How Daylight Saving Time Works

DST delivers the most benefits when the days are long, as they are during summer. If you recall from your high school Earth science class, seasons are reversed in the Northern and Southern hemispheres. When it's summer in North America, it's winter in South America and vice versa. That means DST rules must also be reversed. In the Northern Hemisphere, DST starts in the spring -- typically between March and April -- and concludes in the fall -- between September and November. In the Southern Hemisphere, DST begins between September and November and ends between March and April.

The start and stop dates for DST are completely arbitrary, but over the years, most countries have adopted similar guidelines. The United States follows rules established by the Energy Policy Act of 2005. According to that legislation, daylight saving time begins in the U.S. (a country in the Northern Hemisphere) at 2 a.m. on the second Sunday in March. That's when many Americans move clocks forward by one hour and, if they're diligent citizens, replace their smoke-detector batteries. It ends about eight months later; at 2 a.m. on the first Sunday in November, clocks move back an hour, and standard time reigns again.

Although the U.S. Energy Policy Act thoughtfully provides rules to standardize the implementation of daylight saving time, it doesn't require all states to follow them. In fact, any state or territory can apply for an exemption and, if it's granted, sidestep DST and remain on standard time throughout the year. Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands and most of Arizona (with the exception of the Navajo Indian Reservation) all chose this option. Indiana, which used to observe DST in only 15 of its 90 counties, now "springs forward" and "falls back" across the whole state.

Don't think Americans are alone in their zeal to stretch out summer days. Many other countries practice daylight saving time in some fashion. According to a 2008 paper by the National Bureau of Economic Research, 76 countries currently observe DST, affecting 1.6 billion people worldwide [source: Kotchen]. European nations have been taking advantage of what they call "summer time" for decades, but they didn't standardize it until 1996, when the European Union adopted a common DST schedule that runs from the last Sunday in March through the last Sunday in October.

Next: We'll count down to the second when an hour magically appears or disappears.

Regardless of the specific rules implemented by a country, starting and stopping DST works the same way. A popular start time is 2 a.m., because most people are zonked out, and most businesses are closed. At that time, the clock moves forward exactly one hour. Here's a second-by-second account of what occurs:

1:59:58 -- it's standard time.

1:59:59 -- yep, still standard time.

3:00:00 -- we're on daylight saving time now.

3:00:01 -- daylight saving time rocks on for the next few months.

Notice that every second between 2:00:00 and 2:59:59 disappears completely. To start DST, a full hour must be skipped!

In the fall, when daylight saving time ends, you get the lost hour back because the time from 1:00:00 to 1:59:59 is repeated for one day. Here's what it looks like:

1:59:58 -- daylight saving time still rules.

1:59:59 -- DST's last hurrah.

1:00:00 -- standard time has assumed command.

1:00:01 -- standard time rolls on until the next time ...

Notice that the clock moves from 1:59:59 to 1:00:00, not 2:00:00. In other words, one full hour occurs twice, and the day ends up being 25 hours long. Most people don't ever need to refer to time within this hour, but if they do, say, because a birth or death occurred, they need to mention whether it was before or after the change back to standard time.

It's taken a few years -- and several changes -- to perfect this time-switching model. In the next section, we'll look at the history of daylight saving time to understand how it's evolved.

More than a century after eager beaver Ben Franklin conceived of daylight saving time, other folks, like New Zealander George Vernon Hudson and Englishman William Willett, had the same idea. Willett penned a pamphlet in 1907 called "The Waste of Daylight," in which he proposed that, during the spring and summer months, British clocks be moved ahead by 80 minutes in four 20-minute increments. In 1908, the House of Commons said thanks but no thanks to Willett's plan.

And so clocks ticked off standard time in the world for another 10 years. Then, the first of two global conflicts saw widespread fighting and forced many governments to consider drastic energy-cutting measures. Germany adopted daylight saving time (or war time) rules in 1916 in an effort to preserve its supplies of coal. England quickly followed suit, introducing British summer time and moving all clocks one hour ahead of Greenwich Mean Time during the summer months. In total, 31 nations, including the United States, began shifting their clocks to make better use of daylight hours and to preserve resources for the war effort. After the war, most countries repealed their DST laws and returned to standard time.

Peace, of course, didn't last. When World War II began in 1939, governments once again recognized the value of daylight saving time and implemented appropriate rules. This time, 52 countries shifted their clocks forward, some during the summer months, others for the whole year. The U.S., which fell into the latter category, adopted year-round DST for three years, beginning Feb. 9, 1942, and ending Sept. 30, 1945. After the war, Congress repealed mandatory DST, but left it up to different states and localities to decide whether they would continue the practice. Some did. Some didn't.

Finally, in 1966, the U.S. decided that if states were going to observe daylight saving time, they should conform to a standard set of rules. These rules were contained in the Uniform Time Act, which set the beginning and ending of DST, as well as the time when the changeover should occur. DST was to start the last Sunday of April and end on the last Sunday of October. For the next 20 years, all was calm on the U.S. time front, except when the country extended DST through the winter in response to the Arab oil embargo.

In 1986, U.S. President Ronald Reagan tweaked daylight saving time when he signed Public Law 99-359. The legislation revised the start time of DST to the first Sunday in April. According to language in the law, the edit was made to provide "more daylight outdoor playtime for the children and youth of our Nation, greater utilization of parks and recreation areas, expanded economic opportunity through extension of daylight hours to peak shopping hours and through extension of domestic office hours to periods of greater overlap with the European Economic Community."

In 2005, President George Bush signed legislation that extended DST from the second Sunday in March to the first Sunday in November. This went into effect in 2007. That same year, the entire state of Indiana started observing DST (before, only parts of the state did).

