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It happens every year like clockwork.

COLBERT: We’re all feeling a little off today and we know why.

Well, actually, it is clockwork. The devil’s clockwork, some might say.

COLBERT: It’s Daylight Saving Time. Yeah, it’s my least favorite holiday. For one thing, the parade is horrible

That’s right — I’m talking about Daylight Saving Time. And let me say, right here at the start, that it’s saving, not savings. There’s no “s.” I know — as if we needed more confusion around this event.

Anyway, one day every spring, across most of North America and Europe, and some other scattered places around the world, everyone switches their clocks ahead by one hour, turning one Sunday a year into a 23-hour day. And suddenly, overnight, it’s still light out at seven p.m. Of course, it’s now darker at seven a.m. but eventually that morning darkness goes away and we’re left with our days being lighter, later.

For years, some people have argued that Daylight Saving Time is pointless. And that’s led to efforts across the country to pass legislation that either ends Daylight Saving Time altogether or makes it permanent – in other words, to stop the switch once and for all.

Personally I love the long daylight hours of summer. But what if those longer hours come at a cost? And I don’t mean a financial one — I mean a cost to our health.

From the Freakonomics Radio Network, this is Freakonomics, M.D.

I’m Bapu Jena. I’m an economist and I’m also a medical doctor. Each episode, I dissect an interesting question at the sweet spot between health and economics.

So, how does changing the clocks every spring affect our health? It’s time to look at the data in the cold light of day.

KOLLA: I think, like with most good ideas, this came from my wife.

A few Daylight Saving Time’s ago, Bhanu Kolla’s wife woke up feeling groggy.

KOLLA: She had woken up at the usual time for her, but clocks had changed. She was thrown off and she felt that, “This can’t be right because I’m not at 100 percent.”

Bhanu is a psychiatrist at the Mayo Clinic, and he specializes in addiction and sleep. His wife is also a physician. And her wobbly feeling that day got him thinking: If other doctors had the same experience — you know, felt tired because of Daylight Saving Time, did that mess with their work — did it impact how they cared for patients? Was Daylight Saving Time causing harm? We’ll get to Bhanu’s findings in a minute. But first, we need a little context. Where did this odd tradition come from?

You may have heard about farmers wanting extra daylight, but that’s not how this all started. The first time change actually started in New Zealand in 1916, thanks to an amateur entomologist named George Hudson. See, George Hudson loved insects. Over the course of his life, he amassed the largest collection of bugs in New Zealand. But he had a day job that required him to keep his passion to his leisure hours, which meant every minute of daylight was precious. Out of desperation, though, an idea was born. He first proposed a two-hour clock shift to the Wellington Philosophical Society in 1895. A later sunset meant more daylight to spot bugs after work. It took a while, but eventually others came on board and New Zealand passed a clock-change law about 20 years later.

By then it was World War I. In the hope of saving energy, Germany instituted the spring-forward time change on May 1, 1916. Two years later, the U.S. did the same.

MALOW: Everybody basically adapted this Daylight Saving Time during World War I and World War II. It was called “war time,” and it wasn’t super popular because people didn’t want to wake up in the dark.

That’s neurologist and pediatrician Beth Malow. She directs the sleep division at Vanderbilt University Medical Center. She says that while most of the country ended Daylight Saving Time after the war, New York City held onto it, and that eventually prompted other cities to do the same. Rural areas stayed off that bandwagon, though, leading to serious chaos. Missed trains! Missed business deals! In 1965, Minnesota’s twin cities, St. Paul and Minneapolis, which are right next to each other, actually changed their clocks on different dates. Finally, though, in 1967, the U.S. passed the Uniform Time Act. Arizona and Hawaii opted out, but all the other states entered this great national experiment of changing the clocks twice a year. That isn’t the end of the story though. It’s actually just the beginning.

MALOW: And then in the early ’70s because of the energy crisis, President Nixon decided it would be good to bring back the permanent Daylight Time.

The thinking behind Nixon’s decision to keep the clocks moved ahead was that it would reduce the amount of electricity we consume. Most of us sleep through several hours of morning daylight and we stay awake for several hours after it gets dark outside. But if we sync up our waking hours with daylight, when we don’t need to have as many lights on, that could save a lot of electricity. That was the theory. And the change to permanent Daylight Time did save energy. But there were downsides.

MALOW: People really hated it because in the dead of winter, it was really dark, and kids were having to wait for buses in the dark, and there were even some fatalities, where kids got run over. So, people did not like having permanent Daylight Time in the winter.

