Daylight saving time explained: history, purpose, and why countries are dropping it

Twice a year, roughly a quarter of the world's population shifts their clocks by one hour, disrupting sleep schedules and creating a brief period of cross-border scheduling confusion. Daylight saving time (DST) is one of the few widely adopted policies that has been studied extensively, found to produce mixed or negative results on most of its stated objectives, and yet remained in place in many countries largely through political inertia.

Origins and early adoption

The idea of shifting clocks to capture more usable daylight hours was proposed independently several times in the late 19th century, most famously by British builder William Willett in a 1907 pamphlet. Germany was the first country to implement DST nationally, in 1916 during World War I, primarily as an energy conservation measure. The United Kingdom followed weeks later, and the United States adopted it in 1918.

After World War I, most countries abandoned DST as a peacetime measure. It was readopted widely during World War II and has been retained with varying levels of consistency since. The United States standardised its DST rules nationally only in 1966 with the Uniform Time Act, before which individual states and even cities observed different transition dates, creating significant regional chaos for broadcasters, railways, and anyone coordinating across state lines.

The stated rationale

Proponents of DST offer three main benefits: reduced electricity consumption (shifting activity hours to align with natural daylight reduces evening lighting demand), increased economic activity in the evening (more daylight after work hours benefits retail, recreation, and tourism), and reduced traffic accidents (more daylight during active commuting hours).

Each of these claims has been studied empirically, and the evidence for each is weaker than its proponents suggest.

The energy argument has not held up

The original energy conservation argument assumed that evening lighting savings would outweigh any increase in morning heating or cooling demand. Studies conducted before widespread air conditioning found marginal electricity savings. Post-2000 studies consistently find that DST produces little to no net energy savings and in some regions produces a net increase in energy consumption — because the cooling load for homes and offices that stay warmer into the evening offsets the lighting savings.

A 2008 Indiana study — comparing counties that had recently adopted DST to those that had not — found a 1% increase in residential electricity consumption after DST adoption, with the increase driven by air conditioning. Indiana's climate makes it a reasonable proxy for many US states with significant summer cooling loads.

Health costs of the transition

Roenneberg, Wirz-Justice, and Merrow's research on chronotypes — the individual variation in the timing of the biological clock — shows that the human body does not adapt to abrupt clock shifts as quickly as the clock itself shifts. The transition to DST in spring is a form of induced circadian disruption: for most people whose chronotype is already delayed relative to social time requirements, advancing the clock by one hour produces sleep debt and alertness impairment for one to three weeks.

Janszky and Ljung (2008) examined hospital records in Sweden and found a statistically significant increase in heart attack incidence in the week after the spring DST transition and a corresponding decrease after the autumn transition. The effect was concentrated in the Monday immediately following the transition — consistent with combined sleep disruption and the return to work schedule. Subsequent studies in other countries have found similar patterns for traffic accidents, workplace injuries, and medical errors.

The global trend toward abolition

The European Union voted in 2019 to end mandatory DST transitions across member states, leaving each country to choose its preferred permanent time. Implementation has been delayed by the coordination problem: if neighbouring countries choose different standards (some permanent summer time, some permanent standard time), border regions experience mismatches that create new scheduling problems. As of 2026, the EU has not yet completed the transition, but the political direction is clear.

Outside Europe, several US states — including Arizona (which has not observed DST since 1967, except for the Navajo Nation within its borders) and Hawaii — have long opted out under federal law. Legislation to make DST permanent has passed the US Senate but as of 2026 has not been enacted into law. China, Japan, India, Iceland, and most of Africa do not observe DST.

The "permanent DST vs permanent standard time" debate

If DST transitions are to be abolished, the remaining question is which time to make permanent. The two options are permanent daylight saving time (clocks permanently one hour ahead of solar time in summer) and permanent standard time (clocks aligned with solar time year-round).

Chronobiologists generally favour permanent standard time, arguing that it aligns social schedules closer to the human biological clock — which is calibrated primarily by morning light exposure. Under permanent DST, sunrise in winter at high latitudes occurs very late (9 AM or later in some northern cities), delaying the morning light exposure that anchors the circadian rhythm and potentially worsening the social jetlag that DST critics identify as the primary health cost.

The public debate tends to favour permanent DST because it preserves the long summer evenings that are the most visible benefit of the current system. The epidemiological evidence does not support this preference.

Practical implications for scheduling

Until DST is fully abolished in relevant jurisdictions, the practical implications for anyone scheduling across time zones are:

• US and EU DST transitions occur on different dates in spring (US transitions about two to three weeks before the EU), creating a brief period each year when the US-EU time difference is one hour different from its usual value.
• The autumn transitions also differ, creating a similar window in October/November.
• Recurring calendar events scheduled as "9 AM Eastern" will automatically adjust for DST if calendar software uses IANA time zone identifiers correctly — but manually entered times with bare UTC offsets will not.
• Always confirm the local time for important events within 7 days of a DST transition in either party's time zone.