Americans will give up an hour of sleep in the wee hours this Sunday (March 11), when, at 2 a.m. local time, clocks are set ahead for daylight saving time. For Europeans, that transition won't happen for another two weeks, on March 25.
Daylight saving time is intended to make the best use of the longer daylight hours — the result of a tilt in the Earth's axis — during the coming half year by setting clocks forward one hour in spring and back in fall.
A study, published in 2007 in the journal BMC Biology, combined surveys from 55,000 people in central Europe with data on 50 individuals' sleeping and wakefulness patterns for eight weeks around the shifts to and from daylight saving time.
The researchers found people never fully adjust their circadian rhythms to the hour shift associated with daylight saving time (or, as it is known in Europe, summer time). Springing ahead by an hour, however, was most difficult for night owls — people prone to wake up and go to sleep late, they found.
Another much smaller study, published in 2008, also found evidence that one's status as an owl or a lark mattered. After examining the sleep cycles of nine volunteers, researchers suggest the transition into daylight saving time in spring was more problematic for owls, while the transition out in fall was more problematic for larks, they write in the journal BMC Physiology. [Life's Extremes: Night Owl vs. Morning Lark]
It seems clear the hour time shift can interfer with sleep. And sleep problems have been associated with everything from disasters, such as the Chernobyl nuclear accident and the Challenger space shuttle explosion to health problems, such as obesity and psychiatric problems.
So what effects does the transition have? A cursory search indicates the evidence so far is mixed.
A Finnish study, published the same year in BMC Public Health, used hospitalization data to look for a link between the one-hour shifts twice a year with accidents and manic episodes between 1987 and 2003. The team did not find evidence for a connection.
However, data from the U.S. National Institute for Occupational Safety and Health did show an increase in the number and severity of workplace accidents the Monday after the switch to daylight saving time. On that Monday, workers slept an average of 40 minutes less than other days, the researchers write in a study published in 2009 in the Journal of Applied Psychology.
Finnish researchers also found no evidence that the time transition affected traffic accidents, while a Canadian analysis found an 8-percent increase in traffic accidents on the Monday following the shift to daylight saving time.
There is also evidence that heart attacks increase the first three weekdays after the transition to daylight saving in spring.
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