At 2 a.m. ET on Sunday, clocks in the U.S. turned back one hour as daylight saving time ended, marking the beginning of winter's dark evenings.
The change often renews the longstanding debate about the tradition. In March, the Senate weighed in, unanimously voting in favor of the Sunshine Protection Act, which would make daylight saving time permanent year-round for all states but Hawaii and most of Arizona, which would continue to observe year-round standard time. But the bill has stalled in the House.
Sen. Marco Rubio, R-Fla., who introduced the act, suggested it would reduce crime, encourage kids to play outside and lower the risk of heart attacks and car accidents.
"There’s some strong science behind it that is now showing and making people aware of the harm that clock-switching has," Rubio said on the Senate floor in March.
Indeed, a 2020 study found that fatal traffic accidents in the U.S. rose 6% in the week after daylight saving started. Other studies have found that the switch to daylight saving brings small increases in workplace injuries and medical errors in the days following the change. A 2019 study, meanwhile, found that the risk of heart attacks went up in the week after clocks sprung forward, though other research did not find such an increase.
Those studies mostly looked at the immediate effects of turning clocks forward. But Steve Calandrillo, a law professor at the University of Washington, said people do benefit from sunlight later in the day, since that's when car crashes are more common.
A 2004 study estimated that switching to year-round daylight saving would result in 171 fewer pedestrian deaths each year and 195 fewer deaths among car drivers or passengers. Another study, published Wednesday, predicted that year-round daylight saving time would prevent 33 deaths and around 2,000 injuries among humans each year by reducing deer-vehicle collisions.
Calandrillo's research has also suggested that more sunlight in the evening reduces crime.
"I’ve always said darkness kills, sunshine saves — and darkness kills more people in the evening than it does in the morning," he said.
But the research is mixed overall, and the American Academy of Sleep Medicine supports the opposite switch to permanent standard time, given research showing that our bodies function best with more sunlight in the morning.
“I have received calls from constituents who prefer permanent standard time because they have safety concerns for children who have to wait too long in the dark during winter for the school bus," said Rep. Jan Schakowsky, a Democratic member of the House Energy and Commerce Committee, where the bill currently sits.
"And I have heard from constituents and businesses who prefer permanent daylight saving time because they prefer longer daylight hours.”
Schakowsky said there does not seem to be a consensus among voters yet, but "we know that the majority of Americans do not want to keep switching the clocks back and forth."
Sleep experts don't support year-round daylight saving time
Ideally, the sun should reach its highest point at noon, according to sleep experts. That’s known as solar time. During standard time, people in the central time zone are perfectly aligned with the sun’s clock, whereas daylight saving pushes the U.S. further from solar time.
"Under standard time here in Seattle, we’re about half an hour off from the real solar time, so if we are in daylight saving, we’re almost one hour and a half off," said Horacio de la Iglesia, a biology professor at the University of Washington.
Though sleep experts favor permanent standard time, most would opt for switching back and forth over permanent daylight saving time, de la Iglesia said.
The more mismatch with solar time, the higher the risk of health problems, according to Dr. Karin Johnson, a neurology professor at UMass Chan Medical School-Baystate who is on the board of Save Standard Time, a nonprofit that advocates for permanent standard time.
Johnson said people in the U.S. lose about 19 minutes of sleep per day due to daylight saving time, which could increase their risk of obesity, diabetes and heart disease.
"We really need to align to the sun," she said. "If we want to make social changes and decide we want that extra hour [of light] at the end of the day, then maybe the work schedule should be an hour shorter."
Dr. Kin Yuen, a sleep medicine specialist at the University of California, San Francisco and a fellow at the American Academy of Sleep Medicine, said that when people rise in darkness, hormones like cortisol may not be readily available, so people might feel drowsier.
Then at night, daylight saving can lead people to go to bed later, which can delay the body’s production of melatonin.
"Some people never adapt to daylight saving time, so they are predominantly so-called jet lagged for eight months of the year," Yuen said, adding, "our brains are wired to receive the sun in the morning and perform activities that are consistent with our internal clock."
A June study found that people whose clock times weren’t closely aligned with the sun had 22% higher road fatality rates than those living within 30 minutes of solar time.
These cumulative health risks likely influenced Mexico’s Senate vote last week to eliminate daylight saving time there.
The original argument for delaying daylight doesn't hold up
The U.S. first adopted daylight saving time in 1918 to save oil and electricity during World War I. But now, it isn't associated with energy savings. A 2011 study found that daylight saving actually cost Indiana households an extra $9 million per year in electricity bills because they spent more on heating and cooling, even though people used lights less.
"The real reason for why this policy came to be and we first started using it was because of energy, and right now it’s a completely open question about whether or not it saves energy," said Matthew Kotchen, an economics professor at Yale University who conducted that research.
The country most recently experimented with permanent daylight saving in 1974, but that ended less than a year later, after eight Florida kids died in traffic accidents attributed to the change.
"Every time it’s been tried in places, they often repeal it soon after," Kotchen said.