How to optimize your circadian rhythm

Summary

Life Kit (NPR): “How to Optimize Your Circadian Rhythm”

Air date: December 4, 2025
Host: Marielle Segarra
Guests: Will Stone (NPR Health Correspondent), Dr. Charles Szeisler (Brigham & Women’s/Harvard Medical School), Daniel Windrid (Flinders University, Australia), Dr. Laura Fankin (UT Austin)

---

Main Theme & Purpose

This episode explores ways to optimize your circadian rhythm by understanding the link between light exposure, sleep, and long-term health. Host Marielle Segarra and NPR health correspondent Will Stone dissect groundbreaking research on how light levels—both day and night—affect mortality risk and overall wellness. With insights from leading scientists, the episode offers actionable tips to balance light and dark in our modern environments.

---

Key Discussion Points & Insights

Why Nighttime Light Exposure Matters

• Marielle opens with a relatable struggle: “I have blackout shades and curtains over them, but the street light on my block shines directly into my window... once my eyes adjust at night, it’s not actually that dark in my room.” (00:25)
• Modern society’s pervasive evening light is a significant health issue, with new research showing it can have serious consequences.

The Big Study: Light, Health, and Longevity

• Will Stone summarizes a landmark UK Biobank study tracking ~90,000 people for 8 years:
  • Participants wore wrist devices with light sensors, resulting in 13 million hours of data.
  • The study tracked light exposure and cross-referenced with premature mortality rates. (03:04)
• Daniel Windrid, lead author, highlights:

  “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.” (04:11)

• Research shows a “dose response”: Brighter nights = higher risk; Brighter days = lower risk.

The Magnitude of Night/Day Differences

• Windrid again:

  “People with both bright days and dark nights could be living up to five years longer than people with bright nights and dark days.” (04:45)

• Marielle reacts:

  “Okay, five years. I feel like that freaks me out a little bit.” (04:57)

• Will notes the study demonstrates correlation, not causation, but results remained robust after controlling for income, health, and activity.

Circadian Rhythms: How Light Synchronizes Our Bodies

• Dr. Charles Szeisler reflects on major scientific progress:

  “Thinking back to nearly 40 years ago…light was the most powerful synchronizer of human circadian rhythms...not only is light the most powerful synchronizer...keeping our clock in sync with the 24-hour day is critical for our health and wellness and longevity.” (05:49)

• Our central “clock”—the suprachiasmatic nucleus—responds strongly to light.
• Societal changes have disrupted natural light cycles:

  “In the last one and a half centuries, we’ve completely changed the temporal environment to which we’re exposed...flooding the nighttime with light…and shielding ourselves from light during the daytime.” (07:14 - Szeisler)

Health Effects of Bad Timing

• Misaligned light exposure is associated not just with sleep issues but increased risks for cancer, mental illness, metabolic diseases (diabetes, heart disease, obesity). (07:31)

Practical Advice: How to Optimize Your Rhythm

Day:

• Get outside for at least 45 minutes/day.
  • Outdoor light intensity (up to 50,000 lux) is orders of magnitude higher than most indoor environments (< 300 lux). (11:19)
  • Morning light is especially beneficial for alertness, but any time outside helps.

Night:

• Keep nighttime as dark as possible.
  • The lowest mortality rates were seen among those with “barely any light between about midnight and 6am.” (12:36)
  • Light during the core overnight hours (especially 2:30–3:00am) is most disruptive to your body clock and health.
• If you need to get up at night:
  • Use dim, orange/red nightlights that don't stimulate your circadian system. (13:31)
  • Avoid turning on bright lights or looking at phone/tablet screens (blue light is especially disruptive).

If You Can’t Fully Control Nighttime Light:

• Maximizing daytime light exposure can help “protect” you:
  • “Making sure you get light during the day...can reduce the sensitivity to light exposure at night.” (14:14)
  • Those in dim daytime settings are more affected by nighttime light.

