If you’re at all like me, today will be very painful for you, because it is the first day back to work after the switchover to Daylight Saving Time (DST). Given that the loss of an hour occurs overnight on Saturday, I can usually fake it through Sunday (either that, or sleep in without actually getting more sleep by getting up an hour later by the click than usual), but when Monday rolls around there’s no faking it. As I contemplated a topic to write about today, I searched the archives to see if we’ve ever covered the topic of Daylight Saving Time before on Science-Based Medicine (SBM). I was shocked to see that we have not, as far as I can tell. So I decided that it was time to remedy this situation, because DST is a practice that persists despite little evidence that it achieves its stated goal of decreasing energy use and growing evidence of harm to health and wellbeing. It’s also in the news more than usual this year because recently Florida passed a law that would allow it to observe DST year round—although there is a catch that will prevent Floridians from leaving their clocks set an hour ahead come November, when we all have to “fall back.”

Time zones: A solution necessitated by the railroads

Standardized time is actually a rather recent innovation. Before clocks were invented, people kept time using various instruments, such as sun dials, to observe the sun’s meridian passing at noon. After the invention of reasonably accurate chronometers and clocks in the 18th century, towns and cities set their clocks based on sunsets and sunrises. Not surprisingly, this produced wildly differing times between (and sometimes even within) cities and towns. However, before the 19th century, this was not a particularly pressing problem because, even though sunsets and sunrises occurred at different times in different parts of the country, travel times were so slow that it was barely noticeable, and most people didn’t routinely travel far enough that it even made a difference.

What made the big difference in the 19th century was, of course, the train. In the late 1800s, American railroads maintained many different time zones, and each train set its own clock. Calculating arrival times thus became a serious problem for people traveling by train. Add to that how every city in the US used a different time standard, leading to more than 300 local sun times. To try to compensate, railroad managers established 100 railroad time zones, but this was an unwieldy solution to the problem that didn’t address all the local time zones. Operators of the new railroad lines needed a new time plan that would offer a uniform train schedule for departures and arrivals. In 1883, four time zones were introduced for the continental US.

In 1884, at the behest of the President of the US, 41 delegates from 25 nations met in Washington, DC for the International Meridian Conference. At the Conference the following important principles were established:

  1. It was desirable to adopt a single world meridian to replace the numerous ones already in existence.
  2. The Meridian passing through the principal Transit Instrument at the Observatory at Greenwich was to be the ‘initial meridian’.
  3. That all longitude would be calculated both east and west from this meridian up to 180°.
  4. All countries would adopt a universal day.
  5. The universal day would be a Mean Solar Day, beginning at the Mean Midnight at Greenwich and counted on a 24 hour clock.
  6. That nautical and astronomical days everywhere would begin at mean midnight.
  7. All technical studies to regulate and extend the application of the decimal system to the division of time and space would be supported.

The plan adopted was based on a plan devised in the late 1870s by a Canadian railway planner and engineer named Sir Sandford Fleming, who had outlined a plan for worldwide standard time in the late 1870s. The Meridian Conference established the time zones that are still in use today. It wasn’t until 1918, though, that Woodrow Wilson signed the Calder Act, which mandated standard time. But what about Daylight Saving Time?

Daylight Saving Time

Daylight Saving Time

Enter Daylight Saving Time

It’s frequently noted that the concept of Daylight Saving Time appears to have originated with Ben Franklin, who in 1784 published an essay, “An Economical Project for Diminishing the Cost of Light”, written to the editor of The Journal of Paris. In his essay, Franklin observed that Parisians could save on candles by getting out of bed earlier in the morning, making use of the natural morning light instead. Franklin’s suggestion appears not to have been serious, but rather a joke, and he didn’t actually suggest turning clocks ahead. In reality, DST was “invented” by New Zealand scientist George Vernon Hudson and British builder William Willett. In 1895, Hudson presented a paper to the Wellington Philosophical Society, in which he proposed a 2-hour shift forward in October and a 2-hour shift back in March. Although there was some interest in the idea, it was never adopted as policy.

Daylight Saving Time

World War I era poster promoting Daylight Saving Time.

It was during World War I that DST became a reality for much of the world. Germany and Austria adopted it first, the rationale being to save on fuel use for artificial lighting at home in order to make more fuel available to the war effort. Britain and many other countries in Europe soon followed suit, although after the war ended they reverted back to standard time and DST did not make a reappearance until World War II. But what about the US?

