The distinction between the two options is not merely a matter of preference on the clock face. Permanent daylight saving time keeps evenings lighter, which may support recreation, social activity, commuting, and certain forms of economic activity. But it also pushes sunrise an hour later, leaving more people waking up, travelling to school, and commuting to work in darkness during the winter months. Permanent standard time, by contrast, preserves more morning light and better aligns clock time with the internal biological rhythms that govern sleep, mood, and alertness. The trade-off is that evenings become darker earlier, a shift that many people find unappealing but that the authors argue carries fewer health costs than the alternative.

At the heart of the argument lies circadian science. The human body maintains an internal clock of roughly 24 hours, and this clock depends on environmental cues, particularly light, to remain synchronized with the external day. Morning light advances and stabilizes the circadian system, while evening light delays it. Under permanent daylight saving time, the clock is shifted an hour relative to solar time, which means that during winter, people receive less light in the morning and more in the evening. This combination works directly against circadian alignment. Dr. Ralph Mistlberger, professor of psychology at Simon Fraser University in Vancouver, explains the mechanism plainly: our biological clocks need morning light to stay synchronized with the 24-hour day, and permanent daylight saving time moves that light an hour later just when winter mornings are already darkest.

The consequences of circadian misalignment are not abstract. The analysis highlights effects on mood and sleep as potentially the most significant. An estimated 1 to 3 percent of people in Canada experience seasonal affective disorder, a form of depression linked to reduced light exposure in winter, while as many as 15 percent experience milder seasonal changes in mood, sleep, appetite, and energy. Because permanent daylight saving time delays sunrise, it shortens the natural morning light exposure that helps regulate these seasonal patterns. Geographic studies cited by the authors add a further layer of evidence: people living farther west within a time zone, where solar events occur later by the clock, experience several adverse health outcomes, a natural experiment that mirrors what permanent daylight saving time would impose on the entire population.

Certain groups may be especially vulnerable to the effects of later sunrise. Children and adolescents are of particular concern because puberty naturally shifts sleep timing later, while school start times remain early; delaying sunrise by an hour compounds an existing tension between biology and schedules. The authors also identify shift workers and early-start workers, public transit users, pedestrians and cyclists, and people living in rural or lower-income communities with poorer lighting and transportation infrastructure as potentially affected populations. For these groups, dark winter mornings are not simply an inconvenience but a matter of safety, sleep quality, and daily functioning.

Yet the analysis does not dismiss the appeal of permanent daylight saving time. Longer evening light genuinely makes it easier to exercise, socialize, or shop after work and school, and these benefits are real. Dr. Patrick Baylis, associate professor at the Vancouver School of Economics at the University of British Columbia, notes that the attraction is easy to understand because people like more light after work and school. But he cautions that those evening benefits must be weighed against evidence linking later solar time with less sleep, lower productivity, and worse educational outcomes. In economic terms, the question becomes whether the measurable gains in evening activity outweigh the documented costs to sleep, learning, and health.

International experience offers cautionary lessons for policymakers considering the change. The United States adopted permanent daylight saving time during the 1974 energy crisis but reversed the policy within a year amid public opposition to dark winter mornings and concerns about children travelling to school before sunrise. Russia adopted permanent daylight saving time in 2011 and reversed it in 2014 following public dissatisfaction. Chile similarly reversed a national move to permanent daylight saving time after its 2015 experiment. These reversals suggest that the political durability of permanent daylight saving time may be tested most severely in the first winter, when the consequences of late sunrises become impossible to ignore.

The authors emphasize that whichever policy governments ultimately choose, its effects should be actively monitored rather than assumed away. If permanent daylight saving time is adopted, possible mitigation measures include safer roads and pedestrian infrastructure, later or more flexible school and work start times, increased morning light exposure in schools and workplaces, and systematic monitoring of sleep, mood, injuries, and other health outcomes. Permanent standard time, by contrast, would require fewer compensatory measures, mainly addressing earlier evening darkness in winter and earlier dawn in summer. This asymmetry in required interventions is itself informative: the option that demands less remediation is also the option better aligned with human physiology.

Dr. Alexandra Choi, medical health officer with Vancouver Coastal Health, frames the issue in explicitly public health terms, arguing that time policy is public health policy. Her message to governments is that if they choose permanent daylight saving time, they should plan for the consequences before the first winter arrives, from safer morning travel and flexible start times to monitoring of sleep, mental health, and injuries. This preventive framing shifts the debate from a question of convenience to one of population health management, placing time policy alongside other structural determinants of health that governments routinely regulate.

The analysis closes with a reframing of the entire debate. Dr. Raymond Lam, professor of psychiatry at the University of British Columbia and lead author, observes that researchers and policymakers have focused for years on whether to stop changing the clocks, but that the more important question now is what happens the morning after they stop. As Canadian jurisdictions prepare to legislate the end of clock changes, the scientific consensus emerging from sleep and chronobiology research, echoed by major professional organizations that recommend permanent standard time, suggests that the morning after matters most. Aligning civil time with solar time in winter would preserve the morning light on which circadian health, sleep, and mood depend, even at the cost of darker evenings. The decision, the authors argue, should be made with eyes open to the evidence rather than to the calendar alone.

Subject of Research: Health impacts of adopting permanent standard time versus permanent daylight saving time

Article Title: When the clocks stop changing: What time is best for health?

Article References: When the clocks stop changing: What time is best for health?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: standard time, daylight saving time, circadian rhythm, seasonal affective disorder, sleep, public health, clock changes, morning light, Canada, chronobiology, time policy, CMAJ