The sun rose over Washington, D.C., at 8:27 a.m. on January 7, 1974. Children had already left for school. Some carried flashlights. Eight children in Florida would be dead within weeks of the clock change, and public support for the experiment would crater from nearly 80% down to 42% before spring even arrived. The U.S. tried permanent daylight saving time once before. Congress ended the experiment in less than a year.
That history matters now because the debate is back, louder than ever. The U.S. House passed legislation in July 2026 that would make daylight saving time permanent nationwide, though the bill still needs to clear the Senate and be signed into law before taking effect. The politics are moving fast. The biology, unfortunately, is not on the same side.
What permanent daylight saving time (DST) actually does to your body depends almost entirely on one thing: light. Not how much of it you get in total, but when you get it. That timing difference, morning versus evening, turns out to matter far more to your health than most people realize, and it sits at the center of one of the most consequential public health debates in years.
Why Your Body Clock Doesn’t Care What the Clock Says
Humans and other mammals are guided by circadian rhythms, 24-hour cycles that regulate sleep and other key bodily functions such as appetite and mood. These rhythms are largely dependent on light exposure, synchronizing with natural light-darkness cycles to promote healthy, high-quality sleep. The problem with permanent DST is that it pushes clock time an hour ahead of solar time, meaning your alarm sounds before your body expects it to, every single day, for months at a time in winter.
Circadian rhythm is the body’s innate, roughly 24-hour clock, which regulates many physiological processes. For most people, that internal clock actually runs slightly longer than 24 hours. Bioengineering researcher Lara Weed and psychiatry professor Jamie Zeitzer, PhD, of Stanford University modeled light exposure, circadian impacts, and health characteristics county by county, publishing their findings in a 2025 study in PNAS. Their modeling estimates that permanent standard time would prevent some 300,000 cases of stroke per year and result in 2.6 million fewer people having obesity. Permanent daylight saving time would achieve about two-thirds of the same effect – a meaningful gap, but still an improvement over the current switching system. The study is a modeling analysis based on county-level health data, not a randomized controlled trial, so the figures represent projections rather than direct experimental outcomes.
That two-thirds figure deserves some unpacking. Permanent DST is better than the current system of switching twice a year, but it falls measurably short of what standard time could deliver. The reason comes down to morning light.
The Morning Light Problem
The problem with daylight saving time is that it isn’t aligned with the natural circadian rhythm, making it harder for people to fall asleep at the right time and wake up at the right time. Under permanent DST, this misalignment doesn’t ease up after a few weeks. People “never fully adjust” to daylight saving time, and some experience “chronic” effects with it in place for eight months.
Morning sunlight is the most powerful signal the body receives to set its internal clock. Abrupt clock changes alter the daily patterns of exposure to natural light, which is the main driver of circadian rhythms. Under permanent DST, winter mornings become significantly darker. Permanent standard time gives you earlier sunrises in winter at the cost of earlier sunsets. Permanent DST gives you later sunsets at the cost of months of morning darkness. In cities like Seattle, Grand Rapids, and Williston, North Dakota, under permanent DST, the sun wouldn’t rise until nearly 9 a.m. during winter in Seattle, while in Grand Rapids it would rise as late as 9:15 a.m., and in Williston, residents wouldn’t see the sun until almost 9:45 a.m.
That’s not a minor inconvenience. It’s a biological signal arriving nearly two hours late, every morning, for months. Michael Grandner, director of the Sleep and Health Research Program at the University of Arizona and a volunteer expert at the American Heart Association, told Newsweek that “even a small average reduction in sleep or increase in circadian misalignment, when applied to millions of people over years, can have meaningful public health implications.”
What Circadian Disruption Does Inside the Body – and the Permanent Daylight Savings Health Case
Observational and modeling research consistently finds that the biannual shift in clocks disrupts not only circadian rhythms but also the physiological processes they regulate, which in turn affects overall health outcomes. Permanent DST would eliminate those twice-yearly transitions, but it would lock in a chronic misalignment that runs through every winter.
Changing clocks twice a year disrupts the body’s innate roughly 24-hour clock that regulates sleep, hormone production, and many other physiological processes, leading to measurable increases in stroke risk and obesity. Under permanent DST, the transition shock disappears, but the underlying misalignment lingers. Dr. James Rowley, program director of the Sleep Medicine Fellowship at Rush University Medical Center and president of the American Academy of Sleep Medicine, has noted that circadian rhythms need bright light in the morning to help people wake up and darker evenings to sleep at night. Without proper sleep, there’s an increased risk of cardio-metabolic problems, including heart disease, high blood pressure, and glucose regulation issues.