Daylight Saving Time

World War I poster promoting Daylight Saving Time.

Remember the Calder Act? That same act that mandated the use of standard time in the US also mandated DST. As Real Clear Science notes, the first switchover to DST did not go well:

It didn’t go smoothly. In 1918, Easter Sunday fell on March 31, which led to a lot of latecomers to church services. Enraged rural and evangelical opponents thereafter blamed daylight saving for subverting sun time, or “God’s time.” Newspapers were deluged by letter writers complaining that daylight saving upset astronomical data and made almanacs useless, prevented Americans from enjoying the freshest early morning air, and even browned out lawns unaccustomed to so much daylight.

Within a year, the DST provision of the Calder Act was repealed, but that didn’t mean DST didn’t continue:

In 1920, New York and dozens of other cities adopted their own metropolitan daylight saving policies. The Chamber of Commerce spurred along this movement on behalf of department store owners, who had noticed that later sunset times encouraged people to stop and shop on their way home from work.

By 1965, 18 states observed daylight saving six months a year; some cities and towns in 18 other states observed daylight saving for four, five or six months a year; and 12 states stuck to standard time.

This wasn’t exactly ideal. A 35-mile bus trip from Steubenville, Ohio, to Moundsville, West Virginia, passed through seven distinct local time zones. The U.S. Naval Observatory dubbed the world’s greatest superpower “the world’s worst timekeeper.”

Amazingly, it wasn’t until 1966, when Congress passed the Uniform Time Act, that the system of six months of standard time, six months of Daylight Saving Time became the law of the land. During the OPEC oil embargo of 1973, the US conducted an experiment mandating the use of DST year round to save energy beginning in 1974. (I remember this well, because I remember that winter walking to school in the dark a lot.) The experiment only lasted one year, and back in 1975 everything went back to the system before. Then, in 1986, Congress amended the Uniform Time Act to have DST extend from the first Sunday in April to the last Sunday in October. Finally, in 2005, Congress passed the Energy Policy Act of 2005, which extended DST from the second Sunday in March to the first Sunday in November beginning in 2007. That is where DST in the US stands today.

This brings me back to the recently passed Florida law, HB 1013, or the “Sunshine Protection Act,” which is the logical extension of this seemingly relentless increase in DST. Basically, nothing will happen as a result of the law if it’s signed, because, although states have the power to opt out of DST (as Hawaii and most of Arizona do), they do not have the power to opt out of standard time year round. It would require a change in federal law for Florida’s law to take effect.

The health effects of Daylight Saving Time

I’m not going to discuss (much) the science of whether DST actually saves energy. Suffice to say that the actual energy savings are minimal, if any. The most frequently cited evidence for the energy savings due to DST is a 2008 report to Congress by the Department of Energy which showed that total electricity savings from the extended daylight saving period that began in 2007 amounting to 0.03 percent of electricity consumption over the year, which is a minuscule savings. In contrast, a California Energy Commission study after the extension of DST in 2007 found that the estimated weather- and lighting-corrected energy savings from the extended DST was 0.18%, but the confidence interval ranged from a ranging from a 1.5% savings to a 1.4% increase. Indeed, one reason that Arizona opts out of DST is because of the concern that any savings in energy used for lighting would be more than offset by more use of air conditioning. One study looking at energy use in Indiana after it adopted DST statewide in 2006 showed that DST was actually associated with at least a 1% increase in residential electricity use. Other studies corroborate this effect. At best, DST is a wash; at worst, it increases energy consumption.

More importantly, that lack of benefit in terms of decreasing energy consumption comes at a considerable cost, and that’s because the lost hour of sleep comes not from wakeful time, but rather primarily from resting hours. This results in many problems associated with sleep deprivation and disrupted Circadian rhythms.

