Health evidence favors permanent standard time over permanent daylight saving time to better align human circadian rhythms with natural light, according to a medical analysis published in the Canadian Medical Association Journal (CMAJ). The findings suggest that while permanent daylight saving time offers brighter evenings, it risks worsening sleep disruption and seasonal mood disorders by delaying winter sunrises, The Brighter Side of News reported.
Morning light regulates the internal body clock
Daylight saving time does not increase the total amount of daylight available; instead, it shifts sunrise and sunset one hour later on the clock. This redistribution of light affects the brain’s master circadian clock, which typically operates on a cycle of approximately 24.2 hours. Environmental signals are required to synchronize this internal clock with the 24-hour day.
Morning light generally advances the internal clock, which counteracts its natural tendency to run late. Conversely, evening light delays the clock. Dr. Ralph Mistlberger, a psychology professor at Simon Fraser University, stated that biological clocks need morning light to stay synchronized and that permanent daylight saving time moves that light an hour later during winter mornings when it is already darkest.
This mismatch between biological timing and fixed social schedules, such as school or work, is often referred to as social jet lag. The analysis notes that indoor living and electric lighting already encourage this misalignment.
Winter sunrise delays impact seasonal mood
The CMAJ analysis identifies seasonal mood vulnerability as a primary concern. Between 1% and 3% of Canadians experience seasonal affective disorder (SAD), while up to 15% experience milder seasonal changes that affect energy, appetite, sleep, and mood.
Delayed circadian timing is a leading biological explanation for winter SAD. Because early-morning bright light therapy is used to correct this delay, the authors argue that moving the sunrise even later reduces the natural morning signal. This could increase vulnerability to mood disturbances, particularly for those already struggling with sleep.
Geographic data supports these concerns. Studies associate later sunrises—common for people living on the western edge of a time zone—with shorter sleep durations and various adverse health outcomes.
Evening light benefits contrast with health costs
Permanent daylight saving time remains popular because it provides more usable light after work and school, facilitating outdoor exercise and social gatherings. However, Dr. Patrick Baylis, an associate professor at the UBC Vancouver School of Economics, noted that these benefits must be weighed against evidence linking later solar time with lower productivity, worse educational outcomes, and less sleep.
The impact of these policies varies by geography and latitude. In polar regions, where winter daylight is already extremely limited, a one-hour adjustment has different consequences than it does at midlatitudes. Saskatchewan serves as a practical example; while much of the province uses Central Standard Time year-round, its geographic position makes its solar timing more similar to permanent daylight saving time.
Specific groups face higher risks from darker mornings
Certain populations are more susceptible to the negative effects of delayed morning light:
- Adolescents: Puberty naturally shifts sleep timing later. Later winter dawns can deepen the conflict between biological needs and early school start times.
- Shift Workers and Caregivers: Those with early-start obligations or those who already suffer from sleep deprivation may face increased burdens.
- Commuters: Pedestrians, cyclists, and public transit users may spend more of their morning journeys in darkness, particularly in rural or lower-income areas where street lighting is inadequate.
Lead author Dr. Raymond Lam, a psychiatry professor at the University of British Columbia, stated, “Ending the clock change solves one problem, but it creates another decision: which time should we live on year-round?”
Common questions about permanent time changes
Does permanent daylight saving time provide more sunlight?
It moves one hour of light from the morning to the evening, meaning the sun rises later and sets later according to the clock.
Why is standard time preferred by sleep experts?
Standard time keeps the clock closer to solar time. This ensures that the morning light—which is critical for resetting the circadian clock—occurs earlier, supporting better sleep alignment and mood stability.
Who benefits most from permanent daylight saving time?
People with flexible schedules and those who prioritize evening outdoor activities generally benefit most from the extra hour of light after the traditional workday.
“The health evidence points to standard time, largely because morning light matters for our body clocks, sleep, and mood.” — Dr. Raymond Lam