If shifting the clocks forward for daylight saving time makes you uneasy, there's a good reason: it will disrupt your sleep. When clocks move forward in the spring, we lose an hour. It's like giving the entire nation one timezone's worth of jetlag.

Daylight saving time in the U.S. started as an energy conservation trick during World War I, and became a national standard in the 1960s. But many now feel it has outlived its usefulness. For one, the presumed electricity savings of taking advantage of more daylight in the evening turn out to be unclear or nonexistent.

One hour of lost sleep sounds like a small change, but we humans are fragile, sensitive animals. Small disruptions in our sleep have been shown to alter basic indicators of our health, and dull our mental edge. Researchers have also found evidence of serious public health consequences of hundreds of millions of Americans being robbed of sleep every year.

There's some evidence that finds "springing forward" increases the number of traffic accidents

In 1999, researchers at Johns Hopkins University and Stanford wanted to find out what happens on the road when millions of drivers have their sleep disrupted.

Analyzing 21 years of fatal car crash data from the U.S. National Highway Transportation Safety Administration, they found a very small, but significant, increase of road deaths on the Monday after the clock shift in the spring: The number of deadly accidents jumped to an average of 83.5 on the "spring forward" Monday compared to an average of 78.2 on a typical Monday.

"We didn’t expect to see anything, actually," Richard P. Allen, the Johns Hopkins neurologist who oversaw the study, told me in September. "To me it was really amazing that one hour made that difference."

The paper did not find evidence of increased crashes on the Sunday daylight saving time begins, presumably because people can sleep in that day. But they say they think people are still sleep-deprived on Monday because they're still adjusting to the shift in time — as they would for jet lag.

Allen's findings joined a few smaller studies on the phenomenon. In 1996, researchers in Canada found the spring clock change "resulted in an average increase in traffic accidents [not deaths] of approximately 8 percent." Here's the chat they published in a letter to the New England Journal of Medicine:

Findings from Great Britain from 1980 report a similar uptick in accidents as well.

(Note: The effect isn't consistently found across the world. Researchers in Sweden reported "little evidence that the shift to or from daylight saving time had immediate effects on crash incidence" in a 2000 paper. They concluded the effect, if it exists in their country, was too small to detect.)

It's Allen's view that his findings show that, there are many Americans who are on the margin of poor sleep. Daylight saving time change pushes them over the edge, and creates a body-wide stress. And, it seems, it's not just car accidents. Since Allen's study, evidence has mounted of an increase in incidence of workplace injuries and heart attacks in the days after we spring forward.

Should all driving on Monday be discouraged?

The take-home lesson here isn't that driving on the Monday after the "spring forward" clock change is incredibly dangerous. A 2014 University of Colorado analysis (which has not yet been published ) found that daylight savings time may have caused 302 road deaths over a ten year period. Which is pretty tiny, all things considered. There are around 30,000 traffic deaths in the U.S. each year.

The takeaway is that even small decreases in our sleep times can stress our bodies. And good sleep, increasingly, is seen as a subtle yet critical component of our health. Scientists now think poor sleep may play a critical roll in the development of heart disease, diabetes, and obesity.

To prepare for a rough Monday after the clock change, it's a good idea to get enough sleep in the days leading up to it. Allen stresses that a one hour sleep disruption will be more severe for someone who is already sleep deprived.

"A lot of these accidents occur because we don't have residual sleep [reserves] to survive that insult," he says. "When we're running nearly empty on our sleep-wake status, it doesn't take much to push it into a negative area."