Here’s an example. Multiple studies show that there are more motor vehicle collisions the Monday after the time change to DST. For instance, in 1996, Canadian researchers noted that the number of traffic accidents rose significantly:

The loss of one hour’s sleep associated with the spring shift to daylight savings time increased the risk of accidents. The Monday immediately after the shift showed a relative risk of 1.086 (95 percent confidence interval, 1.029 to 1.145; ξ2 = 9.01, 1 df; P<0.01). As compared with the accident rate a week later, the relative risk for the Monday immediately after the shift was 1.070 (95 percent confidence interval, 1.015 to 1.129; ξ2 = 6.19, 1 df; P<0.05). Conversely, there was a reduction in the risk of traffic accidents after the fall shift from daylight savings time when an hour of sleep was gained. In the fall, the relative risk on the Monday of the change was 0.937 (95 percent confidence interval, 0.897 to 0.980; ξ2 = 8.07, 1 df; P<0.01) when compared with the preceding Monday and 0.896 (95 percent confidence interval, 0.858 to 0.937; ξ2 = 23.69; P<0.001) when compared with the Monday one week later. Thus, the spring shift to daylight savings time, and the concomitant loss of one hour of sleep, resulted in an average increase in traffic accidents of approximately 8 percent, whereas the fall shift resulted in a decrease in accidents of approximately the same magnitude immediately after the time shift.

The observation that the number of crashes declines in the fall transition back to standard time is very compelling.

A more recent study in 2016 published in the American Economic Journal estimated that the impact of DST from 2002-2011 resulted in over 30 deaths at a social cost of $275 million annually and that this result is most consistent with sleep deprivation as a cause. Yet another study (this one from 2004) estimates that never changing the clock all year round would reduce pedestrian fatalities by 171 per year, or 13% of all pedestrian fatalities in the 5:00–10.00 a.m. and in the 4:00–9:00 p.m. time periods, and motor vehicle occupant fatalities by 195 per year, or 3%. A 2014 study from the University of Colorado Boulder found a spike in fatal automobile crashes during the six days following the shift to DST. The list goes on.

Risk on the road and sidewalk is not the only health risk from DST, though. For instance, there is evidence of a small but real increase in heart attacks. Swedish data suggest the risk of myocardial infarction to be significantly elevated for the first three weekdays after the transition to DST. A 2013 study in The American Journal of Cardiology found a similar result, with an increase in heart attack rates the Sunday after the shift to DST. Similarly, a recent University of Alabama study found that heart attacks increase by 10-24% on Monday following the shift to DST. A recent review of the literature published in European Review for Medical and Pharmacological Sciences concludes:

Transitions into and out of Daylight Saving Time (DST) may disrupt circadian rhythms and lead to sleep disturbance and deprivation. A first report observed an association between DST and acute myocardial infarction (AMI), especially after the spring shift and in women. We tried to identify and evaluate the possible association between DST and AMI, using the MEDLINE, EMBASE and Google Scholar electronic database (years 2009-2016), with regards to the searching terms ‘daylight saving time’, ‘daylight saving time’ plus ‘gender’, and ‘daylight saving time’ plus ‘acute myocardial infarction’. In total, 72, 10, and 6 studies were found, respectively. Overall, 6 studies, including a total of 87,994 cases, resulted to satisfy the searching request, and were included in the present analysis. All studies confirmed a higher occurrence of AMI in the spring shift, ranging from 4 to 29%, whereas only 1 study showed a higher occurrence of AMI in the autumn shift. By the way, in 5 studies providing separate analysis, the results by sex were not univocal. In fact, as for the spring shift, 2 studies did not show differences between men and women, 2 reported a higher frequency in men, and 1 in women. Regarding the autumn shift, only 1 study reported a higher occurrence of AMI in women. These results support the presence of an association between DST and a modest increase of AMI occurrence, especially for the spring shift, and with no definite gender specific differences.

My reading is that this is probably a real phenomenon. Not only is it plausible that the shift to DST could lead to an increased risk of a heart attack in the days following, but multiple epidemiological studies are pointing to the same conclusion: A modestly increased risk of heart attack in the days following the shift to DST.

There are other health effects as well. There are far fewer studies of the effect of the DST transition on workplace injuries, but there are reports indicating increased work-related injuries and accidents on the Monday following the DST transition. For instance, a 2009 analysis from Michigan State University of the National Institute for Occupational Safety and Health database of mining injuries for the years 1983–2006 found that on the Monday following the time change miners suffered more workplace injuries and injuries of greater severity, with a 5.7% increase in injuries and a 67.6% increase in days of work lost because of injuries. There was no increase or decrease in injuries on Mondays after the switch back to standard time. This same study found that the miners arrived at work with an average of 40 minutes less sleep the night before.

