Why Do We Change Clocks? The Real Story Behind Daylight Saving Time

Twice a year, hundreds of millions of people in dozens of countries do something that would seem absurd if stated plainly: they collectively agree to pretend it’s a different time than it actually is. Clocks spring forward in spring and fall back in autumn, shifting the nominal hour of sunrise and sunset. The practice disrupts sleep schedules, confuses scheduling software, and — according to a growing body of research — may cause measurable harm to public health. So why do we still do it?

The Beginning: Coal, War, and Germany

The first nation to formally implement Daylight Saving Time was not the United States, not the United Kingdom, and not the country its most famous advocate came from. It was Germany, on April 30, 1916, roughly twenty months into World War I.

The German Empire and its Austro-Hungarian ally were grappling with severe coal shortages. Lighting for homes, factories, and public spaces was coal-powered, and the war’s demands had strained supply dramatically. Someone in the German war bureaucracy recognized that moving clocks forward by one hour would effectively shift an hour of daylight from the morning — when people were typically asleep — to the evening, when they were awake and using lights. The theoretical result: reduced evening lighting consumption.

Britain adopted the same measure just weeks later, in May 1916. The United States followed when it entered the war, passing the Standard Time Act of 1918, which established time zones and mandated Daylight Saving Time nationally. The response from the American public was not enthusiastic. Congress repealed the measure the following year, 1919, overriding President Wilson’s veto.

The Claim That Started the Whole Thing Wasn’t Quite Right

The most widely cited origin story traces back to Benjamin Franklin, who wrote a satirical letter in 1784 suggesting that Parisian economy could be improved if residents woke earlier and used morning sunlight instead of candles. Franklin was joking — the piece was a mock proposal — and he had no concept of government-mandated clock changes.

The first serious modern advocate was William Willett, a British builder who was reportedly irritated that his neighbors slept through summer mornings that he considered beautiful. Willett lobbied the British Parliament for years before his death in 1915, one year before the policy he advocated was finally implemented — without any direct credit to him.

The Energy Savings Turned Out to Be Negligible

The central justification for Daylight Saving Time — reducing energy consumption — has been tested exhaustively in the century since its adoption, and the results are underwhelming.

A frequently cited 2008 study by researchers at UC Santa Barbara used data from Indiana, which only partially observed DST before 2006 and then adopted it statewide. The study found that residential electricity use in Indiana actually increased by about 1 percent after DST adoption, as gains from reduced evening lighting were outweighed by increased air conditioning in warm afternoons and more heating in cold mornings.

The US Department of Energy estimated in 2008 that the extended DST period adopted in 2005 saved about 0.5 percent of national electricity on DST days. That’s a real number, but small — and it refers only to electricity, not total energy.

Most transportation and industrial energy use is completely unaffected by clock changes. The claim that DST saves energy in any meaningful sense has become increasingly hard to defend with modern data.

The Health Costs Are Increasingly Clear

While energy savings remain disputed, the health consequences of the spring clock change are not. Multiple large studies have found consistent patterns in the days following the spring forward:

Heart attacks increase by approximately 24 percent on the Monday after the spring clock change, according to a widely cited study from the University of Colorado. Traffic accidents rise in the days after the shift, as sleep deprivation impairs reaction time and judgment. Workplace injuries increase. Hospital records in several countries show upticks in stroke rates.

The mechanism is sleep disruption. Even one hour’s change to a deeply entrained circadian rhythm creates a form of social jet lag that takes most people several days to resolve. The autumn clock change — falling back — produces a smaller and briefer disruption, and some studies show modest health improvements from gaining an hour of sleep.

Most of the World Never Adopted DST, or Has Since Abandoned It

The global adoption of Daylight Saving Time was never universal. Most of Asia, including India, China, Japan, and most of Southeast Asia, has never observed DST. Most of Africa abandoned it decades ago. Nearly all of South America dropped the practice by the 2010s, with Brazil — which had observed DST since 1931 — ending it in 2019.

Mexico was among the last holdouts in North America. After decades of observing DST nationally, Mexico abolished it in 2023, leaving only a narrow strip along the US border that continues to observe the practice to stay synchronized with American neighbors.

Arizona and Hawaii Don’t Change Their Clocks — And Mostly Manage Fine

Within the United States, the state of Arizona (with the exception of the Navajo Nation) has declined to observe Daylight Saving Time since 1968. The reasoning was practical: in a desert state where summer temperatures regularly exceed 110°F (43°C), adding another hour of daylight in the evening means more people running air conditioning at peak heat. Hawaii, far enough south that day length doesn’t vary dramatically with seasons, has also opted out since 1967.

Both states have adapted. Business travelers and remote workers in Arizona learn quickly to mentally recalculate what time it is relative to California (the same in winter, one hour behind in summer), and the state has functioned without incident for over five decades.

The EU Has Been Trying to Abolish DST Since 2019

The European Parliament voted overwhelmingly in March 2019 to end seasonal clock changes across all EU member states. The proposal would have taken effect in 2021. It has not happened.

The sticking point is a political one: member states must agree on whether to standardize on permanent winter time (their standard UTC offset) or permanent summer time (UTC offset +1 hour). Southern European countries, which enjoy more sun, generally prefer permanent summer time. Northern countries, where winter darkness is more pronounced, are more ambivalent. Without consensus, the existing system — change the clocks twice a year — remains in place by default.

The US Has Tried Too, and Also Stalled

The Sunshine Protection Act, which would make Daylight Saving Time permanent in the United States, passed the Senate unanimously in March 2022 — an unusual display of bipartisan agreement. It then died in the House of Representatives without a vote, having never been scheduled for consideration.

The act has been reintroduced in subsequent sessions of Congress. Arguments for permanent DST emphasize consistent afternoon light for outdoor activities, sports, and retail commerce. Arguments against it, often from the medical community, note that permanent standard time (not permanent DST) better aligns with circadian biology, since light in the morning is more effective at setting the internal clock than light in the evening.

The debate isn’t over. What’s clear is that the simple act of moving a clock’s hands forward or backward remains — a century after it was first tried — one of the most consistently controversial interventions governments make into daily life. If you want to calculate how many days remain until the next clock change, try the date difference calculator or leap year tool on HourZone.io.