You've probably heard the expression, "Spring forward, fall back." This means that starting on the second Sunday of each March and ending on the first Sunday of November in the U.S., daylight saving time takes effect. From the fall to the spring, we're on standard time. We set the clock forward an hour in the spring (losing an hour) and set it backward in the fall (gaining another hour of sleep). First used in World War I, daylight saving time was a way to adjust the daylight hours of the summer to correspond better with the hours that people were awake. In addition, adjusting the clock saved energy and fuel by cutting down on the amount of artificial light people needed. Many other countries also switch to daylight saving time during the summer, although not always on the same dates as the United States.
In March 2007, a U.S. law took effect that extended daylight saving time one month in the spring and a week in the fall. The move was heralded as a way to conserve energy, although the jury's still out on whether energy savings have been realized for consumers and businesses [sources: NPR, Handwerk]. Other benefits are more daylight hours for outdoor and family activities and even the potential to reduce crime and traffic accidents. States don't have to comply with the law, and a few, such as Arizona and certain counties in Indiana, choose not to change their times.
Regardless of when daylight saving time takes effect each year, it's bound to have some effect that's similar to jet lag. In fact, when members of Congress considered extending the fall date for daylight saving time further, they compromised after hearing testimony about the effects of daylight saving time on cows' milking times [source: NPR]. Dairy cows can't tell time, but they likely have internal clocks that work with solar time. You've probably noticed that the sun isn't always exactly overhead at 12 noon even during standard time. In a nutshell, that's the difference between solar time and clock time. Solar time is fixed by the movement of the Earth around the sun. Clock time is a socially agreed-upon time that everyone in a particular time zone shares. Some part of that time zone probably corresponds with solar time, but for those living further west, for example, the sun will be overhead when their watch reads 12:10, or even later. When we change our clocks for daylight saving time, we're moving an hour away from solar time.
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