What permanent daylight saving time could mean for sleep, mood, and mental health

As lawmakers across North America move to end the twice-yearly ritual of changing the clocks, three new papers highlight a complex debate over how clock shifts and permanent time systems affect health. A policy analysis published in CMAJ argues that permanent standard time would better support winter sleep, mood, and biological rhythms than permanent daylight saving time, and a review focused on people with mental illness reaches the same conclusion. A large review of studies tracking health patterns across whole populations, however, finds that seasonal clock changes and summer daylight saving time carry a mix of harms and benefits rather than being uniformly harmful.

Although major sleep societies have recommended adopting permanent standard time, North American governments eliminating the clock switch have largely chosen permanent daylight saving time instead. Yukon adopted permanent daylight saving time in 2020, followed by British Columbia in March 2026 and Alberta and the Northwest Territories in April 2026, while the United States House of Representatives passed a bill in July 2026 to make daylight saving time permanent nationwide. Public opinion does not necessarily match that legislative push: in a 2025 Gallup poll, only 19% of Americans wanted to keep switching the clocks, while 48% preferred permanent standard time and 24% favored permanent daylight saving time.

How clock time interacts with the body’s internal clock

Standard time is a timekeeping system designed so that 12:00 noon on the local clock roughly matches solar noon, the moment when the sun reaches its highest point in the sky along a time zone’s central line of longitude. Daylight saving time shifts local clocks forward by one hour in the spring and returns them to standard time in the autumn, which does not create extra daylight but moves an hour of sunlight from the morning to the evening.

Both schedules interact directly with circadian rhythms, the internal 24-hour biological cycles that regulate sleep, alertness, hormone production, and cellular activity throughout the body. These daily rhythms are coordinated by the suprachiasmatic nucleus, a tiny cluster of nerve cells in the hypothalamus at the base of the brain that acts as the body’s master pacemaker.

Because the human master clock naturally runs slightly slower than a 24-hour day, averaging about 24.2 hours per cycle, it relies on natural daylight hitting the eyes to reset itself each morning. Bright morning sunlight speeds the internal clock up so people feel alert during the day and sleepy at night, whereas evening light slows the clock down and pushes sleepiness later. Modern habits like working indoors and looking at illuminated screens at night already weaken morning light cues and strengthen evening ones, creating a mismatch known as social jet lag, where work or school schedules force people to wake up before their delayed biological clocks are ready.

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Public frustration with biannual clock shifts has grown alongside evidence that changing the schedule disrupts well-being; for instance, a study covered by PsyPost earlier this year found that both seasonal transitions triggered drops in public mood, with the autumn return to standard time producing a more lasting dip than the spring shift. Beyond temporary mood dips, the authors of a 2015 review argued that chronically disrupted or shortened sleep can contribute to the development of depression.

Canada’s choice between two permanent time systems

To evaluate what happens after a jurisdiction abolishes seasonal clock changes, Raymond W. Lam of the University of British Columbia and colleagues at Simon Fraser University and Vancouver Coastal Health conducted the public health policy analysis in CMAJ. Rather than debating whether twice-yearly transitions should end, the authors examined the chronic, year-round consequences of adopting either permanent standard time or permanent daylight saving time across Canada. They calculated winter and summer sunrise and sunset times across six major Canadian cities, synthesized evidence from biological rhythm research, disease-modeling simulations, and economic border studies, and reviewed lessons from countries that previously enacted permanent daylight saving time.

Lam and colleagues showed that under permanent daylight saving time, winter sunrises would occur around or after the start of many school days across much of Canada. By early January, switching from standard time to permanent daylight saving time shifts the latest winter sunrise from 7:51 a.m. to 8:51 a.m. in Toronto, from 8:08 a.m. to 9:08 a.m. in Vancouver, from 8:27 a.m. to 9:27 a.m. in Winnipeg, and from 8:40 a.m. to 9:40 a.m. in Calgary.

