As clocks prepare to shift again this fall, a new analysis is reviving a long-running debate among sleep scientists: if governments are going to stop changing the clocks twice a year, which fixed time should they lock in?
The analysis, published October 5 in the Canadian Medical Association Journal and led by Dr. Raymond Lam, a professor of psychiatry at the University of British Columbia, concludes that the weight of health evidence favors permanent standard time over permanent daylight saving time — a position shared by major sleep and chronobiology societies, but one that cuts against the policy direction several governments, including a number of U.S. states, have leaned toward in recent years.
Why Morning Light Is the Deciding Factor
The case for standard time comes down to a basic feature of human circadian biology: morning light helps synchronize the body's internal clock to a 24-hour day, while evening light delays it. Permanent daylight saving time pushes clock time an hour ahead of solar time year-round, which means darker mornings and later sunrises, especially in winter.
Under permanent daylight saving time, the analysis notes, that circadian misalignment does not go away once people adjust — it persists all winter, because the sun itself does not rise any earlier just because the clock says it should. Permanent standard time keeps clock time aligned with solar time, preserving earlier morning light exposure even as evenings grow darker sooner.
The authors argue this is more than a minor inconvenience. Morning light exposure is one of the most effective tools for regulating sleep timing, mood, and alertness, and chronically reduced morning light has been linked to worse sleep quality and higher rates of seasonal low mood.
The Seasonal Affective Disorder Connection
The analysis points to seasonal affective disorder (SAD) as a concrete illustration of the stakes. An estimated 1% to 3% of Canadians experience clinical SAD, while as many as 15% experience a milder seasonal pattern of low mood, disrupted sleep, reduced energy, and appetite changes tied to shrinking daylight. Because permanent daylight saving time delays morning light exposure further into the winter months, the authors argue it would likely intensify these seasonal effects for people already vulnerable to them.
Adolescents are flagged as a particular concern. Puberty naturally shifts the body's preferred sleep timing later, a mismatch that already collides with early school start times. Pushing clock time an hour further from solar time, as permanent daylight saving time would, risks widening that gap and cutting further into teenagers' morning light exposure and sleep duration.
Acknowledging the Case for the Other Side
The analysis does not dismiss the appeal of permanent daylight saving time. Lighter evenings make it easier to exercise, socialize, shop, or simply spend time outdoors after work and school — benefits that have real value for public life and local economies, and which explain why keeping evening light has broad public support.
The twice-yearly clock change itself carries its own well-documented risks that neither option eliminates on its own. Prior research has tied the spring shift into daylight saving time to a short-term spike in heart attacks — one widely cited analysis found a roughly 24% jump in heart attack incidence the Monday after the spring transition — along with a measurable rise in fatal car crashes in the days that follow, attributed to lost sleep and circadian disruption. Ending the biannual switch, in either direction, would remove that acute twice-a-year disruption; the CMAJ analysis is concerned with which permanent option minimizes the ongoing, lower-grade cost that remains after the switching stops.
What This Means for Patients
For individuals, the actionable takeaway is less about which policy a government eventually chooses and more about protecting morning light exposure now, regardless of the clock. Getting outside or near a bright window soon after waking, especially in fall and winter, helps anchor the body clock and can blunt seasonal dips in mood and sleep quality — the same mechanism the analysis says makes standard time preferable at a population level. People who notice a seasonal pattern of oversleeping, low energy, or low mood as mornings grow darker should talk to a clinician about screening for seasonal affective disorder rather than assuming it is an inevitable part of the season; light therapy, timed appropriately, is an evidence-based treatment. And for anyone advocating on this issue locally, the authors' core message is that policymakers should not treat "end the clock changes" as a settled question — the choice of which fixed time to adopt carries its own distinct, and unequal, health consequences.
