Daylight saving time (DST; also summer time in British English) is the convention of advancing clocks so that afternoons have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn. Modern DST was first proposed in 1907 by William Willett. Many countries have used it since then; details vary by location and change occasionally.
The practice can be controversial Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours, but causes problems for farming, entertainment and other occupations tied to the sun.] Extra afternoon daylight reduces traffic fatalities;] its effect on health and crime is less clear. An early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity; changes in usage patterns have cast doubt on whether DST now saves energy.
DST's clock shifts can serve as fire safety reminders,[8] but they complicate timekeeping and can disrupt meetings, travel, billing, recordkeeping, medical devices, and heavy equipment.[9] Many computer-based systems can adjust their clocks automatically, but this can be limited and error-prone, particularly when DST rules change.[
Benefits and drawbacks
Willett's 1907 proposal argued that DST increases opportunities for outdoor leisure activities during afternoon sunlight hours. Obviously it does not change the length of the day; the longer days nearer the summer solstice in high latitudes merely offer more room to shift apparent daylight from morning to evening so that early morning daylight is not wasted. DST is commonly not observed during most of winter, because its mornings are darker: workers may have no sunlit leisure time, and children may need to leave for school in the dark.
General agreement about the day's layout confers so many advantages that a standard DST schedule usually outranks ad hoc efforts to get up earlier, even for people who personally dislike the DST schedule. The advantages of coordination are so great that many people ignore whether DST is in effect by altering their nominal work schedules to coordinate with daylight, television broadcasts, or remote colleagues.[23]
Energy use
Delaying the nominal time of sunrise and sunset increases the use of artificial light in the morning and reduces it in the evening. 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.[6] Although energy conservation remains an important goal,[24] statistically significant evidence for it has proved elusive. The U.S. Dept. of Transportation (DOT) concluded in 1975 that DST might reduce the country's electricity usage by 1% during March and April,[25] but the National Bureau of Standards (NBS) reviewed the DOT study in 1976 and found no significant energy savings.[21] In 2000 when parts of Australia began DST in late winter, overall electricity consumption did not decrease, but the morning peak load and prices increased.[7] Although a 2007 study estimated that introducing DST to Japan would reduce household lighting energy consumption,[26] a 2007 simulation estimated that DST would increase overall energy use in Osaka residences by 0.13%, with a 0.02% decrease due to less lighting more than outweighed by a 0.15% increase due to extra cooling; neither study examined non-residential energy use.[27] A 2007 California study found no clear evidence that electricity would be saved through the earlier start to DST that year,[28] and although one utility did report a decrease in March 2007, five others did not.[29] DST may increase gasoline consumption: U.S. gasoline demand grew an extra 1% during the newly introduced DST in March 2007.[30]
Economic effects
Clock shifts affect apparent sunrise and sunset times in Greenwich in 2007.[31]Retailers, sporting goods makers, and other businesses benefit from extra afternoon sunlight, as it induces customers to shop and to participate in outdoor afternoon sports. For example, in 1984 Fortune magazine estimated that a seven-week extension of DST would yield an additional $30 million for 7-Eleven stores, and the National Golf Foundation estimated the extension would increase golf industry revenues $200 million to $300 million.[32] Conversely, DST can adversely affect farmers and others whose hours are set by the sun.[3] For example, grain harvesting is best done after dew evaporates, so when field hands arrive and leave earlier in summer their labor is less valuable.[33] DST also hurts prime-time broadcast ratings[4] and theaters, especially drive-ins.
Clock shifts correlate with decreased economic efficiency. In 2000 the daylight-saving effect implied an estimated one-day loss of $31 billion on U.S. stock exchanges.] Clock shifts and DST rule changes have a direct economic cost, since they entail extra work to support remote meetings, computer applications and the like. For example, a 2007 North American rule change cost an estimated $500 million to $1 billion.
