Clocks change for daylight saving time to make better use of daylight in the evening in summer.
Reason
Daylight saving time (DST) moves clocks forward 1 hour in spring to give more evening light, and back in autumn.
History
Germany used it in 1916. The US adopted it in 1918, and the Uniform Time Act of 1966 standardized it.
US dates
Starts on the second Sunday in March and ends on the first Sunday in November.
Exceptions
Arizona (except the Navajo Nation) and Hawaii don’t observe DST.
Debate
The Sunshine Protection Act, which would make DST permanent, has been proposed but not passed.
Daylight saving time
Clocks change to make better use of daylight. In spring, clocks move one hour forward (“spring forward”), and in autumn they move back one hour (“fall back”). Changing the clock shifts an hour of daylight from morning to evening during the warmer months.
History
The idea is credited to Benjamin Franklin’s satirical essay in 1784, but the first modern proposal came from George Hudson in New Zealand in 1895. Germany introduced it in 1916 during World War I to save fuel. The US adopted it in 1918, repealed it after the war and used it again in World War II (“war time”).
Uniform Time Act
The Uniform Time Act of 1966 standardized daylight saving dates across the US. The Energy Policy Act of 2005 extended DST to the period from the second Sunday in March to the first Sunday in November, starting in 2007.
Exceptions
Hawaii and most of Arizona (except the Navajo Nation) do not observe DST, nor do US territories such as Puerto Rico, Guam and American Samoa. About 70 countries use DST worldwide, while most countries near the equator do not.
Health effects
Studies show an increase in heart attacks and strokes in the days after the spring clock change, and disrupted sleep for several days. The American Academy of Sleep Medicine supports permanent standard time over permanent DST.
Energy savings
The savings are small. A 2008 Department of Energy study found a 0.5% reduction in electricity use per day for DST, while other studies show no savings or increases.
Debate
The Sunshine Protection Act, passed by the US Senate in 2022, would make DST permanent but didn’t pass the House. States may adopt permanent standard time without Congress, but need congressional approval to adopt permanent DST. The European Union proposed ending seasonal clock changes in 2019, but no final decision was reached.
History and law
Daylight saving time was first adopted by Germany in 1916 during World War I to save fuel, and the US introduced it with the Standard Time Act of 1918, then repealed it in 1919. The Uniform Time Act of 1966 set a national system, and the Energy Policy Act of 2005 extended it to start on the second Sunday in March and end on the first Sunday in November from 2007. Hawaii and most of Arizona do not observe it. Congress has considered making daylight time permanent, and the Senate passed the Sunshine Protection Act in 2022, but it did not become law.
Health and energy effects
Studies show a rise of about 24 percent in heart attacks on the Monday after the spring clock change, according to a 2013 study from the University of Michigan, and more car accidents and sleep disruption. The American Academy of Sleep Medicine supports permanent standard time. The energy saving effect is small, as the Department of Energy found in 2008 that daylight saving trimmed about 0.5 percent of daily electricity. Debate continues over which permanent time is best. Adjust gradually by shifting bedtime in 15 minute steps.
Based on data from the US Department of Transportation and the American Academy of Sleep Medicine