Advocates of daylight saving time have always pointed to energy conservation as the most important reason to move clocks forward during summer months. Here's the theory: Because 25 percent of all electricity consumed goes to powering lamps and small appliances, having more daylight hours for at least half the year should reduce the amount of electricity we use for lighting and running TVs, DVD players and stereos [source: Aldrich]. Also, DST should serve as an incentive for people to spend more time outdoors. In other words, there's more daylight available after school and work to go for a walk, play tennis or hit the links. If people are outside, they're not inside turning on lamps, appliances and electronics.

It wasn't until the early 1970s, however, that the power-conservation theory was put to the test. As part of the Emergency Daylight Saving Time Energy Conservation Act of 1973, initiated because of the Arab oil embargo, the U.S. Department of Transportation was required to study the effect of DST on electricity demand. To do this, researchers analyzed electricity load data from 22 different utilities for a period of days before and after transitions in and out of DST. Their report, published in 1975, found that daylight saving time reduced national electricity usage by roughly 1 percent compared with standard time.

This study stood as gospel for years, with little research conducted to support or refute it. Then, in May 2001, the California Energy Commission spearheaded a study to analyze the effects of winter DST and double daylight saving time (a two-hour time shift) on the state's electricity usage. The study concluded that winter DST would cut winter peak electricity use by 3.4 percent. Summer double DST would result in smaller reductions, but would still be beneficial because it would shift electricity use from high-demand afternoon hours to low-demand morning hours. The U.S. Department of Energy (DOE) reported similar findings in 2008 after conducting research as part of the Energy Policy Act of 2005. According to DOE data, DST resulted in total electricity savings of about 1.3 terawatt-hours, or 0.03 percent of electricity consumption over the year.

The benefits of daylight saving time go beyond energy conservation, if you believe its supporters. Advocates of the practice argue that allowing drivers to return home in the daylight reduces traffic accidents during the evening rush hour. They also suggest that DST prevents crime because it limits a person's exposure to criminals, who usually conduct their business under the cloak of darkness. Finally, the sports and recreation industries are rabid fans of daylight saving time. In 1986, for example, representatives of the golf industry lobbied for extended DST, arguing that an extra month of DST was worth up to $400 million annually in extra sales and fees [source: Choi].

Some of the biggest boosters of DST have been retail merchants, who believe that more evening daylight would encourage more people to go out and shop or dine. For 25 years, candy makers lobbied to get Halloween included in DST believing this would boost candy sales (because more kids would be out on the street collecting candy). They finally succeeded in 2007 [source: NPR].

Not everyone is so keen to manipulate time. Some say the studies described above are flawed, or they point to other, contradictory research suggesting DST should be eliminated altogether. We'll examine some of the evidence next.

Over the years, many well-known politicians have fiercely championed daylight saving time. U.S. President Warren G. Harding, however, despised it, despite being an avid golfer and a fan of Major League Baseball. In 1922, he made daylight saving time voluntary for private employers but not federal ones in the District of Columbia. Some institutions and businesses shifted their clocks, while others didn't. As a result, the city was thrown into chaos, and the experiment was eventually terminated [source: Fisher].

During Harding's misguided DST experiments, The Washington Post wrote this: "Cities and states could, of course, effect the same saving of daylight by simply keeping the clock on sun time and starting the day's work an hour earlier than customary." Farmers couldn't have agreed more. They already got up early and took advantage of daylight hours, so they didn't need an imposed, time-shifting system. They railed against the practice from the very beginning.

Unfortunately, those farmers fought an uphill battle, especially after 1975, when the U.S. Department of Transportation reported that DST saved energy. The study became the evidential foundation of the pro-DST movement, and little data emerged to refute the idea. Recently, however, some researchers have questioned whether the time shift is truly good for the power grid or for the health of citizens.

Arguably, the most compelling evidence comes from Matthew Kotchen, an economist at the University of California. When the entire state of Indiana began to observe DST in 2006, after spending many years on a half-and-half system, Kotchen seized the opportunity to conduct a before-and-after study of energy use. He and his team found that daylight saving time led to a 1 percent overall rise -- a rise! -- in residential electricity use, costing the state an extra $9 million [source: Kotchen].

Even more troubling, some scientists wonder if DST deleteriously affects human health. A German chronobiologist -- someone who studies natural physiological rhythms and other cyclical phenomena -- has shown that our circadian body clocks never adjust to daylight saving time. According to his research, moving clocks forward and back again interrupts normal sleep cycles, causing a sort of perpetual jet lag that leads to decreased productivity and quality of life and increased fatigue and susceptibility to illness.

That's not the worst of it. Swedish researchers examined heart attack rates in Sweden since 1987 and found that the number of heart attacks rose about 5 percent during the first week of daylight saving time. They published their results in the Oct. 30, 2008, issue of The New England Journal of Medicine, saying that disrupted sleep patterns may be linked to the cardiac episodes.

In 2018 and again in 2021, bills were introduced in the U.S. Congress to make DST permanent. Some states have already approved this change but it can't take effect until Congress gives approval. And what do people think? A 2019 poll of Americans found 40 percent preferred to stay on eastern standard time permanently, 30 percent wanted DST permanently and another 30 percent preferred the current system [source: Davies].

Perhaps Shakespeare was right after all. In King Henry VI, Part 1, he says, "Defer no time, delays have dangerous ends." Even though the famous playwright recorded these words some 193 years before Ben Franklin first conceived of daylight saving time while stationed in France, it could be sage and, ahem, timeless advice.

Originally Published: Oct 18, 2011