The country resumed its alternating schedule in 1976, although years later Daylight Saving Time got extended by about a month at the urging of companies that sell sports equipment and barbecue grills. Later sunset, more time to eat hot dogs!

In the mid 1990s, researchers studying sleep deprivation began wondering about the impact of the lost hour of sleep each spring. It might be a nuisance, sure, but could it also be dangerous?

One of the first studies to bring this issue to, um…light…looked at traffic accidents. If pushing the clocks ahead left us sleep deprived, would that lead to more car crashes? In 2001, a group of researchers from Stanford analyzed 21 years of data and found that fatal car accidents increased on the Monday after we set the clocks forward.

What was also interesting was that the researchers spotted more deadly crashes on the Sunday when we gain an hour, in the fall. For the autumn change, they theorized that people changed their behavior — they stayed out later, they drank more — in anticipation of the bonus hour of sleep they were about to get.

Driving hazards weren’t the only problem that people started investigating. In 2008, two physicians from Sweden found that heart attacks spiked in the three days after the spring change. There were about 10 percent more heart attacks on the Tuesday following “spring forward” compared with the two weeks earlier or two weeks later.

The research that’s followed has been contradictory, though. For example, a 2015 German study indicated no change in the number of heart attacks, based on more than 25,000 cases. And a 2017 study from England and Ireland also suggested no effect on road safety.

Finding an effect like this is hard. It’s because you’re looking for what is really a pretty faint signal and a set of outcomes that are affected by all sorts of variables. Lots of things influence heart attacks and reliably teasing out the effect of one lost hour of sleep is going to be hard. My takeaway is that if there is a health effect of Daylight Saving Time, it isn’t huge.

But what if Daylight Saving Time doesn’t affect our health through our own fatigue, but the fatigue of those who care for us, like our doctors and nurses? A few years ago, I looked at whether turning the clocks ahead led to a spike in surgical deaths due to surgeons being more tired when they were operating. I didn’t see any change and I ended up not trying to publish the finding.

What I found, or didn’t find really, may not be the full story, though. That’s why Bhanu Kolla’s study caught my eye.

KOLLA: Everybody needs a certain amount of sleep. And when there is some amount of sleep deprivation, you are more likely to function at less than your optimal. So, you’re going to take a little more time to react to things and you’re more apt to making errors.

As a concern, that made sense to me. And it’s a smart way to spot the trouble caused by the clock change. Instead of looking for a rare problem, like heart attacks, among the entire population of a country, focus on a common problem — mistakes — among a small population — healthcare providers. This approach is a much better way to find a statistical signal.

And the study design was pretty straightforward. Health care organizations keep track of errors through self-reporting systems where doctors, nurses and anyone else who comes in contact with patients can record mistakes. Bhanu and his colleagues looked at safety-related incidents reported over eight years at a large healthcare organization with locations in multiple states.

KOLLA: We looked at all healthcare errors that happened the week post-the spring-forward transition, and also all the errors the week prior to that, comparing these two weeks to see if there is a change. And we also looked at the week prior to and after the fall back, where the clocks change again, to see if there’s a change.

Bhanu needed to be sure, though, that the errors that they were analyzing were actually human errors — something that a sleepy doctor might do.

KOLLA: A human error would be the doctor prescribing the wrong drug, or maybe the wrong dosage, or maybe administering the drug to the wrong patient. Administering the drug to somebody who has a known allergy to the drug. And it could also mean a wrong site surgery — things like that. So, all errors where there is a human involved it’s not a system error.

An error due to a computer malfunction would be an example of a system error, as opposed to a human error. Those were eliminated from the data. And because the system is voluntary, the only errors that were included were those that the providers considered important enough to record.

So, okay, what did Bhanu and his colleagues find?

KOLLA: The only statistically significant change was in the number of human errors which happened following the spring time change. And in that timeframe, the number of human errors went up by about 18.5 percent as compared to the previous week.

Now, these errors — they didn’t necessarily mean that patients were being harmed. We’re not talking about this many patients suffering from a sleep-deprived doctor.

KOLLA: There is this increased risk, but this doesn’t necessarily translate to adverse outcomes. What this shows us? Yes, we are more likely to make errors. So, it is a little more of a dangerous period, in general — in the hospital. Maybe this is a time for us to be a little more vigilant, have more fail-safes, other policy measures. So, consider things like maybe delaying people’s shifts, start times — having a little more in terms of guard rails to reduce the amount of errors.