The authors also noted that political time zone boundaries often distort local solar time, pointing out that while Saskatchewan officially observes year-round Central Standard Time, much of the province sits geographically closer to Mountain Time and actually experiences light schedules resembling permanent daylight saving time. Similarly, clock policies have the greatest impact in mid-latitude regions where most Canadians live, whereas high-latitude territories like Yukon have such brief winter daylight that a one-hour shift makes little practical difference.

What darker winter mornings could mean for health

According to Lam and colleagues, prolonged winter morning darkness under permanent daylight saving time would likely worsen social jet lag and seasonal mood vulnerability. Winter dips in sleep, concentration, and energy are already most prominent among youth aged 12 to 17. An estimated 1% to 3% of Canadians experience seasonal affective disorder, a form of winter depression thought to involve a delay in the body clock linked to later morning light, while up to 15% experience milder winter slumps in mood, sleep, and energy.

Geographic patterns point in the same direction, with global prevalence of seasonal affective disorder rising by 0.2 percentage points for every 1-degree increase in northern latitude. In United States county-level analyses, residents on the western edges of time zones, who experience later clock sunrises and sunsets similar to permanent daylight saving time, averaged up to 2 additional suicide deaths per 100,000 people annually compared to those on the eastern edges (the epidemiological review described below rated this study as low quality).

In addition, a 2025 modeling study simulating chronic disease prevalence across the United States indicated that while both permanent policies improved health over twice-yearly switching, the health benefits of permanent daylight saving time were one-third smaller than those of permanent standard time, especially at northern latitudes comparable to Canada.

Comparisons of places with different sunset times indicate that an hour of later sunset is associated with nearly one hour less sleep per week and about 5% lower long-run earnings, and a time-zone border study found that workers on the late-sunset side earn about 3.5% less. In Florida, an extra hour of morning sunlight before school was linked to higher adolescent math and reading scores by 0.08 and 0.06 standard deviations, a gain comparable to reducing class sizes.

Trade-offs and lessons from other countries

Although permanent daylight saving time provides brighter evenings that can support outdoor recreation and retail spending for people with flexible schedules, its dark winter mornings could disproportionately affect teenagers, shift workers, and lower-income or rural commuters traveling on poorly lit roads.

Permanent standard time has trade-offs of its own: summer sunrises would come as early as about 4 a.m. in cities such as Vancouver and Montréal, which the authors note could wake some people too early, and winter sunsets would arrive as early as about 4:10 p.m. International experience shows how these trade-offs can play out: the United States enacted permanent daylight saving time in 1974 but repealed it within a year after widespread public backlash over dark winter school commutes; Russia, which adopted permanent daylight saving time in 2011, reversed it in 2014, and Chile reversed its 2015 move after complaints.

Lam and colleagues concluded that health evidence favors permanent standard time, and that provinces choosing permanent daylight saving time should pair it with later school start times, morning indoor light interventions, pedestrian safety upgrades, and ongoing public health monitoring.

A review of 157 studies finds a more mixed picture

While the Canadian analysis examined which permanent clock system is healthier once seasonal transitions end, a systematic review published in the European Journal of Epidemiology questioned whether the existing epidemiological evidence truly justifies abolishing the twice-yearly clock shifts in the first place. Led by first author Aiste Steponenaite of the University of Kent and the University of Greenwich and senior author Philip Lewis of the University of Cologne, the researchers searched five databases through June 2025 and identified 157 human epidemiological studies across 36 countries.

Two independent reviewers rated each study’s methodological quality as low, medium, or high, giving greater weight to large registry analyses, which draw on national health records, and to studies that included placebo tests, which check whether similar health changes happen in years, dates, or neighboring regions without a clock shift.

