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Daylight saving time is meant to shift an hour of daylight, by the clock, from morning to evening—not to create more daylight. The United States adopted it partly in hopes of conserving energy and giving people more evening light for recreation and commerce. Those goals explain why the clocks change, but studies have not established that the practice reliably delivers large energy savings or settles every safety concern.
What daylight saving time changes
Daylight saving time (DST) is the part of the year when clocks are set one hour ahead of standard time. As the National Institute of Standards and Technology (NIST) puts it, “Daylight saving time, or DST, is the period of the year when clocks are moved one hour ahead.” The clock shift changes how daylight lines up with people’s schedules; it does not change the amount of daylight available.
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During DST, sunrise and sunset both occur one hour later by the clock than they would under standard time. That means darker mornings and more evening light by the clock. The U.S. DST interval lasts 238 days, or about 65% of the year, according to NIST.
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Energy conservation was a central rationale
The basic policy idea was that shifting daylight toward evening might reduce the need for artificial lighting and energy use. Energy conservation was a central reason for the U.S. policy, but an intended benefit is not proof that the benefit occurs consistently or at a meaningful scale.
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More evening daylight was expected to support activity
Congress also anticipated that extra evening daylight could encourage recreation and commerce and might affect traffic safety and crime. These were legislative aims and expectations, not guarantees that DST would produce those outcomes.
How U.S. daylight saving time developed
The Standard Time Act of 1918 established U.S. time zones and daylight saving time. The Naval Observatory describes the measure as contentious; Congress repealed DST in 1919 but kept the standard time zones. DST returned nationally during World War II. Afterward, local practices varied until the Uniform Time Act of 1966 standardized the dates while allowing jurisdictions to opt out.
Congress changed the dates through the Energy Policy Act of 2005. The longer March-to-November period took effect in 2007. These milestones describe U.S. policy; other countries set their own rules, and DST dates are not governed by a worldwide regulator.
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When clocks change in the U.S. in 2026
Under the general federal rule, DST begins on the second Sunday in March and ends on the first Sunday in November. In 2026, clocks move forward on March 8 at 2 a.m. local standard time, jumping to 3 a.m. They move back on November 1 at 2 a.m. local daylight time, returning to 1 a.m. and repeating that hour.
- Spring forward: The local clock skips from 1:59 a.m. to 3 a.m.; that calendar day has one fewer clock hour.
- Fall back: The clock repeats the 1 a.m. hour, so local schedules in that interval can be ambiguous unless they specify which occurrence.
Which U.S. places do not observe DST?
NIST lists Hawaii, American Samoa, Guam, Puerto Rico, the U.S. Virgin Islands, and most of Arizona as places that do not observe DST. The Navajo Nation does observe it, including in Arizona. These exceptions are U.S.-specific; do not assume every state, territory, or country follows the same schedule.
Does daylight saving time save energy?
The evidence is mixed, and the result depends on the place, period, and outcome measured. A U.S. Department of Energy analysis of the 2007 DST extension estimated a 0.03% decrease in electricity consumption in 2007, as summarized by the Congressional Research Service (CRS). That is a small estimate tied to one extension and one year—not evidence of large, permanent, or universal savings. CRS also describes studies with inconsistent or contradictory results, including increased electricity demand in some settings.
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CRS’s review is explicitly a selection rather than a comprehensive survey of the literature. It does not establish a single energy result that applies across regions and seasons.
What the U.S. safety evidence shows—and does not show
Studies reviewed by CRS do not resolve the question. A 1975 technical review of the 1970s Department of Transportation (DOT) experiment found no significant energy savings or overall traffic-fatality differences. It reported a statistically significant increase in morning fatalities among school-age children in a comparison period, while cautioning that other factors made attribution difficult.
The DOT’s final 1975 report described indications of favorable effects on electricity use, fatal motor-vehicle accidents, and crime, and suggested a modest overall benefit. However, some 1974–75 accident and fatality data were unavailable, so some impacts had to be inferred. CRS describes the interim report as inconclusive and the final picture as difficult to separate from other changes. Different descriptions reflect different analyses and limitations; the experiment does not prove that DST is beneficial or harmful overall.
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In CRS’s account of the DOT interim report, the report concluded that “there is no unambiguous direct evidence that [the measurable effects of year-round daylight saving time] were either beneficial or harmful.” That conclusion concerns the measured effects of that historical experiment, not every later study or every possible time policy.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Seasonal switching, permanent standard time, or permanent DST?
Ending the twice-yearly clock change is not the same policy choice as choosing which time to keep. Seasonal switching, permanent standard time, and permanent DST distribute daylight differently across morning and evening hours. The evidence summarized by CRS does not settle the trade-offs or establish that one arrangement resolves every energy or safety concern.
| Policy choice | What changes | Questions to weigh |
|---|---|---|
| Seasonal switching | Clocks alternate between standard time and DST during the year. | How morning and evening daylight vary by season and latitude; the disruption of twice-yearly changes; energy use by season and region; traffic, safety, health, sleep, and coordination with neighboring jurisdictions. |
| Permanent standard time | Clocks stay on standard time year-round. | How the daylight schedule would affect morning and evening routines across seasons; regional energy use; safety, health, sleep, and coordination. |
| Permanent DST | Clocks stay on DST year-round. | How the daylight schedule would affect morning and evening routines across seasons; regional energy use; safety, health, sleep, and coordination. |
These are decision criteria, not findings that any one option wins. A detailed health or circadian-health judgment requires dedicated, current medical evidence; the selected CRS material does not establish a comprehensive health consensus.
What to check when a clock changes
Most time codes sent through Network Time Protocol (NTP) carry Coordinated Universal Time (UTC); the operating system applies local time-zone and DST rules. Keeping a computer or phone’s operating system current helps it use the applicable rules. NIST says radio-controlled clocks generally receive DST status in its broadcast, but an old, unsynchronized, or incorrectly configured clock may still need attention.
Quick Recap
- Check that devices are set to the correct local time zone and are configured to update automatically.
- For an older or standalone clock, verify whether it adjusts automatically; if not, set it manually after the change.
- For appointments during the repeated fall-back hour, include the time zone and clarify which occurrence you mean.
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