The debate over ending seasonal clock changes is intensifying as more health experts argue that permanent standard time, rather than daylight saving time, offers better protection for public health. A new Canadian review of international research concludes that keeping winter time throughout the year better supports sleep, mood and long-term well-being.
Several governments have moved in the opposite direction, choosing permanent daylight saving time to provide brighter evenings. British Columbia adopted year-round daylight saving time in March, while Alberta and the Northwest Territories are set to follow and skip the November clock change. In the United States, legislation to make daylight saving time permanent passed the House in July and is now awaiting consideration in the Senate.
Why morning light matters most
The analysis, led by psychiatrist Raymond Lam of the University of British Columbia, compares the effects of permanent standard time and permanent daylight saving time. Researchers reviewed evidence on circadian biology, sleep patterns, mood disorders, academic performance and economic outcomes, alongside real-world policy experiments conducted in several countries.
The human body relies on a central circadian clock in the brain that regulates sleep, alertness, hormone release and metabolism. This internal clock resets each day, primarily in response to natural light. Morning light shifts the body’s clock earlier, while evening light delays it, a distinction that becomes particularly important when work and school schedules remain fixed.
Co-author Ralph Mistlberger, a sleep researcher at Simon Fraser University, compared the body’s internal clock to a wristwatch that loses time each day and needs resetting every morning. When sunrise occurs later according to the clock, people receive less of the morning light needed for this adjustment. Over time, their natural sleep rhythms may shift later, even though school and work schedules remain unchanged, forcing many to wake before their bodies are biologically prepared.
Health costs of later sunrises
To demonstrate the difference, researchers compared sunrise times in Canadian cities. In Toronto, the latest midwinter sunrise would occur at approximately 7:51 a.m. under permanent standard time, compared with around 8:51 a.m. under permanent daylight saving time. Someone waking at 7 a.m. in January would therefore experience daylight an hour earlier under standard time, although they would still wake before sunrise.
The authors argue that earlier morning light may help reduce social jetlag, the persistent mismatch between the body’s internal clock and daily social obligations. Observational research has associated social jetlag with increased rates of obesity, cardiovascular problems, depressive symptoms and accidents, as well as poorer workplace and academic performance.
Seasonal clock changes can also cause short-term disruption. Studies have linked the spring transition to daylight saving time with temporary increases in road accidents and cardiovascular events, potentially resulting from sudden sleep loss and circadian misalignment. Although permanent daylight saving time would eliminate the twice-yearly clock changes, it would also mean later sunrises throughout much of the year.
Risks for mood, teens and safety
The Canadian researchers also examined evidence concerning seasonal affective disorder, a form of depression that typically becomes more severe during winter. One prominent hypothesis suggests that the condition worsens when delayed morning light causes the body’s internal clock to shift later. Bright-light therapy administered early in the day can move the clock earlier, whereas later sunrises under permanent daylight saving time may have the opposite effect.
Adolescents may be particularly vulnerable because their natural sleep schedules tend to shift later during their teenage years. For young people with late chronotypes, early school start times already contribute to widespread sleep deprivation. Delaying sunrise further could make it even more difficult for teenagers and university students to get enough sleep before morning classes.
Previous policy experiments illustrate these concerns. The United States introduced year-round daylight saving time in 1974 but reversed the decision within months following reports of children traveling to school in darkness and concerns about morning traffic safety. Russia adopted permanent daylight saving time in 2011 before abandoning it in 2014, citing public dissatisfaction with dark winter mornings.
Balancing lighter evenings and public health
Advocates of permanent daylight saving time highlight the advantages of brighter evenings, including additional opportunities for outdoor activities and potential increases in retail and leisure spending. More daylight during the late afternoon and early evening may encourage exercise and social interaction after work, particularly at higher latitudes where winter daylight hours are limited.
However, the review suggests that these benefits may come with health costs if they consistently reduce exposure to morning light. Geographic studies conducted within individual time zones have found that people living farther west, where sunrise and sunset occur later according to the clock, tend to sleep less and have slightly poorer health and economic outcomes. Although these observational findings cannot establish causation, they reinforce concerns about persistent differences between social schedules and solar time.
The researchers emphasize that latitude and local circumstances are important. Closer to the equator, where daylight duration changes less throughout the year, the health effects of permanent daylight saving time may be relatively modest. In northern regions, where winter mornings are already dark, shifting the clock by an additional hour could leave children commuting to school and adults driving to work in deeper darkness for weeks.
Policy recommendations and open questions
Based on the available evidence, the researchers recommend that governments favor permanent standard time if they choose to eliminate seasonal clock changes. They argue that keeping civil time more closely aligned with solar time, particularly around sunrise, provides better support for healthy sleep duration and stable circadian rhythms across the population.
If political or economic considerations lead governments to adopt permanent daylight saving time instead, the team recommends measures to reduce potential harm. These include later school and work start times, improved morning light exposure through urban design and indoor lighting, and ongoing monitoring of sleep quality, winter depression, absenteeism and traffic injuries.
The review acknowledges that important uncertainties remain because much of the existing evidence comes from natural experiments and geographic comparisons rather than large, long-term randomized studies. Nevertheless, as more jurisdictions consider abandoning seasonal clock changes, experts argue that public health should remain central to the discussion rather than being overshadowed by preferences for later sunsets.
Published in the Canadian Medical Association Journal, the findings contribute to growing agreement among sleep and circadian researchers that permanent standard time is the more biologically appropriate option. As lawmakers consider competing priorities, the debate is increasingly shifting from whether seasonal clock changes should end to which permanent time arrangement would best support future generations.