Across the 157 studies, roughly one in five of which met high-quality standards, the researchers found that neither the seasonal transitions nor summer daylight saving time were uniformly harmful. Among 17 studies of acute myocardial infarction, commonly known as a heart attack (6 rated high quality and 10 medium quality), the evidence pointed to an elevated risk during the first one to four weeks after the spring shift, corresponding to a relative risk increase of about 4% in a 2024 meta-analysis by two of the review’s authors. Among 16 studies of fatal traffic crashes, the evidence pointed to an increase after the spring shift in some places, mainly the United States. At the same time, a high-quality nationwide United States study found that changes in all-cause mortality, or deaths from any cause, after the spring shift were not statistically significant.

Crimes involving physical harm, such as robberies and assaults, tended to drop briefly after the spring shift and rise after the autumn shift. The autumn transition back to standard time was also linked to a roughly 2.5% relative decrease in all-cause mortality and fewer workplace accidents, each based on a single high-quality study. Living under daylight saving time during the summer months was associated with an approximately 4.5% relative reduction in all-cause mortality in one high-quality United States study and with fewer traffic accidents in Chile.

Across 22 sleep duration studies, 6 circadian rhythm studies, and 15 psychiatric outcome studies, Steponenaite and colleagues found no consistent population-wide changes in sleep duration after the spring shift or in psychiatric hospital admissions and suicides after either transition, although two low-quality studies from Russia did indicate that maintaining daylight saving time through the winter shortened sleep duration. Because summer daylight saving time and the autumn shift were associated with mortality and safety benefits, the authors suggested that retaining seasonal clock transitions while using targeted preventive measures around the spring shift, such as gradual schedule adjustments for heart patients and enhanced road safety enforcement, remains a reasonable option.

Notably, the review also concluded that if transitions were abolished, the limited sleep evidence and circadian theory would favor permanent standard time over permanent daylight saving time, the same choice the CMAJ authors recommend. Where the papers part ways is on whether ending the twice-yearly changes should be the priority, a split visible in the CMAJ analysis itself, which counts the mental-health review among papers supporting an end to clock changes and the epidemiological review among those questioning that consensus.

Why people with mental illness may be especially sensitive

Why did the epidemiological review by Steponenaite and colleagues find no consistent population-level spike in psychiatric emergencies after clock changes, even though sleep loss is closely tied to mental illness? A comprehensive review published in Brain Sciences, led by Liahm Blank, Joshua Khorsandi, Jagdish Khubchandani, and Kavita Batra at the University of Nevada, Las Vegas, and New Mexico State University, offers one possible explanation for this discrepancy by focusing specifically on individuals with chronic mental illness.

The authors searched four databases through February 2026 and synthesized 66 peer-reviewed studies published between 2000 and 2026, grouping them into 12 epidemiological studies that directly evaluated daylight saving time transitions, 36 observational studies linking sleep or circadian disruption to mental health symptoms, and 18 experimental studies examining biological pathways across depression, anxiety disorders, bipolar disorder, PTSD, ADHD, and psychotic disorders.

Blank and colleagues noted that while most healthy adults adjust to a one-hour clock change within two to seven days, individuals with pre-existing psychiatric conditions often have fragile circadian timing systems that may adapt slowly or incompletely. They argue that large hospital and emergency department registries mainly capture severe crises and may miss everyday symptom flare-ups, anxiety, and mood instability. For example, a Finnish nationwide analysis and a United States emergency department study found no statistically significant increase in hospital-treated manic or psychotic episodes in the two weeks after clock transitions.

Even in population registries, some specific mental health risks have emerged, including an 11% relative increase in depressive episodes after the autumn transition and a 6.25% relative increase in United States suicide rates following the spring transition. The epidemiological review rated those same depression and suicide studies as medium quality, and its psychiatric conclusion leaned more heavily on a high-quality study from Austria, Switzerland, and Sweden that found no change in suicide rates around clock changes.

Blank and colleagues also explained that because rapid eye movement sleep, the dreaming stage essential for processing emotions and consolidating memories, occurs mostly in the final hours before waking, losing an hour of morning sleep after the spring transition may selectively cut off rapid eye movement sleep even when total sleep loss seems minor.

