Muzhe Yang on How Daylight Saving Time Affects Your Health

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In this episode of Lehigh University’s College of Business ilLUminate podcast, host Stephanie Veto talks with Muzhe Yang about research on daylight saving and circadian misalignment. Yang the MacFarlane Professor of Economics, and his research focuses on identifying factors that contribute to adverse outcomes among vulnerable populations. 

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Below is an edited excerpt from the conversation. Read the complete podcast transcript [PDF].

Veto: You and your co-authors recently did a study on daylight saving and circadian misalignment. It was published in the American Journal of Health Economics. Can you describe what circadian misalignment is and then tell us about the study? 

Yang: The study is related to daylight saving time and circadian misalignment. And this study also includes reasons why I am interested in health economics. The time zone we live in and the daily routines we have adopted can affect our health. In addition to the healthcare we receive from doctors and hospitals, there is also an unintended consequence of a well-intentioned policy, which is standardizing our local time using time zones in order to facilitate our daily activities. 

When we think about the one-hour clock change, we can think of it as moving from one side of a time zone border to the other side. So the question about whether we should adopt daylight saving time permanently is essentially the question about the causal effect of moving a person from one side of the time zone border to the other side. Now, to explain the unintended consequence of this, I will need to explain what circadian misalignment is about. It is basically when our social schedule, our life, in clock time, such as when we wake up, start work, eat dinner does not line up well with our body's natural rhythm, which is determined largely by the sun. 

A simple way to think about this is that our body cares more about the solar time, when the sun rises and goes down. But our society runs on clock time. When the two are out of sync, we will end up sleeping at times that do not match what our body prefers, and that can affect health over time. So ideally, for a perfect match between solar and clock time, we would want to have the sun show up right above our head at noon. That is at 12:00 p.m. in clock time. This perfect match means that different locations on the Earth should use different clock times, which is just impossible to do. So in reality, we divided the Earth's surface into 24 time zones. Within each time zone, the clock time is the same. And between two time zones, the difference is one hour. And this standardization is supposed to facilitate our daily life, but it has health implications. 

By design, unless we live exactly at the center of a time zone, we won't be able to have the sun show up right above our head at 12:00 p.m. Once we live away from the center of the time zone, the clock time set by that time zone will not be perfectly matched to solar time. That means those living near a time zone border will have circadian misalignment. Now, you may think, "Well, it's kind of fair that both sides of the time zone border have this problem." And yes, this is true, but only in theory. In reality, the distortion is much greater on the east side. And this is something really interesting that I learned while doing this project. 

Globally, the time zone borders always shift to the west. Simply speaking, an example of this westward shift is like a city that should have been in the central time zone, like Louisville, Kentucky. But, it is in the Eastern Time Zone. And people living in that city will see a very late sunset. I think this may be driven by the business community who wants to encourage people to do more shopping after work while there is still daylight. 

If you live on the east side of a westward-shifted time zone border, you will see very, very late sunset and also very, very late sunrise. And this means you have a much greater circadian misalignment than those living on the west side of the border. This is true year round when we are in months that we use daylight saving time from March to October, and also in months that we use standard time from November to February. 

We looked at those living close to a time zone border where their solar time is similar, but clock time is different by one hour. And also those living on the east side have a much greater circadian misalignment than those living on the west side. Our study is also very different from many studies that look at the impact of a one-time event, which is the clock-changing event, either the spring forward, we do in March, or fall back, we do every November. This group of studies is mainly focused on the short-term impact of this short-duration event. What we are looking at is chronic circadian misalignment that lasts year-round whenever we use time zones to standardize our local time. We compare mental health outcomes for people living near the borders, east side versus west side, and we find worse outcomes for the east side. And worse outcomes could be due to sleep deprivation, which is an outcome that is more directly and immediately affected by having clock time being out of sync with solar time. 

On a technical note, our study uses the most recent advancement of the causal machine learning techniques. And it helps us estimate the health effect of this chronic misalignment between solar and clock time. Our study looks at a comparison between the two sides of a time zone border. It is equivalent to estimating the health impact of moving a person from one side of the border to the other side. And it is essentially the comparison between choosing permanent daylight saving time, which is moving to the east side, and choosing permanent standard time, which is moving to the west side. 

Veto: Can you discuss the 6% increase in depression that you and your colleagues found through this study? 

Yang: Yes. When we say we find about 6% higher prevalence of depression on the east side of a time zone border, we are talking about a relative increase compared with a baseline rate, which is approximately 20% to 21% in our data, which we obtained from the CDC. And in our study, this baseline rate is for adults aged 18 or above who have depressive disorders diagnosed by healthcare providers. So, they are not self-reported cases. They are diagnosed cases. 

Imagine a community where 10 out of 100 adults have depression. So a 6% increase would move that number to about 10.6 out of 100. For any one person, that sounds small, but across millions of people, that will become meaningful, especially because depression is costly and deeply tied to quality of life. This finding is also kind of striking if we consider the nature of the exposure examined by our study. It is not a sudden shock like a natural disaster. It is not an acute one-hour daylight saving time transition. It is a kind of subtle but persistent feature of our daily life. Our local clock time is slightly misaligned with the sun day after day, year after year and our interpretation is not that everybody becomes depressed because of time zones. It is that the population level risk shifts in a measurable way. 

So, small changes in circadian misalignment day after day, year after year, can push some people, especially those already vulnerable. This is also why the result may initially seem surprising to some people. The exposure is subtle and chronic, but the effect actually is still measurable for mental health.