A study from researchers at Stanford Medicine underscores the health risks associated with the biannual clock changes due to daylight saving time (DST). Many Americans express dissatisfaction with this practice, which not only disrupts their routines but may also have adverse health effects. The loss of an hour of sleep on the second Sunday in March has been linked to an increase in heart attacks and fatal traffic accidents in the days that follow.
Read More
The researchers evaluated three time policies: permanent standard time, permanent daylight saving time, and the current system of biannual shifts. They assessed how these policies could impact circadian rhythms—a natural internal clock that regulates various bodily functions.
The team concluded that both permanent standard time and permanent daylight saving time would be healthier alternatives to the current system, with permanent standard time offering the greatest benefits for public health.
There is ongoing debate among proponents of two potential policies. Supporters of permanent daylight saving time argue that extended evening light would conserve energy, reduce crime, and provide more leisure time after work. However, a trial of this policy in 1974 faced significant public backlash and was discontinued. Despite this, efforts to implement permanent daylight saving time continue, with proposals introduced in Congress every year since 2018, though none have passed.
Conversely, advocates for permanent standard time, including the American Academy of Sleep Medicine and the National Sleep Foundation, argue that more morning light is crucial for better health outcomes. Although these endorsements are based on the theory that morning light is beneficial, the new study provides data supporting this claim.
As the research indicates, the human circadian cycle is typically slightly longer than 24 hours—around 12 minutes—and can be adjusted by exposure to light. The study's findings suggest that reducing evening light exposure and maximizing morning light exposure allows for better synchronization to a 24-hour cycle.
Using mathematical modeling, the researchers calculated how light exposure would vary under different time policies, taking into account local sunrise and sunset times. They found that permanent standard time would minimize circadian burden for most people over the course of a year. However, morning larks, approximately 15% of the population with shorter circadian cycles, would benefit more from permanent daylight saving time due to increased evening light.
Health implications from the research predict that permanent standard time could lower overall obesity rates by 0.78 percentage points—equivalent to about 2.6 million people—and reduce stroke occurrences by 0.09 percentage points, amounting to 300,000 fewer cases. Permanent daylight time would also yield positive effects, decreasing obesity by 0.51 percentage points and strokes by 0.04 percentage points but to a lesser extent.
Although this study is a significant contribution to understanding the health impacts of time policy, it is not definitive. Factors such as weather, geography, and individual behaviors, which can affect real-life light exposure, were not fully accounted for. Researchers assumed ideal light habits that may not reflect the reality of most people's routines.
Zeitzer emphasizes that while the findings favor permanent standard time for circadian health, broader considerations must also be factored into any policy decisions regarding time changes. The study was funded by the National Institutes of Health (grant F31HL170715).