Written by Carter Trent
Edited by Nicki Steinberger, Ph.D.
EDITOR’S SUMMARY: Resetting the clock each season does more than disrupt your routine—it raises accident and cardiac risks, interferes with sleep, and even affects wildlife. Science favors standard time, but policy lags, leaving you to adapt in the meantime. You’ll see why the practice persists and how to protect your internal timing until lawmakers catch up.
Twice a year, Americans brace themselves for the great clock change—a ritual known as daylight saving time (DST). It involves adjusting clocks one hour back in the fall and one hour forward in the spring. This practice was introduced to align waking hours with seasonal shifts in sunrise and sunset associated with the Earth’s orbit around the sun. Today, DST is observed in much of Europe, North America, and parts of South America—but the majority of countries around the world no longer follow it.
Standard time keeps clocks aligned with the natural light–dark cycle, while daylight saving time pushes them forward to stretch evening light. That one-hour move isn’t small—it resets your internal clock, nudges routines off-center, and asks your body to function before it’s ready. Two states stay on standard time year-round. Arizona does so largely because of intense heat, although the Navajo Nation—spanning parts of Arizona, New Mexico, and Utah—still observes the shift to remain consistent across its lands. Hawaii, positioned near the equator where daylight hardly varies, has no practical need for a clock change.
These places aren’t just outliers; they remind you that following the sun is the baseline—not the exception. When sunrise guides your rhythm, you move through the day in sync with nature—not in a tug-of-war with it. This biannual ritual has measurable consequences for your health. As Dr. Charles Czeisler, professor of sleep medicine at Harvard Medical School, explains:
“That one-hour change may not seem like much, but it can wreak havoc on people’s mental and physical well-being in the short term.”
When Timekeeping Collides With Biology
How can an hour difference create this kind of impact? It boils down to human evolution. Your body evolved into its current state long before the first mechanical clocks were invented in 1275. For millennia, humans relied on the cycle of day and night to function and regulate sleep. Consequently, your body developed biological processes tied to this cycle, called circadian rhythms.
These rhythms regulate many important functions, including sleep, hormone release, temperature regulation, and digestion. For instance, when the sun goes down, circadian rhythms trigger the release of melatonin, a hormone that makes you sleepy. When the sun rises, they increase your body temperature to raise your level of alertness. Daylight saving time throws off this natural flow because you’re adjusting wake and sleep patterns based on a human-defined timetable, not the sun’s natural 24-hour cycle. The one-hour seasonal time adjustment can mean the difference between waking in darkness and waking to sunlight. And morning light matters—a lack of it affects the release of mood-boosting hormones like serotonin. That small shift doesn’t just steal sleep—it disrupts biology.
Another implication is the impact on your alertness and ability to concentrate. Take a teenager for example. Their body’s energy levels are typically at their lowest between 3:00 a.m. and 7:00 a.m. As a result, an adolescent may not feel fully awake until 9:00 a.m. or 10:00 a.m. as their body temperature slowly rises throughout the morning. In this scenario, shifting clocks forward in the spring forces them to get up for school when their bodies are still at a low-energy state and not fully alert. Moving the clock forward presses students into the day while their brains are still in biological “sleep mode,” compounding sleep debt and heightening risks for attention issues, behavioral challenges, and metabolic problems.
This drop in mental sharpness extends to adults too, and the consequences can be the difference between life and death. Research published in the journal Current Biology in 2020, “A Chronobiological Evaluation of the Acute Effects of Daylight Saving Time on Traffic Accident Risk,” found that fatal traffic accidents rise by 6% when clocks move forward in the spring. The time adjustment at this point in the year has a particularly strong effect on your body compared to the fall. You’re not only throwing off your body’s internal timing, you’re also losing an hour of sleep. This loss further contributes to impaired alertness when you have to drive to work or take a child to school. The time change also affects lighting conditions on the road, adding visibility challenges during morning and sunset hours, which coincide with peak commuting hours. In addition to these physical risks, the disruption isn’t limited to alertness and safety.
