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Most Americans will lose an hour of sleep on Sunday, due to the start of daylight saving time. They'll also temporarily fall out of sync with much of the rest of the world. Why? For the sake of saving energy.
Daylight saving time goes into effect for the United States — excluding Hawaii, most of Arizona and some U.S. territories — at 2 a.m. Sunday. Technically, clocks should be pushed forward to 3 a.m. at that time, but folks typically take care of the chore before going to bed or after waking up.
The concept of resetting clocks for the summer goes back to Benjamin Franklin, who suggested the switch to maximize the use of the season's evening daylight and save energy. The fuel-saving strategy first took hold nationwide during World War I and again for World War II.
It's traditionally up to the states to decide whether they adopt daylight saving time, but it's traditionally up to Congress to determine when the switch takes effect. Thanks to the Energy Policy Act of 2005, "summer time" now takes up almost eight months of the year, from the second Sunday in March to the first Sunday in November.
Other countries follow different schedules. So the time difference between New York and London, for instance, will be four hours rather than five for the next three weeks.
Does the shift actually save energy? The answer may depend on local conditions and lifestyle choices. A nationwide statistical study in 2008 suggested that the practice produced a net saving of 1.3 terawatt-hours of electricity for lighting annually. However, more focused studies have shown that the savings can be canceled out by increased demand for heating and cooling.
Studies also suggest there'll be an increased chance of bleary-eyed workers showing up on Monday morning, due to the shift in sleep schedules.
One study found that the shift caused office workers to waste more time on "cyberloafing," while another has linked the onset of daylight saving time to a higher incidence of heart attacks. (And yes, the switch back to standard time appears to reduce heart risk.)
For more about the mysteries of time and sleep, check out our archived story and interactive graphic.
This is an edited version of a report first published in March 2014.