(PHILADELPHIA) -- This weekend, the clocks spring forward into daylight saving time -- the bittersweet adjustment that brightens the evenings while wreaking havoc on sleep schedules.
For most people, the shift is a nuisance. But for some, it provokes weeks of sleep deprivation that take a heavy toll on mood and productivity, according to Dr. Phil Gehrman, clinical director of the University of Pennsylvania's Behavioral Sleep Medicine program.
Since researchers began studying the effects of daylight saving time in the 1970s, the missing hour has been blamed for spikes in car accidents and workplace injuries, as well as dips in stock market returns.
"People think, 'It's only an hour.' But considering that most people aren't getting enough sleep to begin with, they often underestimate what an hour can do," Gehrman said.
The results are similar to jet lag. But no one gets jet lag when they lose an hour traveling one time zone east.
"That's because there's more light in the morning, and that helps you adjust your body clock," said Dr. Alfred Lewy, chairman of psychiatry and director of the Sleep and Mood Disorders Laboratory at Oregon Health and Science University in Portland, Oregon. "But with daylight saving time, the new light-dark cycle works against your body clock. The extra light at the end of the day shifts it the wrong way."
The body clock is a cluster of neurons deep inside the brain in an area called the hypothalamus. It generates the circadian rhythm, or sleep-wake cycle, that spans roughly 24 hours. But it's not precise.
"It needs a signal every day to reset it," Lewy said.
The reset signal is light, which comes in through the eyes and transmits signals -- separate from those involved in vision -- that update the clock. But when the sleep-wake and light-dark cycles don't line up, people feel out-of-sync, tired and even depressed.
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