Screens:

• Bedtime screen use undermines melatonin and sleep.
  • Szeisler:

    "A light emitting ebook...emits a lot of blue enriched light which sends a direct signal to the brain saying it’s daytime." (15:11)

---

Notable Quotes & Memorable Moments

• Daniel Windrid (study author):

  "People with both bright days and dark nights could be living up to five years longer than people with bright nights and dark days." (04:45)

• Host Marielle Segarra, on the implications:

  "Okay, five years. I feel like that freaks me out a little bit." (04:57)

• Dr. Charles Szeisler (on scientific progress):

  "Keeping our clock in sync with the 24 hour day is critical for our health and wellness and longevity." (05:49)

• Will Stone (on practical differences):

  "If you’re in a typical indoor environment...it’s easy to overestimate how much [light] you’re getting." (10:38)

• Will Stone (screen time impact):

  "Yeah, I know it can be kind of a bummer. But...you really can’t think about just the day or just the night in isolation. They’re really connected." (15:28)

• Dr. Laura Fankin (on the importance of contrast):

  "It’s not just that exposure to light at night, but also taking into consideration...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.” (15:48)

---

Actionable Tips Recap (16:55)

• Bright Day:
  • Get outdoors for at least 45 minutes, preferably in the morning.
  • Outdoor light is exponentially brighter than indoors—even on cloudy days.
• Dark Night:
  • Use blackout curtains, reduce evening and overnight screen time.
  • Install dim, orange/red night lights if needed (especially for bathroom trips).
  • Avoid looking at your phone/tablet if you wake up at night.

---

Segment Timestamps

• Study Background & Main Findings: 03:00–05:10
• Expert Insights on Circadian Rhythms: 05:49–07:30
• Health Risks from Light Misalignment: 07:31–08:00
• Practical Advice – Daylight: 09:47–11:29
• Practical Advice – Nighttime: 12:17–13:55
• Screen Use/Ambient Light Discussion: 14:14–15:29
• Importance of Day/Night Contrast: 15:48–16:46
• Recap of Life Kit Tips: 16:55–17:35

---

Final Takeaway

Optimizing your circadian rhythm hinges on both increasing exposure to bright outdoor light during the day and minimizing light (especially artificial light) at night. Prioritize time outside, keep your bedroom as dark as possible, limit screen use before bed, and don’t stress if you can’t achieve total darkness—getting adequate daylight also helps buffer some negative impacts. Your sleep—and perhaps your lifespan—might depend on it.

Transcript

[Capital One Announcer]

This message comes from Capital One. Say hello to stress free Subscription management. Easily track block or cancel recurring charges right from the Capital One mobile app. Simple as that. Learn [e-mail] Subscriptions Terms and Conditions apply.

[Marielle 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, 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 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.

[Sponsor Announcer]

This message comes from Progressive Insurance. You chose to hit play on this podcast today. Smart choice. Make another smart choice with Auto Quote Explorer to compare rates from multiple car insurance companies all at once. Try [e-mail], progressive Casualty Insurance Company and affiliates. Not available in all states or situations. Prices vary based on how you buy. This message comes from Capital One with the Venture X card. Earn unlimited double miles, a $300 annual capital one travel credit and access to airport lounges. Capital One what's in your wallet? Terms apply [e-mail] this message comes from Capital One.

[Capital One Announcer]

Say hello to stress free subscription management. Easily track block or cancel recurring charges right from the Capital One mobile app. Simple as that. Learn [e-mail] subscriptions terms and conditions apply.

[Sponsor Announcer]

This message comes from Apple Card. Apple Card members can earn unlimited daily cash back on everyday purchases wherever they shop. This means you could be earning daily cash on just about anything, like a slice of pizza or a latte from the corner coffee shop. Apply for Apple Card in the Wallet app to see your credit limit offer in minutes subject to credit approval. Apple Card issued by Goldman Sachs Bank USA Salt Lake City Branch terms and [e-mail] you know, there are a lot.

[Marielle Segarra]

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 [e-mail] peopleschoice and may the best pod win.

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

[Will Stone]

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. 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.

[Marielle Segarra]

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

[Will Stone]

That's right.

[Marielle Segarra]

Yikes.

[Will Stone]

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 Windrid explain. It was his study. Windrid is a postdoctoral researcher at Flinders University in Australia.

[Daniel Windrid]

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.

[Will Stone]

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

[Daniel Windrid]

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.

[Marielle 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?

[Will Stone]

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. 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 Szeisler. About this study. I wanted to get his take because he's done research in this area for a long time. Szezler's at the Brigham and Women's hospital and Harvard medical School.

