Do Daylight Savings Impact Health?

Daylight savings: the extra hour of sleep we get as the clocks fall back every autumn, and the hour we lose as they leap forwards every spring. I grew up in the UAE, a country whose proximity to the equator means it does not operate a daylight savings system, so the concept was something I only came across when I moved to the UK for uni 5 years ago. And, if I'm being honest I never thought much about it until I came across an article on EuerkAlert! detailing the potential impacts (or lack thereof) it could have on our health. The findings of the article I will dive into soon, but first I thought I would provide a brief summary of what daylight savings actually is and where it came from (spoiler: the myth of it originating to help farmers who get up early have more daylight is popular yet untrue).

Image from Unsplashed

So what actually are daylight savings?

Daylight savings refers to the the changing of the clocks biannually (forwards in the spring and back again in the autumn) which has been adopted by over 70 countries across the globe, mostly in Europe, North and South America, and Oceania due to their higher latitudes which gives more pronounced seasonal changes in hours of daylight. The exact dates the clocks change varies between countries, but in the UK they go forwards by 1 hour on the last Sunday in March and drop back an hour on the last Sunday in October. In the UK the time when the clocks are an hour ahead (GMT +1) between March and October is referred to as British Summer Time or BST.

Where did daylight savings come from?

One of the most widespread myth about the clocks changing is that they originally did so in order to give farmers more light in the mornings. This is, however, not strictly correct. Whilst it is true that in Britain, Daylight Savings Time (DST) was introduced in 1916 to give people who work outside (such as farmers) as much sunlight as possible to work in, DST was actually first introduced in Germany in 1916 (just before Britain) as a way of conserving energy during the first world war. More daylight hours in the day meant less time with lights being turned on (and therefore using precious energy). Even before this though, the first modern recorded use of daylight savings was in the town of Port Arthur in Ontario, Canada who turned their clocks forwards for the first time in 1908. The idea is also recorded to have been suggested even earlier than that in the late 1890s by both Kiwi scientist George Vernon Hudson and British builder William Willet but was never followed through. Prior to this, ancient civilisations are also thought to have used similar practices. One example is the Roman water clocks that used different scales for different months to adjust to daily solar time.

Now onto the meaty stuff... how could DST impact our health?

Now this is an ongoing debate. There are a number of proposed ways that DST could impact our bodies, mostly by the impact on our circadian rhythms. The controversy here starts with a paper published in 2025 by Weed and Zeitzer of Stanford University that claimed negative health outcomes, ranging from acute heart attacks and strokes to chronic conditions such as obesity, linked to DST. However, more recently, in 2026, Professor Martín-Olalla and Professor Pérez published a letter analysing the methodology of the paper by Wood and Zeitzer, concluding that the study was not supported by sufficient mathematical evidence.

Before we get into the scientific beef though, lets touch on what impacts DST could plausibly have on our health. Nearly all living organisms have an approximately 24-hour circadian rhythm (presumably all evolved because of the 24 hour day and night cycle we have here on Earth). This rhythm regulates our body's behavioural and physiological timings to match the outside world. In humans, the circadian clock is involved in the regulation of many parts of our biology so the health impacts of disrupting this rhythm have been examined multiple times. Examples include increases in heart attacks and car accidents, alongside decreases in sleep and work engagement. The circadian rhythm in the modern day is impacted by multiple factors not relevant to our ancestors such as moving between time zones and artificial lighting. These also have impacts on our behaviour and health.

The original article (Wood and Zeitzer, 2025) gained a lot of interest globally. It used a database known as PLACES, which is managed by the US Centre for Disease Control, to analyse data on obesity and strokes, concluding that a permanent shift to daylight savings time (vs either a shift to DST for half the year or a year round standard time) in the US would lead to a decrease in the prevalency of both. They did note that this data is highly dependent on both latitude and longitude as well as short term, not looking at chronic impacts on health. To estimate the impacts they built a model of the circadian impact of both permanent standard time and permanent daylight savings time as well as a biannual shift. The model was based on the using the time at which body temperature is at a minimum to estimate the biological clock and compare it with the Earth's rotation.

This paper gained a lot of publicity in 2025 because of its big claims - that the biannual time shift that comes with daylight savings is bad for our health. However, a year after that paper was published, Martin-Olalla and Perez published their response via a letter in PNAS (which is the same scientific journal that the original paper was published in). Martin-Olalla and Perez reported a big error in the methodology of the 2025 study. The health effects were calculated as a magnitude difference (or readjustment) regardless of whether they were positive or negative in change. Professor Perez described this, "it is as if, while driving, we recorded every small adjustment made by moving the steering wheel back and forth to stay in the lane, but then added them all up in the same direction to report a large value instead of allowing them to compensate for each other ... This alone refutes the study's conclusions." The model therefore captures any "noise" in the data and not net/overall change. At the end of their letter, Martin-Olalla and Perez question the prior expectations of the authors of the original study suggesting that they did not have a prior hypothesis to justify their analytical method.

So overall, what are my thoughts on this?

Well, I certainly think this is an interesting topic to look at and there is not doubt you can feel your body and mood change based on the time of day and length of daylight hours. A link between daylight savings and health doesn't sound too far out of the question. That being said, the methodology of the original paper does seem to have been rightly brought into question and I don't think yet that we have any actual evidence of links between DST and health conditions, especially not globally or in the long term.

I also think that if we are worried about "messing up our circadian rhythms" the we as society have bigger fish to fry. The prevalence of artificial lighting, staring at laptop, TV, and phone screens just before bed, and hours of shift working all probably have worse impacts on our bodies natural clocks than the DST practice that seems to have been going on for centuries. Ultimately though, this is not my field of science so there is a lot out there I don't know - just an interesting topic to look into!

Sources:

Press Release - EurekAlert!

Original Paper - Weed and Zeitzer (2025)

Letter Reply - Martin-Olalla and Mira (2026)

Other info from: https://www.timeanddate.com/