This Sunday, most of the United States and Canada changes from Daylight Saving Time (DST) to Standard Time: at 2:00 a.m. local time, clocks fall back to 1:00 a.m. This event, which happens every year on the first Sunday in November, is the reverse of what happens in the spring: on the second Sunday in March at 2:00 a.m., clocks spring forward to 3:00 a.m.
Effectively, DST moves an hour of daylight from the morning to the evening. The modern idea of daylight saving was first proposed by the New Zealand entomologist George Vernon Hudson in 1895 in a paper to the Wellington Philosophical Society . It was first implemented on April 30, 1916, by Germany and its war-time ally Austria-Hungary as a way to conserve coal.
This annual ritual does not happen everywhere and does not happen everywhere at the same time. In the U.S. and Canada, each time zone switches at a different time. DST is not observed in Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands, the Commonwealth of Northern Mariana Islands, and Arizona. The Navajo Nation participates in the DST policy, even in Arizona, due to its large size and location in three states. However, the Hopi Reservation, which is entirely surrounded by the Navajo Nation, doesn’t observe DST. In effect, there is a donut-shaped area of Arizona that does observe DST, but the “hole” in the center does not.
The timing of the changeover, 2:00 a.m., was originally chosen because it was practical and minimized disruption. Most people were at home and this was the time when the fewest trains were running. It is late enough to minimally affect bars and restaurants, and it prevents the day from switching to yesterday, which would be confusing. It is early enough that the entire continental U.S. switches by daybreak, and the changeover occurs before most early shift workers and early churchgoers are affected.
In the U.S., the dates of the changeover were set in 2007. Widespread confusion was created during the 1950s and 1960s when each U.S. locality could start and end DST as it desired. One year, 23 different pairs of DST start and end dates were used in Iowa alone. For exactly five weeks each year, Boston, New York, and Philadelphia were not on the same time as Washington D.C., Cleveland, or Baltimore—but Chicago was. And, on one Ohio to West Virginia bus route, passengers had to change their watches seven times in 35 miles!
The Minnesota cities of Minneapolis and St. Paul once didn’t have twin perspectives with regard to the clock. These two large cities are adjacent at some points and separated only by the Mississippi River at others, and are considered a single metropolitan area. In 1965, St. Paul decided to begin its Daylight Saving Time period early to conform to most of the nation, while Minneapolis felt it should follow Minnesota’s state law, which stipulated a later start date. After intense inter-city negotiations and quarreling, the cities could not agree, and so the one-hour time difference went into effect, bringing a period of great time turmoil to the cities and surrounding areas.
Indiana has long been a hotbed of DST controversy. Historically, the state’s two western corners, which fall in the Central Time Zone, observed DST, while the remainder of the state, in the Eastern Time zone, followed year-round Standard Time. An additional complication was that five southeastern counties near Cincinnati and Louisville unofficially observed DST to keep in sync with those cities. Because of the longstanding feuds over DST, Indiana politicians often treated the subject gingerly. In 1996, gubernatorial candidate Rex Early firmly declared, “Some of my friends are for putting all of Indiana on Daylight Saving Time. Some are against it. And I always try to support my friends.” In April 2005, Indiana legislators passed a law that implemented DSTstatewide beginning on April 2, 2006.
The North American system is not universal. The countries of the European Union use Summer Time, which begins the last Sunday in March (one or two weeks later than in North America) and ends the last Sunday in October (one week earlier than in North America). All time zones change at the same moment, at 1:00 a.m. Universal Time (the successor of Greenwich Mean Time).
The only African countries and regions which use DST are the Canary Islands, Ceuta and Melilla (Spain), Madeira (Portugal), Morocco, Libya, and Namibia.
In Antarctica, there is no daylight in the winter and months of 24-hour daylight in the summer. But many of the research stations there still observe Daylight Saving Time anyway, to synchronize with their supply stations in Chile or New Zealand.
Proponents of DST generally argue that it saves energy, while opponents argue that actual energy savings are inconclusive. DST’s potential to save energy comes primarily from its effects on residential lighting, which consumes about 3.5% of electricity in the United States and Canada . Delaying the nominal time of sunset and sunrise reduces the use of artificial light in the evening and increases it in the morning. As Franklin’s 1784 satire pointed out, lighting costs are reduced if the evening reduction outweighs the morning increase, as in high-latitude summer when most people wake up well after sunrise. An early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity. Although energy conservation remains an important goal, energy usage patterns have greatly changed since then, and recent research is limited and reports contradictory results. Electricity use is greatly affected by geography, climate, and economics, making it hard to generalize from single studies .
 G. V. Hudson (1895). “On seasonal time-adjustment in countries south of lat. 30°”. Transactions and Proceedings of the New Zealand Institute 28: 734.
 Myriam B.C. Aries and Guy R. Newsham (2008). “Effect of daylight saving time on lighting energy use: a literature review”. Energy Policy 36 (6): 1858–1866. doi:10.1016/j.enpol.2007.05.021.