[Dr. Charles Szeisler]

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 of 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.

[Marielle Segarra]

It's interesting that what freaks me out is exciting to researchers. But.

What do you take from this?

[Will Stone]

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 super chasmatic 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.

[Dr. Charles Szeisler]

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.

[Will Stone]

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.

[Marielle Segarra]

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

[Capital One Announcer]

This message comes from Memorial Sloan Kettering Cancer Center. With nearly 2,000 clinical trials underway at Memorial Sloan Kettering Cancer center, the next breakthrough in cancer research is just around the corner. Like a recent clinical trial where 100% of participants achieved complete remission from rectal cancer. That outcome is now inspiring new trials that could change the standard of care for other types of cancer. Give the gift of hope this holiday [e-mail] holidaymatch and your gift will be tripled. This message comes from Dell. It's time for Cyber Monday, one of Dell technologies biggest sales of the year. Enjoy huge savings on gift worthy accessories and select PCs like the Dell 16 plus featuring Intel Core Ultra processors. Shop [e-mail] deals this message comes from.

[Sponsor Announcer]

Capital One with the Venture X card. Earn unlimited double miles, a $300 annual Capital One travel credit and access to airport lounges. Capital One what's in your wallet? Terms apply [e-mail] this message comes from MongoDB. You're a developer who wants to innovate. Instead you're stuck fixing bottlenecks and fighting legacy code. MongoDB can help. It's a flexible, unified platform that's built for developers by developers. MongoDB is asset compliant, enterprise ready with the capabilities you need to ship AI apps fast. That's why so many of the Fortune 500 trust MongoDB with their most critical workloads. Ready to think outside rows and columns. Start [e-mail] build.

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

[Will Stone]

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.

[Marielle Segarra]

Because the light outdoors is stronger.

[Will Stone]

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 Windrid explains, there's a massive difference.

[Daniel Windrid]

In the light intensity between an indoor environment, which might be, let's say, 2 or 300 lux, and an outdoor environment, which might be 50,000 lux.

[Will Stone]

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 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.

[Marielle 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.

[Will Stone]

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 Windridge described as barely any light between about midnight and 6am 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 3:00am that was associated with the highest risk of mortality. And Woodred 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.

[Marielle 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.

[Will Stone]

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.

[Marielle Segarra]

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

[Will Stone]

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.

[Marielle Segarra]

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

[Will Stone]

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 and 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, 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.

[Dr. Charles Szeisler]

Bed, a light emitting ebook, 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.

[Marielle Segarra]

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

[Will Stone]

Yes, basically.

[Marielle Segarra]

Dang, you're taking away all my fun stuff.

[Will Stone]

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

[Laura Fankin]

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.

[Marielle 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.

[Will Stone]

That's exactly right. And one final point that Vankan made to me, which I think is important is that by paying attention to our light habits, 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.

[Marielle Segarra]

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

[Will Stone]

Of course. Thank you.

[Marielle Segarra]

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 cloud. 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 night lights 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 [e-mail] LifeKit and if you love Life Kit and you just cannot get enough, subscribe to our [e-mail] LifeKitnewsletter Also, we love hearing from you, so if you have episode ideas or feedback you want to share, email [e-mail] this episode of Life Kit was produced by Sylvie Douglas. Our visuals editor is Beck Harlan and our digital editor is Malika Garib. Megan Kane is our supervising editor and Beth Donovan is our executive producer. Our production team also includes Andy Tagle and Claire Marie Schneider and Margaret Serino. Engineering support comes from David Greenberg. I'm Mariel Segarra. Thanks for listening.

[Sponsor Announcer]

This message comes from Capital One with the Venture X card. Earn unlimited double miles, a $300 annual capital one travel credit and access to airport lounges. Capital One what's in your wallet? Terms apply [e-mail] this message comes from Capital One.

[Capital One Announcer]

Capital One offers checking accounts with no fees or minimums. What's in your wallet? Terms apply. See capitalone.combank for details. Capital One NA Member FDIC this message.

[Sponsor Announcer]

Comes from NPR sponsor Shopify. Start selling with Shopify today. Whether you're a garage entrepreneur or IPO ready, Shopify is the only tool you need to start, run and grow your business without the struggle. Go to shopify.com npr.