But that’s still not all.

There are a number of other adverse health consequences that have been linked to the DST transition. Consistent with an increased risk for myocardial infarction, a recent study found an 8% increase in the risk of stroke on the Mondays and Tuesdays following the change to DST in the spring. For another example, a 2008 study found higher male suicide rates the week after the switchover to DST, although it also found higher suicide rates after the fall shift back to standard time, suggesting that it’s sleep change, not sleep deprivation, that could be responsible. A more odd effect of the spring transition is an effect known as cyberloafing, which was identified in a 2012 study. Cyberloafing is just what you think it is: Loafing around on the Internet, social media, and the like, such as checking Facebook and Twitter, answering personal e-mail, or shopping online. The study couldn’t show causation, but the authors speculated that lack of sleep led to lack of focus and motivation.

Are the benefits worth the risks? Probably not

In fairness, it should be noted that not all health effects of the springtime transition to DST are bad ones. Perhaps the best example of a beneficial effect of the spring DST transition is that it is associated with more physical activity in children. In a 2014 study published in the International Journal of Behavioral Nutrition and Physical Activity, investigators examined data from 23,188 children aged 5-16 years from 15 studies in nine countries brought together in the International Children’s Accelerometry Database. Of these 439 were of particular interest because they contributed data from both immediately before and after the clocks were changed. Activity was recorded using Actigraph accelerometers and average physical activity level represented as accelerometer counts per minute, which was the primary outcome. Results were adjusted for confounders such as weather characteristics including daily precipitation, humidity, wind speed and temperature. The authors concluded that a later hour of sunset (i.e. extended evening daylight) was independently associated with increased daily activity across the full range of time of sunset. They also observed that it was in the late afternoon and evening that the duration of evening daylight was most strongly associated with hourly physical activity levels. The authors concluded that the increased activity, although modest, was not trivial.

Another example is that DST is associated with lower robbery rates because fewer potential victims are out after dark, with the authors of one study estimating that the 2007 DST extension resulted in $59 million in annual social cost savings from avoided robberies.

Of course, a number of industries benefit from a later sunset, such as the New York department store owners who in 1920 lobbied for DST in New York City. Tourism is a huge industry here in my home state, and there’s no doubt that resorts benefit. Indeed, the tourism industry is the reason why bills to end DST in Michigan never go anywhere. (Heck, bills to move the start dates of various public schools to before Labor Day, as they are in a number of states that I’ve lived in during my lifetime, rarely go anywhere because of power of the tourism industry in this state.) Certainly, the golf industry benefits by as much as $400 million a year.

Besides benefiting various tourism and recreation industries and maybe a small benefit in terms of increased physical activity in children (there’s only been one study that I’m aware of finding that), who benefits from DST? The answer is, as far as I can tell, no one, and even these benefits come at too high a cost in health and energy use. As we are learning, sleep is important, and Americans, being chronically sleep deprived year round, are especially vulnerable to even minor disruptions in sleep, such that even a disruption as seemingly minor as the spring transition to DST and its associated “loss” of an hour can have ripple effects on health that we have been beginning to appreciate over the last decade or so. That’s why I’m with Jon Oliver when he asked: “How is DST still a thing?”

Not only is the transition to DST associated with a growing number of adverse health effects, but DST doesn’t even do what it was advertised to do 100+ years ago: Save energy. We at SBM advocate for science-based medicine and science-based health policy—actually, science-based policy in general. DST fails in every way to be science-based. It should have been scrapped decades ago, in favor of either year round standard time or DST, no clock changes ever necessary.

Now excuse me while I cyberloaf and take a cat nap at my desk. Fortunately for my patients, I have no cases today or tomorrow.


Posted by David Gorski

Dr. Gorski's full information can be found here, along with information for patients. David H. Gorski, MD, PhD, FACS is a surgical oncologist at the Barbara Ann Karmanos Cancer Institute specializing in breast cancer surgery, where he also serves as the American College of Surgeons Committee on Cancer Liaison Physician as well as an Associate Professor of Surgery and member of the faculty of the Graduate Program in Cancer Biology at Wayne State University. If you are a potential patient and found this page through a Google search, please check out Dr. Gorski's biographical information, disclaimers regarding his writings, and notice to patients here.