[edit] Public safety
In 1975 the U.S. DOT conservatively identified a 0.7% reduction in traffic fatalities during DST, and estimated the real reduction to be 1.5% to 2%,[25] but the 1976 NBS review of the DOT study found no differences in traffic fatalities.[21] In 1995 the Insurance Institute for Highway Safety estimated a reduction of 1.2%, including a 5% reduction in crashes fatal to pedestrians.[5] Others have found similar reductions.ingle/Double Summer Time (SDST), a variant where clocks are one hour ahead of the sun in winter and two in summer, has been projected to reduce traffic fatalities by 3% to 4% in the UK, compared to ordinary DST.It is not clear whether sleep disruption contributes to fatal accidents immediately after the spring and autumn clock shifts. A correlation between clock shifts and accidents has been observed in North America but not in Sweden. If this twice-yearly effect exists, it is far smaller than the overall reduction in fatalities
In the 1970s the U.S. Law Enforcement Assistance Administration (LEAA) found a reduction of 10% to 13% in Washington, D.C.'s violent crime rate during DST. However, the LEAA did not filter out other factors, and it examined only two cities and found crime reductions only in one and only in some crime categories; the DOT decided it was "impossible to conclude with any confidence that comparable benefits would be found nationwide."] Outdoor lighting has a marginal and sometimes even contradictory influence on crime and fear of crime.[42]
In several countries, fire safety officials encourage citizens to use the two annual clock shifts as reminders to replace batteries in smoke and carbon monoxide detectors. This is especially important in autumn, just before the heating and candle season causes an increase in home fires. Similar twice-yearly tasks include reviewing and practicing fire escape and family disaster plans, inspecting vehicle lights, checking storage areas for hazardous materials, and reprogramming thermostats.[43][44][8] This is not an essential function of DST, as locations without DST can instead use the first days of spring and autumn as reminders.[45]
[edit] Health
DST has mixed effects on health. In societies with fixed work schedules it provides more afternoon sunlight for outdoor exercise.[46] It alters sunlight exposure; whether this is beneficial depends on one's location and daily schedule, as sunlight triggers vitamin D synthesis in the skin, but overexposure can lead to skin cancer. Sunlight strongly influences seasonal affective disorder; DST may help in depression by causing individuals to rise earlier,[47] but some argue the reverse.[48] The Retinitis Pigmentosa Foundation Fighting Blindness, chaired by blind sports magnate Gordon Gund, successfully lobbied in 1985 and 2005 for U.S. DST extensions,[2][49] but DST can hurt night blindness sufferers.[50]
Clock shifts reduce sleep duration and efficiency,[51] particularly for late-risers after clocks spring forward; effects can last for weeks.[52] The government of Kazakhstan cited health complications due to clock shifts as a primary reason for abolishing DST in 2005.[53]
[edit] Complexity
DST's clock shifts have the obvious disadvantage of complexity. People must remember to change their clocks; this consumes time, particularly for mechanical clocks that cannot be moved backward safely.[54] People who work across time zone boundaries need to keep track of multiple DST rules, as not all locations observe DST or observe it the same way. The length of the day becomes variable. Disruption to meetings, travel, broadcasts, billing systems, and records management is common, and can be expensive.[55] Near an autumn transition from 02:00 to 01:00, a clock reads times from 01:00 to 02:00 twice, possibly leading to confusion.[56]
Computer-based systems may also require downtime or restarting when clocks shift. Ignoring this requirement damaged a German steel facility in 1993.[9] Medical devices may generate adverse events that could harm patients, without being obvious to clinicians responsible for care.[57] These problems are compounded when the DST rules themselves change, as in the Year 2007 problem. Software developers must test and perhaps modify many programs, and users must install updates and restart applications.[10]
Some clock-shift problems could be avoided by adjusting clocks continuously[58] or at least more gradually—for example, Willett originally suggested weekly 20-minute transitions—but this would add complexity and has never been implemented.
