How daylight saving time works, and why clocks still change twice a year

Daylight saving time does not create extra daylight. It moves the clock forward by one hour for part of the year so that a larger share of daylight falls in the evening rather than the early morning, when most people are still asleep, according to the US National Institute of Standards and Technology (NIST).

In the United States, clocks move forward one hour at 2:00 a.m. on the second Sunday of March and move back one hour at 2:00 a.m. on the first Sunday of November, under rules set by the Energy Policy Act of 2005, NIST says. In 2026 those dates fall on March 8 and November 1; Time magazine confirmed the March 8 start date, while November 1 follows from NIST's rule that the change occurs on the first Sunday of November.

Most member states of the European Union change to summer time on the last Sunday of March and back to standard time on the last Sunday of October, under the EU's existing clock-change rules, according to the European Parliament's legislative tracker. For 2026 that means clocks move forward on March 29 and back on October 25, according to Greek City Times, which reported a local shift from 3:00 a.m. to 4:00 a.m. for the spring change.

Where the idea came from

New Zealand entomologist George Hudson proposed a two-hour seasonal clock shift in 1895 so he would have more daylight after work for insect collecting, according to National Geographic. British builder William Willett independently proposed a similar idea to the UK Parliament around 1902, arguing it would stop the country "wasting daylight," but Britain initially rejected it.

Germany became the first country to implement the practice, in 1916, during World War I, as a way to conserve coal and energy for the war effort, National Geographic reports. The United States followed on March 9, 1918, when Congress passed its first daylight saving law, which also established the country's time zones, according to the same report.

Why the clock moves at 2 a.m. and why it works at all

The switch happens overnight because it falls after bars and most late-night venues have closed and before early Sunday transport schedules and church services begin, minimizing disruption, Time magazine reported.

The scientific basis for the practice rests on the tilt of Earth's axis, which produces far more pronounced seasonal daylight changes the farther a location is from the equator, National Geographic reported. That is why daylight saving time has more effect at higher latitudes and little practical value near the equator, where day length barely varies through the year.

The energy and health debate

The original, and still most-cited, rationale for daylight saving time is energy conservation: shifting activity into daylight hours in the evening was expected to reduce the need for electric lighting. Current research shows the measure no longer meaningfully cuts energy use, National Geographic reported.

Sleep researchers have raised separate health concerns about the spring transition specifically. Jennifer Martin, a former president of the American Academy of Sleep Medicine, said "the spring time change leads to society-wide sleep deprivation," according to Time magazine, which also cited studies linking the spring clock change to a higher short-term risk of stroke and motor vehicle accidents. National Geographic similarly reported a temporary rise in heart attack risk and car accidents around the transition, lasting one to two days, compared with roughly eight months of extended evening daylight the rest of the year.

Efforts to end the twice-yearly change

The European Commission proposed a directive to discontinue seasonal clock changes on September 12, 2018, after a public consultation drew 4.6 million responses, of which 84% favored ending the biannual switch, according to the European Parliament's legislative tracker. The European Parliament adopted a position backing an end to the changes by 2021, voting 410 to 192 with 51 abstentions on March 26, 2019.

The file has stalled in the Council of the European Union since early 2020 over disagreements on coordinating which member states would keep permanent summer time versus permanent standard time, the tracker states. Following discussions in October 2025, the European Commission said it would commission a further study to support future decisions, but the twice-yearly change remains in place across the EU as of 2026, the tracker confirms.

In the United States, Hawaii and most of Arizona do not observe daylight saving time at all, keeping standard time year-round, National Geographic reported. The report also noted a local complication in northeastern Arizona: the Navajo Nation observes the time change, the surrounded Hopi Reservation does not, and a Navajo enclave within Hopi territory does observe it.

Key dates at a glance