Daylight saving time, also called daylight time, is a plan in which clocks are set one hour ahead of standard time for a certain period. During daylight saving time, the sun rises an hour later in the morning, and sets an hour later in the evening. Daylight saving time does not increase the duration of daylight. It simply changes the way that timekeeping aligns with the natural cycle of day and night.
Daylight saving time is most useful in countries far to the north or south, where daylight varies greatly across the seasons. It is not as useful in countries near the equator. Many countries in Europe, North America, and South America use daylight saving time during the summer. In Europe, daylight saving time is often called Summer Time.
Most U.S. states observe daylight saving time. Where it is observed in the United States, it begins on the second Sunday in March and ends on the first Sunday in November. A state may decide to remain on standard time. States that lie in more than one time zone may use daylight time in one zone and not the other.
The chief purpose of daylight saving time is to save energy by reducing evening use of artificial lighting. As a result, countries often first adopt daylight time during a war or other crisis. The United Kingdom, for example, first used daylight time during World War I (1914-1918). The United States adopted the plan in 1918 but repealed it in 1919. The United States also observed it from Feb. 9, 1942, to Sept. 30, 1945, due to World War II.
After the war, many U.S. states established some type of daylight saving time. Beginning in 1967, the entire nation observed daylight time from the last Sunday in April to the last Sunday in October. In the 1970’s, a reduction in Arab oil exports caused a fuel shortage in the United States. To conserve energy, Congress enacted daylight time from Jan. 6 to Oct. 27, 1974, and from Feb. 23 to Oct. 26, 1975. From 1987 to 2006, daylight time began on the first Sunday in April. Since 2007, daylight time has run from the second Sunday in March to the first Sunday in November.
Some scientists and doctors have criticized daylight saving time for disrupting the human body’s natural daily cycle. Daylight saving time can make it more difficult to get enough sleep. Over time, poor sleep worsens an individual’s health. Additionally, sleep deprivation increases the risk of accidents such as car crashes. Some people think eliminating daylight saving time would be safer and healthier for the population.
See also Biological clock; Standard time.
