Canadian analysis weighs the health trade-offs of permanent standard time versus daylight saving time

An extra hour of evening daylight comes with a winter cost: sunrise arrives an hour later by the clock. For people heading to school or work early, that can mean starting more of the day in darkness.

As Canadian jurisdictions consider ending twice-yearly clock changes, a new analysis in CMAJ argues that choosing the replacement matters just as much. The authors conclude that permanent standard time better supports circadian alignment, sleep and mood than permanent daylight saving time.

The analysis weighs existing research rather than reporting a new clinical trial. It recognizes benefits from brighter evenings, while warning that costs differ across communities.

“Ending the clock change solves one problem, but it creates another decision: which time should we live on year-round?” says lead author Dr. Raymond Lam, a psychiatry professor at the University of British Columbia. “The health evidence points to standard time, largely because morning light matters for our body clocks, sleep, and mood.”

Daylight saving time does not create additional daylight. It shifts sunrise and sunset one hour later on the clock, moving light from the morning toward the evening.

That redistribution affects the brain’s internal timekeeping system. The master circadian clock typically takes about 24.2 hours to complete a cycle, so environmental signals help keep it synchronized with the 24-hour day.

Morning light generally advances that clock, counteracting its tendency to run late. Evening light delays it, potentially widening the gap between biological timing and fixed school or work schedules.

Indoor living and evening electric lighting already encourage that mismatch, often called social jet lag. Permanent daylight saving time would prolong winter morning darkness and extend evening light, working against alignment at both ends of the day.

“Our biological clocks need morning light to stay synchronized with the 24-hour day,” says Dr. Ralph Mistlberger, a psychology professor at Simon Fraser University. “Permanent daylight saving time moves that light an hour later just when winter mornings are already darkest.”

The authors identify seasonal mood vulnerability as an important consideration. An estimated 1% to 3% of people in Canada experience seasonal affective disorder, or SAD, while as many as 15% experience milder seasonal changes.

Those changes can affect mood, sleep, appetite and energy. Delayed circadian timing is a leading biological explanation for winter SAD, and early-morning bright light therapy can help correct that delay.

Moving sunrise later reduces the natural morning signal instead. The authors argue that this could increase vulnerability to seasonal mood disturbances, particularly among people already struggling with sleep or circadian timing.

Geographic studies provide another line of evidence. People living farther west within a time zone encounter later sunrise by the clock, and studies associate that timing with shorter sleep and several adverse health outcomes.

These findings are associations and do not establish that changing a jurisdiction’s clocks will cause every reported outcome. Still, they support the biological concerns behind recommendations for permanent standard time from major sleep and chronobiology organizations.

Permanent daylight saving time remains attractive because it preserves more usable light after school and work. Later dusk can make outdoor exercise, social gatherings and some shopping easier.

“The attraction of permanent daylight saving time is easy to understand. People like more light after work and school,” says Dr. Patrick Baylis, an associate professor at UBC’s Vancouver School of Economics. “But those evening benefits have to be weighed against evidence linking later solar time with less sleep, lower productivity, and worse educational outcomes.”

Much of the economic and educational evidence compares regions or time-zone boundaries. Those comparisons depend on assumptions about how similar the populations are.

Location also complicates policy labels. Communities near a time zone’s western edge already experience later sunrise than communities farther east, even while following identical clock time.

Saskatchewan illustrates the distinction. Although much of the province observes Central Standard Time year-round, its geographic position places it closer to Mountain Time, making its solar timing more comparable to permanent daylight saving time.

Latitude matters too. In polar regions, winter daylight is already extremely limited, so a one-hour adjustment has different consequences than it does at the midlatitudes where most Canadians live.

Adolescents may face particular difficulties because puberty naturally shifts sleep timing later. Early school schedules already conflict with that shift, and later winter dawn could deepen the mismatch.

The authors also highlight early-start and shift workers, people with caregiving obligations, and those who already sleep too little. Evening chronotypes and people with sleep disorders may be especially sensitive to delayed morning light.

Travel conditions add another concern. Pedestrians, cyclists and public transit users may spend more of their morning journeys in darkness, particularly where street lighting and sidewalk infrastructure are inadequate.

Rural and lower-income communities could therefore face disproportionate burdens. People with flexible schedules may be better positioned to enjoy the evening benefits.

These resources examine clock policies, circadian disruption and how daily schedules affect sleep and health.

Permanent standard time is the optimal choice for health and safety: an American Academy of Sleep Medicine position statement: Explains why the academy recommends year-round standard time and reviews the biological basis for that position. (Journal of Clinical Sleep Medicine, 2024)

The effects of daylight saving time and clock time transitions on sleep and sleepiness: a systematic review: Reviews 27 studies, highlighting sleep disruption around clock changes and the methodological limits of the evidence. (Sleep Medicine Reviews, 2025)

Daylight Saving Time Transitions and Risk of Heart Attack: A Systematic Review and Meta-Analysis: Examines heart attack risk following seasonal clock changes, providing context for transition-related health concerns. (Deutsches Ärzteblatt International, 2024)

School Start Times, Sleep, and Youth Outcomes: A Meta-analysis: Synthesizes research on school timing, sleep and developmental outcomes, helping explain why adolescents deserve special consideration. (Pediatrics, 2022)

Sunset time and the economic effects of social jetlag: evidence from US time zone borders: Investigates how later sunset timing relates to sleep and economic outcomes across US time-zone borders. (Journal of Health Economics, 2019)

Research findings are available online in the Canadian Medical Association Journal.

The original story "Canadian analysis weighs the health trade-offs of permanent standard time versus daylight saving time" is published in The Brighter Side of News.

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