Each year, most households in the United States reset their clocks as part of the twice-yearly toggling between daylight saving time and standard time. The ritual of “springing forward” in March and “falling back” in November is considered an inconvenience by many Americans. In fact, 64 percent of them would like to eliminate these biannual disruptions, according to a March 2022 poll by YouGov, an international market research and data analytics company based in the United Kingdom.
The problem, according to neurologists and sleep specialists, is that our bodies' natural clocks are out of sync with daylight saving time. It denies us the morning light we need to wake up and delays the cues of darkness that tell us we need to rest, says Logan D. Schneider, MD, a sleep neurologist at the Stanford Sleep Medicine Center in Redwood City, CA. Daylight saving time also increases the gap between our biological clocks and our social clocks.
Our internal clocks optimize various bodily functions throughout the 24-hour day, including digestion, hormone secretion, and the sleep-wake cycle, says Dr. Schneider. Light is one of the strongest drivers of these internal clocks; greater exposure to light in the morning and less exposure in the evening support synchronization of our bodies' functions. During standard time, the sun is directly overhead around noon in most places, which best matches our biological sleep-wake cycle.
Reducing exposure to morning light and increasing it during the evening, as happens during daylight saving time, has been shown to cause sleep deprivation, which can trigger inflammation and activation of genes associated with different cancers, says neurologist Beth Ann Malow, MD, FAAN, director of the Vanderbilt University Medical Center Division of Sleep Medicine in Nashville. In addition, the misalignment of our natural circadian rhythms can contribute to obesity, diabetes, and heart disease, she says.
Several studies bear this out. A 2019 review in the Journal of Clinical Medicine found that the risk of heart attacks increased “modestly but significantly” after the transition to daylight saving time. A paper published in PLOS Computational Biology in 2020 concluded that the shift to daylight saving time was associated with elevated risks of cardiovascular disease, mental and behavioral problems, and immune-related disorders.
Even slight misalignments between the body clock and the social clock can have serious health consequences. In an article published in Cancer Epidemiology, Biomarkers & Prevention in 2017, researchers found health disparities within time zones: People who live in the westernmost parts of a time zone, where sunrise and sunset occur minutes later, experience more health problems and shorter lives on average than their counterparts who live on the time zone's eastern edge. The authors stated that “circadian disruption is a probable human carcinogen.”
Based on these studies and other evidence, the American Academy of Sleep Medicine (AASM) strongly agrees that daylight saving time should be eliminated. In a 2020 position statement, the organization called for the adoption of permanent standard time because it “aligns best with human circadian biology” and protects the health and safety of Americans. The statement was endorsed by more than 20 medical, scientific, and civic organizations.
Keep the Switch
Changing the clocks twice a year is better than making daylight saving time permanent, says Karin G. Johnson, MD, FAAN, a sleep specialist at Baystate Medical Center in Springfield, MA. She notes that the AASM, the Society for Research on Biological Rhythms, the Sleep Research Society, and the advocacy group Save Standard Time all agree that permanent daylight saving time would be more harmful than the current practice. “The only advantage of making daylight saving time permanent,” Dr. Johnson says, “would be to stop the acute effects we feel during the change. It does not eliminate the long-term consequences, which can be harder to appreciate.”
Permanent daylight saving time would push winter sunrises in some areas of the country, such as Atlanta and Minneapolis, to as late as 9 a.m., after the start of the average school day, affecting students' routine, says Dr. Malow, who testified on the subject in front of the House Committee on Energy and Commerce in March 2020.
A Social Invention
Daylight saving time was first introduced in the U.S. toward the end of World War I to save energy. Moving an hour of sunlight from morning to evening meant people didn't need to use as much electricity at night. It went into effect in March 1918 and was repealed six months later after the conflict ended.
During World War II, Congress again approved springing forward, stating that the move would save fuel and “promote national security and defense,” as workers could toil later into the evening in support of the war effort, according to the U.S. Department of Defense. The switch was nicknamed “war time,” and time zones were rebranded Eastern War Time, Pacific War Time, and so on. The federal law was repealed in 1945.
Nearly 30 years later, in December 1973, President Richard Nixon signed a bill launching a two-year trial period of permanent daylight saving time. Before the initiative began, almost 80 percent of Americans supported the idea of an extra hour of afternoon light, according to newspaper reports at that time.
