Daylight saving time (DST)—also summer time in several countries in British English, and European official terminology (see Terminology)—is the practice of advancing clocks so that evenings 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.
The modern idea of daylight saving was first proposed in 1895 by George Vernon Hudson  and it was first implemented during the First World War. Many countries have used it at various times since then. Although most of the United States used DST throughout the 1950s and 1960s, DST use expanded following the 1970s energy crisis and has generally remained in use in North America and Europe since that time.
The practice has been both praised and criticized. Adding daylight to evenings benefits retailing, sports, and other activities that exploit sunlight after working hours, but can cause problems for evening entertainment and other occupations tied to the sun. Although an early goal of DST was to reduce evening usage of incandescent lighting (formerly a primary use of electricity), modern heating and cooling usage patterns differ greatly, and research about how DST currently affects energy use is limited or contradictory.
As many modern societies operate on the basis of “standard time” rather than solar time, most people’s schedules are not governed by the movements of the earth in relation to the sun. For example, work, school and transport schedules will generally begin at exactly the same time at all times of the year regardless of the position of the sun. However, in non-equatorial regions the total number of hours of sunlight in a day will vary a great deal between winter and summer. As a result, if “standard time” is applied year round, a significant portion of the longer sunlight hours will fall in the early morning while there may still be a significant period of darkness in the evening. Because many people will tend to sleep in the early morning hours, these hours of sunlight are wasted for them, whereas if they are shifted to the evening via DST, they can then be used. As days shorten again in autumn/winter, sunrises get later and later, meaning that people could then be waking up and spending a significant portion of their mornings in the dark, so clocks are then returned to the “standard” time. In theory, people who need to or want to could simply wake up earlier to take advantage of the sunlight then, but this is impractical because of the inflexibility of clock-based schedules.
The actual effects of DST can vary significantly by location depending on its latitude and position relative to the centre of its time zone. For example, DST does not have much practical effect in extremely northern or southern locations because the very long/short days mean that the artificial manipulation of time has little or no real impact on daily life since sunrise/sunset times are already dramatically out of synch with modern working hours. DST is also of no use for locations near the equator because they see only a very small variation in daylight through the year.
During his time as an American envoy to France, Benjamin Franklin, publisher of the old English proverb, “Early to bed, and early to rise, makes a man healthy, wealthy and wise”, anonymously published a letter suggesting that Parisians economize on candles by rising earlier to use morning sunlight. This 1784 satire proposed taxing shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise. Franklin did not propose DST; like ancient Rome, 18th-century Europe did not keep precise schedules. However, this soon changed as rail and communication networks came to require a standardization of time unknown in Franklin’s day.
Modern DST was first proposed by the New Zealand entomologist George Vernon Hudson, whose shift-work job gave him leisure time to collect insects, and led him to value after-hours daylight. In 1895 he presented a paper to the Wellington Philosophical Society proposing a two-hour daylight-saving shift, and after considerable interest was expressed in Christchurch, New Zealand, he followed up in an 1898 paper. Many publications credit DST’s proposal to the prominent English builder and outdoorsman William Willett, who independently conceived DST in 1905 during a pre-breakfast ride, when he observed with dismay how many Londoners slept through a large part of a summer’s day. An avid golfer, he also disliked cutting short his round at dusk. His solution was to advance the clock during the summer months, a proposal he published two years later. The proposal was taken up by the Liberal Member of Parliament (MP) Robert Pearce, who introduced the first Daylight Saving Bill to the House of Commons on 12 February 1908. A select committee was set up to examine the issue, but Pearce’s bill did not become law, and several other bills failed in the following years. Willett lobbied for the proposal in the UK until his death in 1915.
Starting on 30 April 1916, Germany and its World War I allies were the first to use DST (German: Sommerzeit) as a way to conserve coal during wartime. Britain, most of its allies, and many European neutrals soon followed suit. Russia and a few other countries waited until the next year and the United States adopted it in 1918.
Broadly speaking, Daylight Saving Time was abandoned in the years after the war (with some notable exceptions including Canada, the UK, France, and Ireland for example). However, it was brought back for periods of time in many different places in the coming decades, widely during the Second World War. It became widely adopted, particularly in North America and Europe starting in the 1970s as a result of the 1970s energy crisis.