How to optimize your circadian rhythm

There's research showing that too much light at night and not enough daylight is taking years off our lives. NPR health correspondent Will Stone has tips to tune up your body's internal clock. This episode originally published December 17, 2024. Follow us on Instagram: @nprlifekit https://www.instagram.com/nprlifekit/?hl=en Sign up for our newsletter here. https://www.npr.org/newsletter/life-kit Have an episode idea or feedback you want to share? Email us at [email protected] mailto:[email protected] Support the show and listen to it sponsor-free by signing up for Life Kit+ at plus.npr.org/lifekit http://plus.npr.org/lifekit See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

  This message comes from NPR sponsor Viori with the Halo Essential Wide-Like Pant. Receive 20% off your first purchase at viori.com/NPR on any US orders over $75 and free returns. Exclusions apply. Visit the website for full terms and conditions.

• 0:16Marielle Segarra?

  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, OK, 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,

• 1:01Marielle Segarra?

  and there's research showing this can end up having big consequences for our overall health. I talked with NPR health correspondent Will Stone last December when these findings first came out. We'll have that conversation after the break.

• 1:14Speaker 02

  Stay tuned for the following announcements and messages from our sponsors. This message comes from Charles Schwab with their original podcast, Choiceology. Hosted by Katie Milkman, an award-winning behavioral scientist and author of the bestselling book, How to Change, Choiceology is a show about the psychology and economics behind people's decisions. Hear true stories from Nobel laureates, historians, authors, athletes, and more about how to make better decisions to help avoid costly mistakes. Download the latest episode and subscribe at schwab.com/podcast or wherever you listen. This message comes from Google Workspace with Gemini. One subscription that does it all. Personalized AI and premium business versions of Gmail, Google Docs, Drive, and

• 1:59Speaker 02

  more. Try it free for 14 days at g o o dot g l e slash Workspace with Gemini.

• 2:06Speaker 03

  The 2026 midterm elections are upon us. On November 3, voters will determine what the next two years of their local leadership and the balance of national government will look like. Listen to the NPR News Now podcast for all the updates you need ahead of and after November 3 from NPR News.

• 2:30Marielle Segarra?

  You know, there are a lot of great NPR shows, but we all know which one's the best. Wink, wink. NPR is celebrating our most memorable podcast episodes of the year, and you get to crown the winner of the People's Choice Award. So vote for us, Life Kit, at npr.org/People'sChoice. And may the best pod win.

• 2:51Marielle Segarra?

  All right, well, so tell us more. What about this research grabbed your attention?

• 2:55Speaker 04

  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.

• 3:38Marielle Segarra?

  So these would be associations between light and your chance of dying?

• 3:42Speaker 04

  That's right.

• 3:42Marielle Segarra?

  Yikes.

• 3:43Speaker 04

  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.

• 4:02Speaker 05

  We found that people exposed to the brightest nights had a 21% to 34% higher risk of premature mortality, whereas people exposed to bright days had 17% to 34% lower risk of premature death.

• 4:18Speaker 04

  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 Windred says, you know, that risk added up if you were falling short on both ends.

• 4:36Speaker 05

  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.

• 4:48Marielle Segarra?

  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 should we be?

• 4:57Speaker 04

  Well, yeah, it's a great question. I mean, the first thing to say is upfront, 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. 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.

• 5:40Speaker 06

  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.

• 6:11Marielle Segarra?

  It's interesting that what freaks me out is exciting to researchers. But what do you take from this?

• 6:20Speaker 04

  I think the key point here is that we have these circadian rhythms, and I'm sure, you know, folks have heard about this before. These are the patterns in our biology that fluctuate, you know, 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 in a region called the suprachiasmatic nucleus. 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.

• 7:05Speaker 06

  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.

• 7:22Speaker 04

  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.

• 7:41Marielle Segarra?

  When we're back, Will explains what we can do in light of this research. That's when Life Kit returns.

• 7:54Speaker 00

  Stay tuned for the following announcements and messages from our sponsors.

