12 Surprising Effects of Daylight Saving Time
Every March, clocks "spring forward" across much of the United States, robbing people of one precious hour of sleep. (In 2019, Daylight Saving Time will occur March 10.) While hearing those same people complain about being tired is one not-so-surprising effect of Daylight Saving Time, the possibility of a longer prison sentence for those going before a judge on "sleepy Monday" is less expected. Here are 12 surprising effects of Daylight Saving Time—the good, the bad, and the scientifically ambiguous.
1. Increased Spending
In 2016, JP Morgan Chase decided to look into the economic consequences of Daylight Saving Time (DST) by examining Los Angeles and Phoenix, two cities that are large, relatively close to each other, and have stable weather. Critically, Phoenix doesn't observe DST while Los Angeles does [PDF].
Among their findings, DST was "associated with a 0.9 percent increase in daily card spending per capita in Los Angeles at the beginning of DST." Perhaps more surprising, the end of DST was associated with a per capita daily spending reduction of 3.9 percent.
2. A Higher Risk of Heart Attacks
Many studies have shown that DST is associated with an increase in heart attacks, with one study showing a 24 percent increase in the number of heart attacks on the Monday after DST at a group of Michigan hospitals. According to the University of Michigan, Mondays are bad for heart attacks in general (researchers believes the stress of beginning a new workweek and changes to the sleep-wake cycle are the reason why), but DST makes everything worse. Interestingly, the Tuesday following the end of DST was associated with a 21 percent drop in patients.
3. Missed Appointments
Somewhat relatedly, a 2017 study found that the percentage of missed medical appointments increased significantly following DST. But as with heart attack risk, the missed appointments decreased in the fall—at least temporarily.
4. More Car Accidents … Maybe (at Least for a Few Days)
Another field where studies aren't as consistent as one might expect is traffic accidents. In 2001, an American study found that there was a significant increase in accidents on the Monday after the shift to DST. A 2018 New Zealand study echoed the sentiment, finding that on the first day of DST road accidents increased 16 percent. In contrast, a Swedish study found that DST didn't have any important effect in that country.
Of course, there's more to DST than just those first couple days. After DST has gotten started, there's more light on the road later in the day. Several studies have found this light reduces accidents substantially, so much so that one study concluded that a year-round DST would reduce motor vehicle occupant fatalities by 195 per year.
It's so complicated that a 2010 analysis in Minnesota listed 10 studies that found positive effects of DST on road safety, and six studies that showed negative effects in both the spring and fall changes.
5. Longer Prison Sentences
Researchers frequently use DST to study sleep deprivation in populations, as it's a period of time when we all wake up an hour before we’re used to. One of these studies focused specifically on judicial punishment in U.S. federal courts. The researchers looked at "sleepy Monday" (the Monday after the time change) and compared the sentence lengths to other Mondays. They found that on "sleepy Monday," judges handed out 5 percent longer sentences. But don't think you can get a lighter sentence during the fall switch; the researchers found no effect on sentencing at that time. But the researchers point out that this probably isn't limited to judges—even managers may find themselves in the mood for doling out harsher punishments.
6. More Mining Injuries
According to one study of mining injuries from 1983 to 2006, the Monday directly after the switch to DST was associated with 5.7 percent more workplace injuries and 68 percent more workdays lost because of injuries, indicating that there are more injuries that are more severe after the switch [PDF]. There isn't, however, a corresponding decrease in the fall.
7. Fewer Koala Collisions
One study decided to look at how DST affected human-wildlife interaction, specifically koala-vehicle collisions [PDF]. Because koalas are largely nocturnal, they often cross the road in the evening or at night. By shifting traffic patterns to times when it wasn't dark, the researchers found that DST could "decrease collisions with koalas by 8 percent on weekdays and 11 percent at weekends" (although the difference between weekend and weekdays wasn't significant, the researchers proposed that a slight increase in morning collisions lessened the benefit during the weekday). The researchers hope that further study can be done on human-animal interactions and DST.