What they didn't consider until it went into effect is that the extra hour of afternoon light resulted in an additional hour of morning darkness. Some schoolchildren left their homes when the sky was “jet black” and had to carry flashlights, according to a report in the Washington Post. Just weeks into the change, eight children in Florida were killed in traffic accidents linked to sleep-deprived drivers negotiating dark morning roads. The tragedies made national news and prompted the state's governor to ask Congress to cancel the trial. Elected leaders in other states also started having second thoughts.
The idea that a permanent time change would save energy did not prove true. In Chicago, a spokesperson for Commonwealth Edison Company said the time change had saved “less than one-tenth of 1 percent in electricity. Much more energy has been saved by voluntary conservation.” A mother in Texas noted that while she might no longer need to turn her lights on in the afternoon, she was now being forced to turn them on earlier in the morning. “If all the lights are on when my children get ready for school, how much energy is being saved?”
Support for the time change plummeted; only 40 percent of Americans were still in favor two months after implementation. In October 1974, President Gerald Ford signed legislation ending the trial period.
Pro Daylight Saving Time
The current push to make daylight saving time permanent is supported by about half of Americans, says Karen Gamble, PhD, professor of psychiatry and behavioral neurobiology at the University of Alabama's Heersink School of Medicine in Birmingham. But that support may be because of the emotional attachment to the time of year with which it's associated. “Daylight saving time is always in the summer, and we think of barbecues, picnics, evening get-togethers, and baseball. You might think if we have daylight saving time year-round, we're going to have those activities year-round, but we're not,” Dr. Gamble says. “No matter what you do, the days are short in winter and it's cold, and changing the clocks won't make the days longer or change reality.”
Since 2015, about 30 states have passed legislation that would end the twice-yearly clock adjustments and make daylight saving time permanent immediately if Congress votes to change the law.
In general, a permanent switch to daylight saving time is supported by chambers of commerce, the travel industry, retail stores and gas stations, and recreation-based businesses, including golf courses and theme parks. All believe an extra hour of sunlight in the evening will make them more profitable.
“Businesses think they will do better with people being out later, and some will do better, but sleep loss and sleep disruption affect the productivity and safety of workers,” says Dr. Johnson. On average, people lose about 19 minutes of sleep every night because of the extra hour of daylight during daylight saving time, resulting in a significant loss of productivity, according to a 2019 study in the Journal of Health Economics.
Workers who start before 7 a.m. typically lose even more sleep—as much as 36 minutes. “Later sunlight exposure and lack of sunlight earlier make it harder to fall asleep on time, and people still have to wake up for work or school earlier than their bodies want to, leading to chronic sleep loss,” says Dr. Johnson. Especially susceptible groups include teenagers, night owls, and people who start work or school before sunrise. “These are the people we're harming [the most],” she says. “Instituting permanent daylight saving time may very well increase health disparities.”
For those with neurologic disorders, the short- and long-term impact of permanent daylight saving time will be similarly negative, says Dr. Johnson. “The underlying brain dysfunction will likely exacerbate the cognitive and memory effects of sleep and circadian rhythm disruption,” she says. Dr. Malow says her patients with narcolepsy and autism have told her that daylight saving time intensifies their underlying conditions, but there are no hard data corroborating that.
Studies on the negative effects of daylight saving time have focused on the general population, but the disruption of circadian rhythms could affect people whose rhythms are already dysfunctional, such as those with Alzheimer's disease or conditions sensitive to circadian patterns, such as migraine and epilepsy. A small study in Headache in 2018 found a link between circadian misalignment and delayed sleep and more frequent and severe migraine attacks.
Some doctors have argued in support of permanent daylight saving time because they believe it will help those who have seasonal affective disorder, but it's actually permanent standard time that will make a positive difference, Dr. Johnson says. “That sounds counterintuitive. People think, ‘I'll get more light exposure and that will help.’ But sleep deprivation outweighs that, and morning light treats mood disorders more effectively,” she says, citing multiple studies that support that conclusion, including one published in Biological Rhythm Research in 2017 that looked at seven years of sleep timing in children and adolescents and noted increased social jet lag and mood problems when clocks were pushed ahead one hour. “It's not just a simple calculus based purely on light. There are so many factors that go into this.”
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