getting some light at night, is there anything else you can do about it? Actually, there is. And this was pretty cool when I learned about this. Research suggests that

making sure you get light during the day, you know, which we already talked about as being, you know, really beneficial. It can actually be protective in a way. It can reduce the sensitivity to light exposure at night. And this comes from some studies that have measured the effect of nighttime light on the hormone melatonin. This is the hormone that promotes sleep and goes up as we get ready for bed. Scientists have found

Those who spend their daytime in dimly lit conditions had a much greater suppression of their melatonin when they encountered light during the night. And then that was compared to people who were exposed to more light during the day. They weren't as affected when they had artificial light at night.

Now, Seisler says this isn't like a total get out of jail free card, especially if you are spending a lot of time staring at a screen before bed. A light emitting e-book such as a tablet or iPad or whatever that emits a lot of blue enriched light, which sends a direct signal to the brain saying it's daytime. I guess it's the same thing for your TV or your phone. Yes, basically. Dang.

You're taking away all my fun stuff. Yeah, I know it can be kind of a bummer. But I think the real takeaway from this research is that you really can't think about just the day or just the night in isolation. They're really connected. This is what Laura Fonken emphasized to me. She's a neuroscientist at the University of Texas at Austin. I think a really important point is not just the

that exposure to light at night, but also taking into consideration if you work in an office environment where you're in a cubicle where there's no windows around, you're also not getting normal daytime light levels. So then it's really that your body can't sense that contrast between night and day very well. So you want to like help your body out basically to know the difference between when it's time, when it's sleepy time and when it's wakey time. Yeah.

That's exactly right. And one final point that Fonkin made to me, which I think is important, is that by paying attention to our light habits, you know, we may end up kind of inadvertently making other changes that are helpful for us. So if you're now prioritizing getting outside a bit more to get your light, you could end up moving around more, going for a walk or run. And that's just one more way that our circadian rhythms and our relationship to light is really intertwined with our overall health.

All right. Well, thank you so much for being here. Of course. Thank you. Okay. Time for a recap. Try to keep your days bright. Aim for at least 45 minutes of time outside each day. Even if you work in a bright room, you want to head outside because the light out there is orders of magnitude higher, even if it's a cloudy day. And our brains are particularly sensitive to light in the morning. So if you can get out early, that's best.

Try to keep your nights dark. Blackout curtains and orange nightlights can help. Also, reduce your screen time in the evenings. And if you get up in the middle of the night, really try not to look at your phone.

For more Life Kit, check out our other episodes. We have one on everything we get wrong about sleeping and another on the importance of taking a 10-minute walk after dinner. You can find those at npr.org slash life kit. And if you love Life Kit and you just cannot get enough, subscribe to our newsletter at npr.org slash life kit newsletter. Also, we love hearing from you. So if you have episode ideas or feedback you want to share, email us at lifekit at npr.org.

This episode of Life Kit was produced by Sylvie Douglas. Our visuals editor is Beck Harlan, and our digital editor is Malika Garib. Megan Cain is our supervising editor, and Beth Donovan is our executive producer. Our production team also includes Andy Tegel, Claire Marie Schneider, and Margaret Serino. Engineering support comes from David Greenberg. I'm Mariel Seguera. Thanks for listening. ♪

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