Ending Clock Changes? Health Experts Say There's a Winner Between Standard And Daylight Saving Time

Keeping the clocks on summer time all year means lighter evenings, but it also delays winter sunrise by an hour on the clock. People getting up early for work or school spend more of the morning in darkness.

A new Canadian analysis concludes that permanent standard time is the better choice for health if seasonal clock changes are abolished, because it preserves earlier access to morning light.

British Columbia adopted permanent daylight saving time in March. Alberta and the Northwest Territories have also adopted year-round daylight saving time and will skip the November clock change. In the US, the House passed a bill to make daylight saving time permanent on July 14, and it was referred to a Senate committee the following day.

Psychiatrist Raymond Lam of the University of British Columbia and his colleagues assessed the health consequences of the two options: standard time, usually observed in winter, and daylight saving time, which runs an hour ahead.

Sleep and circadian rhythms researcher Ralph Mistlberger of Simon Fraser University co-authored the peer-reviewed analysis, which reviewed published research on biological rhythms, sleep, mood, education, and economic effects. The team also examined countries' experiences with permanent time policies and compared sunrise and sunset times across Canadian cities.

In Toronto, the latest winter sunrise, in early January, would come at 7:51 am under permanent standard time and 8:51 am under permanent daylight saving time.

A Toronto resident waking at 7 am in early January would get up before sunrise under either setting, but daylight would arrive an hour sooner under standard time. Neither option changes the length of the day.

The brain's main circadian clock coordinates sleep, alertness, and other daily functions. It needs daily adjustment to stay in step with the day. Morning light helps move it earlier, while evening light pushes it later.

"Imagine a wristwatch that runs 10 minutes slow each day, and every morning you make the 10 minute correction by setting it forward," Mistlberger explained to ScienceAlert.

With less morning light, sleep rhythms can slip later while school and work still begin at the same hour. Getting up on time may mean cutting sleep short.

Social jetlag, the gap between the body clock and school or work schedules, has been linked to poorer physical and mental health, as has insufficient sleep. Seasonal clock changes can also disrupt sleep.

The analysis also draws on geographic comparisons: people living farther west within a time zone get later sunrises by the clock. Studies have linked this position with shorter sleep and some poorer health and economic outcomes, though these associations do not prove causation.

The researchers considered seasonal affective disorder, a form of depression often experienced in winter. One leading explanation involves the body clock slipping later when morning light is delayed. Morning bright-light therapy helps move it earlier; a later sunrise on the clock works in the opposite direction.

Teenagers and young adults may be especially vulnerable because their body clocks naturally shift later at that age.

"Later school and work times would help," Mistlberger said. Brighter indoor lighting in the morning could also help, he added.

Several countries have tried year-round daylight saving time before. The US introduced it in 1974 and abandoned it within the year, following concerns about dark winter mornings and children traveling to school before sunrise. Russia adopted it in 2011 and reversed it in 2014.  

Permanent daylight saving time would keep evenings lighter, allowing more time for outdoor activity and socializing. Standard time brings earlier winter dusk, while early summer sunrise may wake some people too early. Latitude and location within time zones also determine how much the choice matters in a given place.

Much of the evidence comes from observational research and geographic comparisons, leaving uncertainty about permanent policies' long-term health consequences.

The researchers recommend permanent standard time. If governments choose year-round daylight saving time, they advise considering later school and work starts, improving morning light exposure, and tracking whether sleep problems, winter depression, absenteeism, and road injuries rise.

The research has been published in Canadian Medical Association Journal.