Overview

Daylight saving time (DST), also called summer time in some countries, is the seasonal practice of advancing clocks during the warmer months so that evenings have more apparent daylight and mornings have less. In most implementations the clock is moved forward by one hour near the start of the period and moved back again near its end. The stated intent is to make better use of daylight in the late afternoon and early evening and to align waking hours with daylight.

How it works

When a jurisdiction observes DST, standard civil time is shifted ahead (commonly by one hour) for a defined part of the year. The change is usually scheduled to occur at night or in the early morning to reduce disruption to daily activities. Modern devices such as computers and smartphones often update automatically when configured for a location that observes DST. Exact start and end dates are determined by law or regulation and can change with new legislation.

History and motivations

Interest in seasonal clock adjustment dates from proposals in the late 19th and early 20th centuries. Several countries adopted DST during wartime to conserve fuel and to make more daylight available for industry and transportation. Economists, public health researchers and energy analysts continue to study whether DST produces measurable savings or broader social benefits. Over time, many jurisdictions have modified, abolished or reinstated variants of the practice.

Arguments for and against

Proponents cite potential benefits for retail and leisure activities, reduced evening traffic accidents, and possible energy savings in some settings. Opponents point to sleep disruption, short-term increases in certain health risks around transition dates, logistical complexity for scheduling and computing systems, and mixed evidence on net energy savings. The magnitude and direction of effects often depend on latitude, lifestyle patterns and local climate.

Geographic variation and examples

DST is commonly used in parts of Europe and North America, but most countries worldwide do not observe seasonal clock changes. Practices vary widely within countries and across regions. For introductory guidance about typical seasonal timing see summer months and timing.

  • United States: most states observe DST, though some areas do not; notable exceptions include Arizona and Hawaii.
  • Australia: some states and territories use summer time while others remain on standard time year-round; examples of non-observing jurisdictions include Queensland, Western Australia and the Northern Territory.
  • Canada largely observes DST, but most of Saskatchewan remains on standard time year-round; some territories have adopted independent policies.
  • United Kingdom and many European states follow coordinated rules within regional frameworks, though individual policy debates continue.
  • Some countries in northern Europe and Eurasia either never adopted DST or have ceased observing it; examples discussed in public sources include Iceland, Russia, Belarus, and adjustments in parts of Ukraine.

Practical impacts

Switching clocks affects transportation timetables, broadcasting schedules, international communication and computing systems that coordinate across time zones. Sectors such as agriculture, which are often perceived as strongly affected, typically follow natural daylight and market schedules rather than civil clock time; this nuance is important in public debate. Health studies focus on short-term sleep disruption around transitions and potential long-term circadian effects.

In recent years, some legislatures have debated abolishing the twice-yearly clock change in favor of year-round standard time or a permanent advanced time. The outcomes vary: some regions have proposed or passed measures to stop the annual transitions, while others retain them pending further study. Clear public communication and coordination with neighboring jurisdictions are common challenges when rules change.

Further information: For official rules, historical records and current local practice consult government timekeeping authorities and national legislation portals. Regional overviews and country-specific details are available through public resources for seasonal timing, Europe, North America, the United States, Australia, the United Kingdom, Canada, and individual subnational links such as Arizona, Hawaii, Queensland, Western Australia, Northern Territory, Saskatchewan, Iceland, Russia, Belarus, and Ukraine.