Anxiety in the days after the switch
Even a small cut in sleep changes how your brain handles stress: emotion-reactive regions ramp up while regulatory networks quiet down. The result is a measurable rise in anxiety the next day—not just feeling “off,” but a physiological uptick in threat sensitivity after insufficient sleep. When you wake in darkness, you delay the morning light your brain relies on to stabilize daytime neurotransmitters. That delay can heighten unease in the days after the clock reset, as studies note seasonal and light-linked variations in serotonin that track with changes in mood and tension. Your stress-readiness system also expects a strong cortisol rise soon after waking. A sudden shift in the light cycle can nudge that peak later or flatten it, leaving you more reactive and less grounded throughout the day; controlled studies and scientific reviews show that post-awakening light directly affects this cortisol response.
Accidents, heart health, and animals
The same factors raising the risk of deadly car crashes also cause a spike in workplace injuries. Data collected between 1983 and 2006 by the U.S. Department of Labor showed a 6% increase in workplace accidents following a daylight saving time change. A study published in the Journal of Applied Psychology—“Saving Daylight, Increasing Danger: Daylight Saving Time Leads to Less Sleep, More Injuries on the Job”—found that work-related incidents nearly quadrupled after DST began, and the severity of those injuries was greater as well.
Along with the threat of vehicle and workplace accidents, research has shown a higher incidence of heart problems and stroke as a result of the seasonal clock change. One study, “Daylight savings time and myocardial infarction,” uncovered a 24% rise in heart attacks in the spring, attributed to a reduction in sleep. Another analysis, “Changes in ischemic stroke occurrence following daylight saving time transitions,” found a greater chance of stroke in the days following DST, especially for those over age 65. According to Dr. Maja Jurisic, medical director at the occupational health services provider Concentra:
“It takes about a week for circadian realignment to occur each time someone shifts his/her sleep-wake cycle… During the process of realignment, one’s sense of well-being, one’s mood, and performance efficiency are all negatively affected.”
The effects extend beyond humans. Wildlife pays a price, too. A study from the University of Washington, “Permanent daylight saving time would reduce deer-vehicle collisions,” examined more than one million incidents and found a 16% surge in the week after the fall time shift. Deer continue to move at dusk, but drivers are suddenly on the road in darkness, creating a dangerous overlap. As lead researcher Calum X. Cunningham explains:
“Humans today have this ‘evening bias’ in our activity: we get up later and stay active later than what the sun is telling us to do. As long as people are living their lives ‘by the clock,’ which animals do not, people need to be aware of risks and make adjustments where we can.”
Environment America highlights the same point about animals remaining on natural time:
“Deer and other animals’ internal clocks don’t change just because humans change their external clocks. Drivers should be extra diligent when driving, especially at night. If you see a deer — or even a squirrel — along the side of the road, slow down. You never know when they might run. An animal darting into the road can surprise even the most vigilant and coordinated driver. That’s why the best way to minimize collisions is to keep wildlife off our roads in the first place. Wildlife crossings can take different forms, such as a tunnel or culvert under a road or a bridge over a road. When they are built with fencing, wildlife crossings can reduce wildlife-vehicle collisions by up to 97%.”
Age plays a major role in how well someone adapts to seasonal time shifts. As you get older, the mechanisms that keep your internal sleep–wake cycle steady become less flexible. Deep sleep becomes lighter and more fragmented, the timing of melatonin release shifts earlier, and your ability to recover from even small schedule disruptions declines. These changes make it harder to “absorb” a sudden loss of sleep or a shift in morning light.
“Age 40 seems to be a threshold age,” Dr. Jurisic explains, “after which it takes even longer to make adjustments in circadian rhythms.” As that adaptability wanes, you feel the effects of the seasonal clock switch more acutely—from slower mental processing to greater daytime sleepiness. For older adults, even a one-hour mismatch between biological timing and social timing can amplify stress on the cardiovascular system, affecting blood pressure, heart rate, and overall stability throughout the day.
A Policy Rooted in History, Not Health
Given the adverse effects of daylight saving time, why does this practice persist at all? Its origin stretches back over 100 years. Different places around the world experimented with seasonal time shifts. In 1908, the Canadian towns of Port Arthur and Fort William, which were located in Central Time, agreed to switch to Eastern Time in the summer months to provide more daylight for recreation. In 1916, Germany adopted DST as a wartime effort to conserve energy during World War I. The U.S. followed Germany’s example with the Standard Time Act of 1918. It was repealed at the end of the war, only to make a comeback during World War II before being repealed again. The modern clock shifts known today were established in 1966 as part of the Uniform Time Act. Before the passage of this law, every state set its own rules for whether to adopt daylight saving or standard time, along with the start and end dates for each. This created chaos, especially for interstate transportation and commerce. The federal policy was meant to provide uniformity for Americans.
