This weekend, many countries will move their clocks one hour back to return to winter, or “standard,” time — the one more closely aligned with the sun — after months of the summer schedule.
Clock changes happen twice a year and although there have been multiple attempts to end them, every spring and autumn the clocks still shift by an hour.
The seasonal time change was brought in to save energy during the First World War and was revived in many countries in the 1970s.
Researchers now warn, however, about the health impacts, especially of the spring change.
Yet deciding whether to keep the practice is only the first step.
“Ending the clock change solves one problem, but it creates another decision: which time should we live on year-round?” said Raymond Lam, professor of psychiatry at the University of British Columbia and lead author of a recently published analysis on the health effects of daylight saving time.
While eliminating clock changes resolves short-term disruption, the permanent system chosen determines chronic, long-term public health outcomes, the authors wrote.
“The health evidence points to standard time, largely because morning light matters for our body clocks, sleep, and mood.”
Both choices have pros and cons: permanent daylight saving time keeps evenings lighter, which may support recreation, social activity, commuting and some economic activity.
However, it also pushes sunrise an hour later, leaving more people waking, travelling to school and commuting to work in darkness during the winter.
“Our biological clocks need morning light to stay synchronized with the 24-hour day,” said Ralph Mistlberger, professor of psychology at Simon Fraser University in Canada. “Permanent daylight saving time moves that light an hour later just when winter mornings are already darkest.”
Permanent standard time preserves earlier winter dawns, which naturally align with human circadian rhythms by providing essential morning light to synchronise the biological clock.
Morning exposure to light helps compensate for the biological clock's natural tendency to lag, reducing sleepiness and supporting alertness, sleep quality and mood, the analysis noted.
The analysis identifies standard time or winter time as the best option for health.
“Time policy is a public health policy,” said Alexandra Choi, medical health officer with Vancouver Coastal Health.
“If governments choose permanent daylight saving time, they should plan for the consequences before the first winter — from safer morning travel and flexible start times to monitoring sleep, mental health, and injuries.”