The Health Implications of Daylight Saving Time
Every spring, Americans adjust their clocks forward to daylight saving time, and every fall, back to standard time. While this process is a long-standing tradition, it is often met with inconvenience and concerns regarding its effects on health. Research indicates that the biannual time shift not only disrupts sleep but is also correlated with an increase in heart attacks and fatal traffic accidents in the following days.
Research by Stanford Medicine
A recent study conducted by researchers at Stanford Medicine reveals that the health impacts of changing time policies extend beyond immediate effects, highlighting potential long-term hazards as well as better alternatives.
The researchers evaluated three different time policies: permanent standard time, permanent daylight saving time, and the current biannual transition. The goal was to understand how these policies affect individuals’ circadian rhythms, which regulate various physiological processes in the body.
The findings suggest that, from a circadian perspective, the current practice is suboptimal. Both permanent standard time and permanent daylight saving time are better options than the seasonal switching, with permanent standard time potentially benefiting the largest number of individuals.
By analyzing light exposure and health characteristics at the county level, the researchers estimate that maintaining permanent standard time could prevent approximately 300,000 cases of stroke annually and reduce the prevalence of obesity by 2.6 million individuals. Permanent daylight saving time may offer about two-thirds of these benefits.
“We found that choosing either standard time or daylight saving time is significantly better than making biannual changes,” notes Dr. Jamie Zeitzer, professor of psychiatry and behavioral sciences and the senior author of the study published in the Proceedings of the National Academy of Sciences on September 15. The lead author of the study, Lara Weed, is a graduate student in bioengineering.
Debate Over Time Policies
The discussion surrounding seasonal time adjustments is marked by divergent opinions regarding which time policy to adopt. Supporters of permanent daylight saving time argue that extended evening light could enhance energy savings, deter crime, and provide additional leisure time after work hours. However, a trial of permanent daylight saving time conducted in 1974 was met with public disapproval and was discontinued within a year, partly due to concerns from parents regarding children traveling to school in darkness.
Despite increased adoption of daylight saving time from six months to seven months and the recurring introduction of a bill for permanent daylight saving time in Congress since 2018, a definitive resolution remains elusive.
Conversely, proponents of permanent standard time argue that increased morning light aligns better with health optimization. Organizations including the American Academy of Sleep Medicine, the National Sleep Foundation, and the American Medical Association have endorsed year-round standard time, citing that early morning light may promote overall wellness.
“While the theory support for permanent standard time is grounded in reasoning, it lacked empirical data until now,” Dr. Zeitzer explained.
The Importance of Circadian Synchronization
The human circadian cycle is not precisely 24 hours; for most individuals, it averages around 24 hours and 12 minutes. It can, however, be influenced by light exposure. Proper light exposure is essential for maintaining an accurate circadian cycle, whereby morning light accelerates this cycle and evening light can slow it down. Insufficient synchronization of the circadian cycle has been linked to a range of adverse health outcomes.
The researchers utilized mathematical models to assess how light exposure under each time policy impacts circadian burden, which refers to the extent to which individuals must adjust their internal clocks to align with a 24-hour day. Their analysis revealed that permanent standard time leads to the least circadian burden for the majority of the population.
The findings also indicated that while most individuals would benefit from permanent standard time, those who identify as “morning larks”—approximately 15% of the population—would face less circadian burden under a permanent daylight saving time due to their natural cycles being shorter than 24 hours.
Health Outcomes Linked to Circadian Burden
To connect circadian burden with specific health outcomes, the research team analyzed county-level data from the Centers for Disease Control and Prevention regarding the prevalence of conditions such as arthritis, cancer, chronic obstructive pulmonary disease, coronary heart disease, depression, diabetes, obesity, and stroke.
Their models suggest that maintaining permanent standard time could reduce the nationwide prevalence of obesity by 0.78%, equating to approximately 2.6 million fewer individuals suffering from this condition, while also lowering stroke incidence by 0.09%, corresponding to 300,000 fewer cases. Under permanent daylight saving time, obesity prevalence might decrease by 0.51% (around 1.7 million individuals) and stroke occurrences by 0.04% (around 220,000 cases).
Notably, the models predicted no significant changes in conditions such as arthritis, which do not have a direct association with circadian rhythms.
Final Thoughts
While this study is one of the first comprehensive analyses of the long-term health effects of different time policies, Dr. Zeitzer notes that it is not the final word on the subject. Various factors such as weather, geography, and human behavior that could influence real-life light exposure were not considered in this analysis.
Moreover, the researchers based their models on idealized light habits, assuming an average sleep schedule of 10 p.m. to 7 a.m. and consistent exposure to daylight and indoor light. Real-world variability in sleep patterns and light exposure can vastly differ.
Nonetheless, while the data suggests a benefit of permanent standard time, it remains a vital consideration among many factors in the ongoing debate regarding time policy. As Dr. Zeitzer points out, time policy ultimately determines which clock hours represent sunrise and sunset without changing the total amount of available daylight.
Key Health Takeaway
Research indicates that both permanent standard time and daylight saving time are healthier options than the current practice of biannual time shifts. Permanent standard time may offer the most significant health benefits, potentially reducing obesity and stroke incidence significantly across the population.