So, the jury is still out on how Daylight Saving Time may affect our health, either directly, or through fatigue of the providers who care for us. And we still don’t know why changing clocks wreaks such havoc on our bodies. What’s happening in our brains that makes this relatively tiny change feel so powerful? That’s after the break.

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So, the evidence that Daylight Saving Time messes with our health has been mixed. Traffic accidents might be more common. Heart attacks and strokes might be more common too. And Bhanu Kolla’s study made an interesting case that turning the clocks ahead an hour each spring does do something to our brains. But what? Let’s get back to Beth Malow, the sleep specialist from Vanderbilt.

MALOW: Melatonin is produced in our brains. There’s a structure of the brain called the pineal gland, and it is suppressed by light. So, during the day, when it’s light outside and our brains are taking in light, it actually suppresses our production of natural melatonin. When it gets to be dark, let’s say eight o’clock, nine o’clock at night, earlier in the winter, we turn off the suppression, and we turn on the melatonin production. So, darkness promotes the release of melatonin in our brains, which then allows us to fall asleep. It turns the switch on that allows us to fall asleep. So, if we have some problem where we’re not getting that dark or it’s delayed because of, let’s say, Daylight Saving Time, then we’re not going to be able to go to sleep as early as we might normally need to.

The disruption doesn’t stop at melatonin mayhem.

MALOW: When we’re off an hour we’re off from our outside environment. That has been shown to trigger a whole host of genes.

It’s true: some research in humans has shown that sleep loss can put us so off kilter that it actually changes our biology, right down to the expression of genes within our cells. These changes may be linked in some way to certain neurological disorders or even to insulin resistance a key feature of diabetes. Now, this isn’t saying that Daylight Saving Time causes diabetes. I’m definitely not saying that. What it means, though, is that changing the clocks puts some wear and tear on the body, and sometimes that damage might contribute over time to serious illnesses.

But how does this one-hour change cause damage, even if it’s minor? Well, it’s all about what started this practice in the first place: sunlight.

MALOW: Patterns and habits are so critical. So, something that throws you off for an hour is not necessarily in all of us, but in people who are susceptible — going to interfere with their sleep cycle, sometimes for weeks. And it’s funny because a lot of us think, “It’s kind of like going from Chicago to Boston — you know, flying from the Central time zone to the Eastern time zone. I’ll — I’ll acclimate, and I’ll get used to it. It’s only an hour.” But it’s different because you haven’t changed the light. In other words, we’ve changed our clocks, but we haven’t changed our natural environment. So, when we move from Chicago to New York, we’re also changing the light. Here, we’re just changing the clock without changing what’s going on in our environment. So, we’re really off. I mean, we’re off for a week. We’re off for a month. We’re actually off for eight months.

In other words, the impact of the time change isn’t necessarily about getting more or less sleep. That may matter. But it’s also about getting more or less light. Change the clock and you change your relationship with light. And that change, even if it’s temporary, may affect people differently.

MALOW: And some people are going to be affected more than others because of their genetic makeup or because of their age or because of their underlying conditions or even just because of their jobs and what they can and cannot do at seven in the morning.

It may help to think about light as an environmental exposure, like air pollution. That’s how Trang VoPham approaches it. Trang is an epidemiologist at Fred Hutchinson Cancer Research Center, and like Beth, she sees light at the heart of the clock change disruption.

VOPHAM: So, why would where you live in a time zone matter for health? Well, our bodies rely on circadian rhythms to play critical roles in many biological processes, from when we go to sleep and wake up to when we get hungry and eat. And each of us — we have a master pacemaker located in our hypothalamus, which controls the circadian rhythms in our tissues, and this pacemaker primarily relies on exposure to light to align our circadian rhythms with our environment.

Mess with that rhythm by, say, working the night shift, and you can cause a “circadian misalignment.” When you work at night and sleep during the day, that internal system, that pacemaker, isn’t getting the food it needs to stay on track. And the consequences, some believe, could be real: shift work, for example, has been linked to an increased risk of cancer, heart disease, gastrointestinal disorders, and other serious health issues. Of course, how much of this is because of the misaligned circadian rhythms versus the health behaviors and underlying risk factors among people who do shift work isn’t fully known.