The mental health dimension is just as real. A 2026 narrative review in Brain Sciences, drawing on more than 60 studies from chronobiology, psychiatry, and public health, found that DST creates abrupt circadian disruptions that can impair sleep regulation, hormonal balance, cognitive performance, and emotional stability. Research has also linked sleep disturbances to higher rates of anxiety, depression, and suicidal ideation, according to a 2025 analysis. Interestingly, a 2025 UK study in PLOS ONE found that the week after the autumn clock change, when people gain an hour, was actually associated with fewer cases of anxiety, depression, and sleep disorders, suggesting the direction of the shift matters as much as the shift itself.
Understanding how irregular sleep affects your heart alongside circadian misalignment is particularly relevant for anyone already managing cardiovascular risk factors. A related piece on sleep and heart health covers the cardiovascular mechanisms in more detail.
The Traffic and Safety Risk Most People Aren’t Thinking About
The collective loss of an hour of sleep on the second Sunday in March has been linked to more heart attacks and fatal traffic accidents in the ensuing days. Permanent DST eliminates that single annual spring transition, which is genuinely good news. Under permanent daylight time, the nationwide prevalence of obesity would decrease by 0.51%, or 1.7 million people, and stroke by 0.04%, or 220,000 cases – still a meaningful gain at population scale.
But the safety math on dark winter mornings cuts the other way. Across the country, children waited at bus stops before dawn and commuters traveled to work in the dark during the 1974 trial. Reports of schoolchildren carrying flashlights and parents accompanying children to bus stops before sunrise became a recurring feature of news coverage. Several traffic accidents involving children received widespread attention, further intensifying public concern.
As the policy took effect and the realities of darker winter mornings became apparent, the National Opinion Research Center at the University of Chicago reported that support dropped to 42 percent by February 1974, down from 79 percent just two months earlier. Congress reversed the experiment before the year was out.
A 2020 study in Current Biology found that even the spring DST transition increases fatal motor vehicle accident risk by 6%, with the highest risk in morning hours and in western regions of time zones, where solar time and clock time diverge most. Permanent DST would entrench that geographic mismatch for half the country’s winter mornings.
What the Major Medical Organizations Actually Recommend
Many sleep medicine organizations, including the American Academy of Sleep Medicine, continue to recommend permanent standard time as the healthier long-term option, as it better aligns with human biology. The American Academy of Sleep Medicine notes that losing an hour of sleep during the spring transition increases risk of motor vehicle accidents, medical errors, cardiovascular events, and mood disturbances, and the organization has consistently endorsed eliminating those transitions through permanent standard time specifically, not permanent DST.
The Stanford modeling found that, from a circadian perspective, the current system of seasonal switching is the worst possible choice. Either permanent standard time or permanent daylight saving time would be healthier than the current back-and-forth, with permanent standard time benefiting the most people. The National Sleep Foundation and the American Medical Association have both endorsed year-round standard time for the same reasons.
The core takeaway from the science is that permanent DST is a partial solution. It removes the twice-yearly clock-change shock but replaces it with a persistent winter misalignment that the medical consensus says is harder on the body than morning-aligned standard time.
Read More: New Sunset, Sunrise Times in US if Daylight Savings Bill Passes
What This Means for You
If permanent DST becomes law, the practical implication is straightforward: your body will be living roughly an hour behind the clock every winter morning, indefinitely. Sunrise in many parts of the country will arrive after you’ve already been awake for an hour or more, depriving your circadian clock of its most important resetting signal right when it needs it most.
Daylight saving time isn’t aligned with the natural circadian rhythm, making it harder to fall asleep at the right time and wake up at the right time, and permanent DST doesn’t fix that – it just makes it stable. For people in the northern tier of the U.S. or on the western edge of time zones, the effect will be most pronounced. If the legislation passes, the most practical protection is behavioral: keep your sleep schedule consistent, prioritize getting outside within 30 minutes of waking even on dark winter mornings, and treat morning light exposure as non-negotiable. Your circadian system will try to adjust to the new clock. Your job is to give it every environmental cue you can to help it do so.
The debate over which permanent time to choose isn’t settled, and Congress may yet decide the political appeal of long summer evenings outweighs the sleep science. But the body’s position on the matter is already clear.
Disclaimer: This information is not intended to be a substitute for professional medical advice, diagnosis, or treatment and is for information only. Always seek the advice of your physician or another qualified health provider with any questions about your medical condition and/or current medication. Do not disregard professional medical advice or delay seeking advice or treatment because of something you have read here.
AI Disclaimer: This article was created with the assistance of AI tools and reviewed by a human editor.
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