The William Willett Memorial Sundial is always on DST.DST inherits and can magnify the disadvantages of standard time. For example, when reading a sundial, one must compensate for it along with time zone and natural discrepancies.[59] Also, DST complicates time-of-day approaches for avoiding sun exposure, necessitating even more the shadow rule of avoiding sun exposure whenever your shadow is shorter than you are.[60]
[edit] Politics
Daylight saving has caused controversy since it began.[1] Winston Churchill argued that it enlarges "the opportunities for the pursuit of health and happiness among the millions of people who live in this country".[61] Robertson Davies, however, detected "the bony, blue-fingered hand of Puritanism, eager to push people into bed earlier, and get them up earlier, to make them healthy, wealthy and wise in spite of themselves,"[62] and wags have dubbed it "Daylight Slaving Time"[63] or "Daylight Stupid Time".[64] Historically, retailing, sports and tourism interests have favored daylight saving, while agricultural and evening entertainment interests have opposed it, and a war or economic crisis is often associated with its adoption.
The fate of Willett's 1907 proposal illustrates several political issues involved. The proposal attracted many supporters, including Balfour, Churchill, Lloyd George, MacDonald, Edward VII (who used half-hour DST at Sandringham), the managing director of Harrods, and the manager of the National Bank. However, the opposition was stronger: it included Prime Minister Asquith, Christie (the Astronomer Royal), George Darwin, Napier Shaw (director of the Meteorological Office), many agricultural organizations, and theater owners. After many hearings the proposal was narrowly defeated in a Parliament committee vote in 1909. Willett's allies introduced similar bills every year from 1911 through 1914, to no avail.[65] The U.S. was even more skeptical: Andrew Peters introduced a DST bill to the U.S. House in May 1909, but it soon died in committee.[66]
Retailers generally favor DST. United Cigar Stores hailed a 1918 DST bill.World War I changed the political equation, as DST was promoted as a way to alleviate hardships from wartime coal shortages and air raid blackouts. After Germany led the way, the United Kingdom first used DST on May 21, 1916.[67] U.S. retailing and manufacturing interests led by Pittsburgh industrialist Robert Garland soon began lobbying for DST, but were opposed by railroads. The U.S.'s 1917 entry to the war overcame objections, and DST was established in 1918.[68]
War's end swung the pendulum back. Farmers continued to dislike DST, and many countries repealed it after the war. Britain was an exception: it retained DST nationwide but over the years adjusted transition dates for several reasons, including special rules during the 1920s and 1930s to avoid clock shifts on Easter mornings.[69] The U.S. was more typical: Congress repealed DST after 1919. President Woodrow Wilson, like Willett an avid golfer, vetoed the repeal twice but his second veto was overridden,[70] and only a few U.S. cities retained DST locally thereafter.[71] Wilson's successor Warren G. Harding opposed DST as a "deception". Reasoning that people should instead get up and go to work earlier in the summer, he ordered District of Columbia federal employees to start work at 08:00 rather than 09:00 during summer 1922. Many businesses followed suit though many others did not; the experiment was not repeated.
Since Willett's day the world has seen many enactments, adjustments, and repeals of DST, with similar politics involved. The history of time in the United States includes DST during both world wars, but no standardization of peacetime DST until 1966.[74] In the mid-1980s, Clorox (parent of Kingsford Charcoal) and 7-Eleven provided the primary funding for the Daylight Saving Time Coalition behind the 1987 extension to U.S. DST, and both Idaho senators voted for it on the basis of fast-food restaurants selling more French fries made from Idaho potatoes;[2] in 2005, the Sporting Goods Manufacturers Association and the National Association of Convenience Stores successfully lobbied for the 2007 extension to U.S. DST.[49] In early 2007, Western Australia continued to debate a trial use of DST and several politicians changed positions after public sentiment swung against it. In the UK the sport and leisure industry supports a proposal to observe SDST's additional hour year-round.
Observance practices