SUMMARY
- The shift to daylight saving time has been linked to increased heart attacks, strokes and fatal traffic crashes.
- US lawmakers have introduced several bills to eliminate clock changes, but none have become law.
- Scientists say disruptions to circadian rhythms and sleep loss explain most health concerns.
WASHINGTON — As the United States prepares to move clocks forward for daylight saving time on Sunday, March eight, medical researchers warn the shift may increase risks of heart attacks, strokes and fatal car crashes.
Despite decades of debate and growing scientific evidence linking the biannual clock change to negative health effects, federal lawmakers have repeatedly failed to end the practice.
Most Americans will lose an hour of sleep when clocks “spring forward” at two a.m. Sunday, marking the start of daylight saving time across much of the country.
While originally introduced to conserve energy and extend evening daylight, the time shift has increasingly drawn criticism from medical researchers who say the disruption to sleep patterns can harm public health.
Yet more than a century after its introduction, the policy remains in place nationwide except in Hawaii, most of Arizona and several US territories.
The modern concept of daylight saving time dates to World War I, when governments sought ways to reduce energy consumption by extending evening daylight hours.
The United States formally standardized the system through the Uniform Time Act of 1966, which established nationwide clock changes unless states opted out.
Congress later expanded the duration of daylight saving time. Since 2007, clocks have moved forward in March and back again in November.
Several regions do not observe the change, including Hawaii, most of Arizona, Puerto Rico, Guam, the US Virgin Islands, American Samoa and the Northern Mariana Islands.
In recent years lawmakers have proposed eliminating the twice yearly shift. Sen. Marco Rubio, R-Florida, repeatedly introduced legislation to make daylight saving time permanent, though it has stalled in Congress.
In February 2026, Rep. Greg Steube, R-Florida, proposed the Daylight Act of 2026, which would shift US time zones forward thirty minutes and make that change permanent.
Medical researchers say the strongest argument against daylight saving time centers on sleep disruption.
“Sunlight is the most powerful synchronizer of our circadian rhythms,” said Adam P. Spira, professor of mental health at the Johns Hopkins Bloomberg School of Public Health.
Exposure to evening light after the time shift can delay sleep and reduce overall rest, he said.
According to Johns Hopkins University, studies show higher rates of heart attacks and strokes in the days following the spring time change. Sleep loss also affects mood, cognitive performance and metabolic health.
“Even small disruptions to sleep can trigger measurable cardiovascular stress,” said Dr. José Ordovás, senior scientist at Tufts University’s Human Nutrition Research Center on Aging.
Researchers also link the clock change to public safety risks. A 2020 study by scientists at the University of Colorado Boulder and the Fred Hutchinson Cancer Research Center found a six percent increase in fatal traffic accidents after the spring shift.
| Impact Area | Research Finding | Source |
|---|---|---|
| Heart attacks | Increased risk in days after spring shift | Johns Hopkins University |
| Fatal car crashes | Six percent rise after time change | University of Colorado Boulder study |
| Sleep loss | Most Americans lose about one hour initially | Sleep medicine researchers |
| Circadian disruption | Evening light delays melatonin release | Johns Hopkins research |
Sleep specialists say certain groups face greater risk from the time change. Children commuting to school before sunrise, night shift workers and residents on the western edges of time zones experience the greatest circadian disruption, according to Johns Hopkins researchers.
“People living farther west within a time zone already experience later sunsets and sunrises,” Spira said. “The shift to daylight saving time intensifies that misalignment.”
The American Academy of Sleep Medicine has called for eliminating the seasonal clock change and adopting permanent standard time to better align sleep patterns with natural sunlight.
Political momentum to alter daylight saving time has grown in recent years but remains divided.
Former President Donald Trump said in March 2025 that public opinion remains split between those who prefer more evening light and those who want brighter mornings for school and commuting.
Without congressional action, Americans will continue adjusting clocks twice each year under current federal law.
More than one hundred years after its introduction, daylight saving time remains deeply embedded in US policy despite mounting scientific concerns.
While lawmakers continue to debate potential reforms, the biannual clock change persists as researchers warn its health effects may extend far beyond a lost hour of sleep.
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