Since then, many attempts to change how the U.S. handles daylight saving have come and gone. The latest proposal, the Sunshine Protection Act, would make DST permanent—meaning you would spring forward once and stay there, with no future clock changes. But Congress remains stalled, largely because many experts argue standard time better supports human health and biology.
Closer to home, several states have taken up the issue of time changes as well. For example, the Alabama legislature approved observing daylight saving time year-round in 2021, but the law cannot take effect unless Congress amends or repeals the Uniform Time Act to allow states to make that choice. In California, voters approved a measure authorizing the state legislature to adopt permanent DST, but like Congress, lawmakers remain split on whether to choose daylight saving time or standard time. Perhaps that’s a good sign. The scientific research supports standard time as the way to go, since it aligns more closely with your body’s natural design. Sleep expert Adam Spira with Johns Hopkins University states:
“The Sunshine Protection Act would result in permanent misalignment of our internal clocks with the time on our social clocks—the clocks on our walls, wrists, and phones. That would be bad for the health of the U.S. population.”
He explains that evening light exposure disrupts circadian rhythms and raises health risks: “Exposure to more light closer to bedtime makes it harder to fall asleep at our usual bedtime, and can reduce the amount of sleep we’re able to get each night.”
Staying Aligned in a Misaligned System
Even if the Sunshine Protection Act never comes to pass, there’s still the current DST to contend with, which can take about a week for your body to adjust to fully. Fortunately, you can adopt several actions to reduce its effects.
To prepare for an upcoming time change, adjust your sleep schedule to achieve seven to nine hours of sleep each night. Gradually shift your bedtime and wake-up time—about 15 to 20 minutes per night in the days leading up to the change. This steady approach helps your body adapt more smoothly. Preparation is particularly vital in the spring, when the transition to DST results in the loss of an hour. The fall shift, which grants an extra hour, is less taxing; it also offers a chance to strengthen sleep routines so the next spring transition feels less abrupt.
Sleep hygiene improvements include avoiding behaviors that can disrupt nighttime rest. For example, don’t consume food, caffeine, or alcohol right before bed. Also, limit the use of electronic devices at night. The blue light emitted by screens disrupts your body’s production of melatonin, making it harder to fall asleep. Strategic napping can help when you need it—keep naps brief, ideally early in the afternoon, and limit them to about 20 minutes to avoid disrupting nighttime sleep.
Make sure to go outside and get morning sunlight when you wake up. Light exposure is one of the key ways your body maintains healthy sleep–wake cycles. Sunlight also supports the hormone cortisol, boosting alertness while helping regulate stress. The same goes for exercising in the morning, as it assists in raising your body temperature. According to Dr. Andrew Huberman, neuroscientist and tenured professor in the Departments of Neurobiology and Ophthalmology at Stanford University School of Medicine:
“‘View morning sunlight!’ is one of my common refrains. Viewing sunlight within the first hours of waking (as soon as you can, even if through cloud cover) increases early-day cortisol release (the ideal time for elevated cortisol) and prepares the body for sleep later that night. A morning spike in cortisol will also positively influence your immune system, metabolism and ability to focus during the day.”
In summary
Daylight saving time reveals a mismatch between human biology and human policy. A one-hour clock change may sound harmless, but research tells a different story: more crashes, more heart events, more workplace injuries, and more fatigue-driven mistakes. Despite substantial evidence of harm, historical precedent continues to stand in the way of reform. Policy still outpaces what’s now understood about human physiology, even as growing research strengthens the case for standard time.
While you’re constrained by systems and schedules beyond your control, you’re not without agency. Supporting your natural sleep–wake patterns can soften the effects of daylight saving time and protect long-term health—even within a structure slow to change. Biology doesn’t wait for legislation, and honoring your body’s natural cues remains one of the simplest, most effective ways to stay aligned.
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Published on November 06, 2025.
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