Koalas aren't the only ones crossing a road that benefit from DST; pedestrians might be safer as well. One study found "no significant detrimental effect on automobile crashes in the short run" and in the long run was associated with "a 8 to 11 percent fall in crashes involving pedestrians … in the weeks after the spring shift to DST." Meanwhile, another study found that a year-long DST would mean 171 fewer pedestrian fatalities a year.
8. Decreased Satisfaction With Life in General (and Increased Use of the Word Tired)
In both the UK and Germany, studies have shown that life satisfaction deteriorates in the first week after the switch to DST in the spring. One study even quantified the deterioration in Germany with money. For the entire sample, the cost was calculated to be €213 (about $262), but for people in full employment—with relatively inflexible schedules—that increases to €332 ($408). And for the men in the sample, the cost of transition was €396 ($487).
Meanwhile, a Facebook analysis looked at the "feelings" people were sharing on the platform. On the Monday after the start of DST, the use of the word tired increased by 25 percent, with similar increases for "sleepy" and "exhausted" (as well as "wonderful" and "great"). In just the period from 5 a.m. to 12 p.m. on Monday, "feeling tired" usage increased an average of 86 percent, from a 12 percent increase in the non-DST Arizona up to a 231 percent increase in Delaware. By Thursday, "tired" is back to normal.
9. Sleepier Kids (Maybe)
The studies surrounding DST and school children are surprisingly inconclusive. On the one side, a 2009 article in Sleep Medicine looked at 469 Germans from 10 to 20 years old and divided them up into 'larks' (those who go to bed early and wake up early) and 'owls' (those who go to bed late and wake up late). They found that after the DST transition the group was sleepier for three weeks after the transition, with owls showing higher daytime sleepiness, and proposed that tests shouldn't take place in the week following the switch over to DST.
A 2017 article in Economics of Education Review, however, looked at 22,000 Europeans students and found that, at least for low-stakes tests, the effect wasn't statistically significant.
10. More Cyberloafing on the Job
Another study looked at people's Google search trends for the Mondays before the switch to DST, immediately after the switch, and a week after, with a specific focus on sites like Facebook, YouTube, and ESPN (i.e. entertainment sites that people probably aren't Googling for their jobs). They found that on the Monday after the switch, people searched for 3.1 percent more entertainment websites than the Monday before DST, and 6.4 percent more than the subsequent Monday. While the researchers caution they can't be sure this was all "cyberloafing," the fact that there was nothing else special about these Mondays meant it very likely was.
11. Mistimed Insulin Shots
It might seem that in this age of smartphones and connected devices that figure it all out, the twice-yearly ritual of finding all the clocks to change is a thing of the past. But that's not necessarily true. In a 2014 article in the Journal of Diabetes Science and Technology, the authors pointed out an easy clock to miss: insulin pumps. Because most commercial pumps aren't GPS-enabled and lack internal time change mechanisms, they have to be manually set up. The study authors discuss an international college student with an insulin pump that came from a country that didn't observe DST, meaning the clock was an hour off. They say that no significant harm resulted, but it just serves as a reminder to make sure you check all your clocks.
12. Higher Energy Bills
One of the main rallying cries for DST is that it saves energy, but studies have been mixed. In 1975 the Department of Transportation issued a report about whether a short-lived, year-long DST experiment had been worthwhile [PDF]. They declared "modest overall benefits might be realized by a shift from the historic six-month DST system," but cautioned that these benefits were difficult to isolate. Optimistically, though, they said DST might help reduce 1 percent of electricity use.
But as modern researchers have noted, electricity usage has shifted since then. Chief among the changes: Only 46 percent of the new single family households completed in 1975 had air conditioning, compared to 93 percent in 2016 [PDF].
Indiana provided a good place to test this change, because in 2006 they decided to observe DST as an entire state (individual counties had observed DST before). A study ultimately concluded that while DST does save electricity in lighting, this is more than offset by increased demands for heating and cooling, resulting in Indiana households being hit by $9 million per year in higher electricity bills [PDF]. However, the study only looked at residential electricity consumption, not commercial or industrial.
Around the same time, the Department of Energy also looked into DST and found that during a four-week extension, electricity use decreased about half a percentage point per day. Ultimately, Stanton Hadley at the Oak Ridge National Laboratory told Live Science, "I could see the answer being either way."
This story originally ran in 2018.