• 7:58Speaker 02

  This message comes from Charles Schwab with their original podcast, Choiceology. Hosted by Katie Milkman, an award-winning behavioral scientist and author of the bestselling book, How to Change. Choiceology is a show about the psychology and economics behind people's decisions. Hear true stories from Nobel laureates, historians, authors, athletes, and more about how to make better decisions to help avoid costly mistakes. Download the latest episode and subscribe at schwab.com/podcast or wherever you listen. This message comes from Google Workspace with Gemini. One subscription that does it all. Personalized AI and premium business versions of Gmail, Google Docs, Drive, and more.

• 8:39Speaker 02

  Try it free for 14 days at goo.gle/workspacewithgemini.

• 8:47Marielle Segarra?

  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're just inside all day, working in an office, and you're not getting so much daylight.

• 9:07Speaker 04

  Yeah, I think this is a really important point. We have limited control over our physical environment 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.

• 9:36Marielle Segarra?

  Because the light outdoors is stronger?

• 9:39Speaker 04

  Yeah, exactly. If you are in a place with tons of windows, you might be getting plenty of light during the day. But 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.

• 10:17Speaker 05

  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.

• 10:29Speaker 04

  And this is what I heard from, you know, 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, you know, late afternoon is fine. When you get home from work is fine. And Sizer 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

• 11:13Speaker 04

  of the day. Even if it's cloudy, you will still be getting a lot of light.

• 11:17Marielle Segarra?

  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.

• 11:36Speaker 04

  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, you know, what Windridge described as barely any light between about midnight and 6:00 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 3:00 a.m., that was associated with the highest risk of mortality. And Windridge 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.

• 12:21Marielle Segarra?

  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.

• 12:31Speaker 04

  Definitely don't look at your phone. And actually, Seisler says they have these kind of special nightlights that are kind of a muted orange-red kind of shade. And they turn on, you know, when you walk by them, so you don't, you know, trip over, 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.

• 12:55Marielle Segarra?

  I've tried eye masks too, but I feel like they always fall off by the morning.

• 12:59Speaker 04

  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.

• 13:06Marielle Segarra?

  But, okay, so is there anything? If you are getting some light at night, is there anything else you can do about it?

• 13:15Speaker 04

  Actually, there is. And this was pretty cool when I learned about this. Research suggests that

• 13:21Speaker 04

  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, you know, 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, Seizler says this isn't like a total

• 14:05Speaker 04

  get out of jail free card, especially if you are spending a lot of time staring at a screen before bed.

• 14:11Speaker 06

  A light emitting ebook, such as a tablet or iPad or whatever, emits a lot of blue enriched light, which sends a direct signal to the brain saying it's daytime.

• 14:22Marielle Segarra?

  I guess it's the same thing for your TV or your phone?

• 14:25Speaker 04

  Yes, basically.

• 14:26Marielle Segarra?

  Dang. You're taking away all my phone stuff.

• 14:29Speaker 04

  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 Fonkin emphasized to me. She's a neuroscientist at the University of Texas at Austin.

• 14:48Speaker 07

  I think a really important point is not just 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.

• 15:08Marielle Segarra?

  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?

• 15:17Speaker 04

  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 a run. And that's just one more way that our circadian rhythms and our relationship to light is really intertwined with our overall health.

• 15:46Marielle Segarra?

  Yep. All right. Well, thank you so much for being here.

• 15:50Speaker 04

  Of course. Thank you.

• 15:55Marielle Segarra?

  OK, 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.

• 16:28Marielle Segarra?

  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/lifekit. And if you love Life Kit and you just cannot get enough, subscribe to our newsletter at npr.org/lifekitnewsletter. Also, we love hearing from you, so if you have episode ideas or feedback you want to share, email us at [email protected]. This episode of Life Kit was produced by Sylvie Douglas. Our visuals editor is Beck Harlan, and our digital editor is Malika Gharib. Megan Cain is our supervising editor, and Beth Donovan is our executive producer. Our production team also includes Andy Tagle, Claire Marie Schneider, and Margaret Sorino. Engineering support comes from David Greenberg. I'm Mary El Segarra. Thanks for listening.