Back to Trang. A few years ago, she was part of a research team that looked at whether fatal car accidents increased right after the spring time change. Her focus was on time zones whether people at the western edge of a time zone were more susceptible to this problem then people at the eastern edge of a time zone. Why would this geographic location matter? Well, because the western edge of a time zone already has a later sunrise time. So, when you turn the clocks in a way that makes the mornings darker, it stays darker for longer at the western edge compared to the eastern edge. And the data confirmed this difference.

VOPHAM: We found that the spring Daylight Saving Time transition was associated with an increased risk for fatal traffic accidents in the U.S. And the association between the spring Daylight Saving Time transition and fatal traffic accident risk was more pronounced farther west in a time zone.

Now, we can’t be sure whether the effect here was because people in the westernmost edge of a time zone were driving in the dark later in the morning and driving in the dark is just risky or whether circadian misalignment was the culprit. This field of research is really new. But it’s an idea. And it’s hard to study these things in humans. Trang has actually tried to unpack some of these issues about how light and circadian misalignment could impact health in mice.

VOPHAM: So, there is an association between circadian misalignment and liver cancer. There’s been studies showing that chronic circadian disruption has been associated with increased bile acid levels in mice. So, that is one pathway through which circadian misalignment may impact liver cancer risk, because increased bile acid levels have been shown to increase liver cancer risk.

The effects of all this don’t require drastic measures. No one should move from one side of their time zone to the other as a way to prevent cancer. What interested me about the studies we talked about today is that they help clarify some of the tradeoffs involved with Daylight Saving Time. I personally enjoy the bright summer evenings and I think that some degree of health tradeoffs might be worth it. As an economist, the right way to answer this question would be to weigh the health risks associated with Daylight Saving Time against the utility people get from the longer lit nights. But people will differ on this. Bhanu and Beth are a little more direct about their feelings.

KOLLA: Yes, I’m firmly in the camp that it’s ridiculous for us to keep changing the clock two times a year for no good reason.

MALOW: The health aspects speak out against not going back and forth. And then I think there is a strong case that could be made, if you’re going to land on something, to land on permanent, Standard Time on the basis of the health aspects.

Remember, Standard Time is what we have from fall until spring so, Beth is suggesting doing away with the change that gives us such late sunsets in summer. Most U.S. states are considering a permanent switch to one of the time changes. But there’s a pretty even split between states that are leaning toward permanent Daylight Saving Time what we have in spring and summer and Permanent Standard Time what we have in fall and winter.

This is a rare issue where both sides of the political aisle are actually united. In several states, democrats and republicans have vowed to work together to stay permanently on either Daylight Saving Time or Standard Time.

Of course, it’s not that simple. Although federal law allows states to opt out of Daylight Saving Time, they are not allowed to opt into it permanently. There is a bipartisan bill in Congress called the Sunshine Protection Act that would make Daylight Saving Time permanent, with some exemptions.

The bill has been under review since February of 2021, so we’ll just have to wait and see whether lawmakers decide to spring forward with a vote.

Okay that was my last pun, not ever, just for this episode of Freakonomics, M.D. I hope you enjoyed our discussion today. You can find links to all the studies we mentioned at

As always, I want to thank you for listening. It’d be great if you could give us a review on Apple Podcasts or wherever you’re listening. It helps new people discover the show.

And like I said earlier, if you have any thoughts on the show, follow Freakonomics, M.D. on Twitter at DrBapuPod — that’s D-R B-A-P-U P-O-D — and share your thoughts! You can still shoot me an email at

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Freakonomics, M.D. is part of the Freakonomics Radio Network, which also includes Freakonomics Radio, No Stupid Questions, and People I (Mostly) Admire. All our shows are produced by Stitcher and Renbud Radio. This episode was produced by Jessica Wapner and mixed by Eleanor Osborne. Our senior producer is Julie Kanfer. Our staff also includes Alison Craiglow, Greg Rippin, Gabriel Roth, Rebecca Lee Douglas, Morgan Levey, Zack Lapinski, Ryan Kelley, Mary Diduch, Jasmin Klinger, Emma Tyrrell, Lyric Bowditch, Jacob Clemente, Alina Kulman, and Stephen Dubner. Our music was composed by Luis Guerra. To find a transcript, links to research, and a newsletter sign-up, go to If you like this show, or any other show in the Freakonomics Radio Network, please recommend it to your family and friends. That’s the best way to support the podcasts you love. As always, thanks for listening.

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This was a lighthearted episode. I can’t stop with the puns! I’m addicted!

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