Daylight Saving Time (DST), also summer time in several countries, is the practice of temporarily advancing clocks during the summertime 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.

In 2011, Daylight Saving Time comes to an end in U.S. on the morning of Sunday, November 6, when you move the clocks back one hour.

In 2007, U.S. introduced the extended DST, after the Energy Policy Act of 2005 came into effect and the clocks were set back one hour on the first Sunday of November instead of the last Sunday of October.

The start of DST was also changed from the first Sunday of April to the second Sunday of March.

There’s been a number of conflicting reports about how much energy is saved from Daylight Saving Time.

In the 70’s, studies showed US saved 1% of energy nationally, which was a big motivation for adopting DST. On the other hand, states like California argue the energy savings are negligible, while another report published in 2008 by the U.S. Department of Energy concluded 4 weeks extra of daylight savings time could conserve 1.3 trillion watt-hours per day, enough to power 100,000 homes for a year, reports Scientific American.

Benjamin Franklin first came up with the idea in 1784, but DST wasn’t used until World War I to conserve energy.

In 2011, Daylight Saving Time comes to an end in US on the morning of Sunday, November 6, when you move the clocks back one hour

The U.S. observed year-round DST during World War II and implemented it during the energy crisis in the 1970’s, notes the Scientific American.

Modern DST was first proposed by the New Zealand entomologist George Vernon Hudson in 1895, whose shift-work job gave him leisure time to collect insects, and led him to value after-hours daylight. George Vernon Hudson 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.

Not everyone across the U.S. observes Daylight Saving Time, including Hawaii, most of Arizona, Puerto Rico, the Virgin Islands, American Samoa, Guam and the Northern Marianas.

DST is generally not observed near the equator, where sunrise times do not vary enough to justify it. Some countries observe it only in some regions; for example, southern Brazil observes it while equatorial Brazil does not. Only a minority of the world’s population uses DST because Asia and Africa generally do not observe it.

A post by Chris Kline on ABC15.com discusses why most of Arizona doesn’t observe the time change: “According to an Arizona Republic editorial from 1969, the reason was the state’s extreme heat. If Arizona were to observe Daylight Saving Time, the sun would stay out until 9 p.m. in the summer (instead of 8 p.m., like it does currently).”