Across specific psychiatric conditions, modest clock disruptions may interact with baseline sleep difficulties, as sleep disturbances are reported in up to 90% of people with PTSD, and wearable-tracker research in schizophrenia has found severe circadian misalignment in roughly half of clinically stable patients. Similarly, an estimated 73% to 78% of individuals with ADHD have a delayed sleep-wake cycle, and one experiment in children with ADHD found that losing one hour of nightly sleep for a week pushed attention scores from below the clinical cutoff into clinically impaired ranges.

Blank and colleagues proposed three biological pathways that may underlie this sensitivity. First, delayed morning light may alter signals from the eyes to the hypothalamus and disrupt the nighttime release of melatonin, a hormone that promotes sleep. Second, sleep loss may activate the body’s main stress-response system, raising levels of the stress hormone cortisol and of inflammatory proteins. Third, variations in clock genes, which keep time inside individual cells, may make it harder for cells in people with bipolar disorder or schizophrenia to adjust to an abrupt time shift.

Converging with the CMAJ policy analysis, Blank and colleagues favored adopting permanent standard time to protect vulnerable psychiatric populations and recommended that clinicians help high-risk patients shift their sleep schedules by 15 to 20 minutes per day over the three to four days before a transition.

Caveats and next steps

As with all research, there are some caveats to keep in mind when interpreting these three papers. Nearly all population-level findings on daylight saving time rely on observational or quasi-experimental data, which take advantage of natural events such as clock changes, rather than on randomized trials. That means researchers cannot establish direct cause and effect, and studies comparing opposite edges of time zones must assume that neighboring communities do not differ in other lifestyle or economic ways.

Each paper also has its own limitations. The CMAJ paper is a solicited policy analysis rather than a systematic review and relies partly on unpublished data for its claims about evening leisure benefits. In the systematic review of 157 epidemiological studies, roughly four out of five papers were rated as low or medium quality, and placebo tests using non-transition dates or regions made some previously publicized effects disappear, such as supposed clock-shift impacts on stock market decisions.

The mental-health paper was a narrative review, which summarizes studies without the strict search and selection rules of a systematic review, and a single author screened studies for inclusion. Only 12 of its 66 included papers directly measured outcomes during daylight saving time transitions; the rest inferred risk from broader sleep and circadian research that often relied on small clinical samples or self-reported sleep questionnaires. Because sleep disruption and mental illness influence each other in both directions, it remains difficult to separate how much the clock shift itself drives symptom changes.

Looking ahead, the CMAJ authors urged provinces such as British Columbia and Alberta to treat their policy changes as an opportunity to monitor school attendance, traffic injuries, cardiovascular events, and mental health visits over time, while the other two teams called for longer-term studies with objective sleep measures and placebo tests.

The paper, “Analysis of health impacts associated with adopting permanent standard time or permanent daylight saving time to inform policy in Canada,” was authored by Raymond W. Lam, Ralph Mistlberger, Patrick Baylis, and Alexandra Choi.

The paper, “A systematic review of epidemiological studies into daylight-saving time & health identifying beneficial & adverse effects,” was authored by Aiste Steponenaite, Jonas P. Wallraff, Ursula Wild, Lorna Brown, Ben Bullock, Gurprit S. Lall, Sally Ferguson, Russell G. Foster, Jennifer Walsh, Greg Murray, Thomas C. Erren, and Philip Lewis.

The paper, “When the Clock Shifts: A Comprehensive Review of Daylight-Saving Time (DST), Circadian Disruption, and Neuropsychological Risk in Chronic Mental Illness,” was authored by Liahm Blank, Joshua Khorsandi, Elizabeth England-Kennedy, Srikanta Banerjee, Karen Kopera-Frye, Roberto Sagaribay, Jagdish Khubchandani, and Kavita Batra.