What does the science say about Daylight Saving Time?

Health & Wellness

A: TL; DR. Permanent Standard Time is best for our sleep and circadian biology, which is fundamental to overall health and well-being.

Sleep and circadian scientists advocate for ending the twice annual shifting of the clock. However, rather than switching to permanent Daylight Saving Time (which has more light at the end of the day), evidence shows that permanent Standard Time (which has more light at the beginning of the day) is linked with better health outcomes.

In March 2023, Senator Rubio reintroduced a bill that would move the country to permanent Daylight Saving Time. If this bill passes, the US would abolish the disruptive twice yearly clock change. There is general agreement from both the public and the scientific community that changing the clocks twice a year is inconvenient, unhealthy, and risky.

• The National Sleep Foundation’s 2021 Sleep in America® Poll found that nearly three-quarters of Americans prefer a consistent year-round time system.

• The shift to Daylight Saving Time in the spring is associated with increases in motor vehicle crashes, cardiovascular morbidity, stroke, and hospital admissions.

A more nuanced discussion is necessary about whether the clocks should shift to permanent Daylight Saving Time (as proposed) or to permanent Standard Time (as preferred by sleep and circadian scientists, safety experts, and educators). It is easy to understand the appeal of having longer, brighter evenings. Permanent DST may seem like you get more hours per day of sunlight, but in reality, the plan just shifts total bright hours from when we need them in the morning to later in the afternoon. It is both less healthy and less safe to shift our daylight hours to the evening.

• Our circadian rhythms rely on bright natural light in the morning to wake us up and to synchronize important biological processes, with dimmer light in the evening to make us sleepy and ready for bed. Morning light resets the body’s biological clock and improves sleep quality and duration, bringing with it a range of physical and mental health benefits. In contrast, evening light suppresses the sleep-promoting hormone melatonin and pushes bedtimes later, reduces sleep duration, and leaves your body out of sync with the environmental clock.

• Later sunrises and sunsets are associated with shorter duration and worse quality sleep and more irregular sleep patterns. They are also associated with higher rates of obesity, depression, cardiovascular disease, and cancer. For example, a 2017 study showed that living on the west side compared to the east side of the same time zone is associated with increases in many types of cancer, including stomach, liver, prostate and non-Hodgkin lymphoma for men, and lung, breast, esophagus, and colorectum for women, and chronic lymphocytic leukemia for both genders (see Gu et al. article link below).

• Teenagers and others with night owl tendencies and people with work start times before 8 AM will be disproportionately affected.

• Morning light also improves safety, especially for school children who wait for their school buses in the dark. In fact, the United States experimented by switching to permanent DST in 1974 and quickly reverted to a bi-annual clock change later that same year because of the unpopular dark mornings and an increase in morning vehicular crashes and injuries. A recent study also found that later sunrises and sunsets that were misaligned from the sun being overhead at noon by more than 30 minutes were associated with 21.8% more fatal car crashes.

A consistent year-round time system is agreed upon by the National Sleep Foundation, American Academy of Sleep Medicine, National Safety Council and National Parent Teacher Association, and other leading sleep and science-based organizations. The conversation about clock changes is not about whether you want more or less sunshine in your day, but rather what time of day the sunshine is preferable. Science indicates that morning sunshine is best for the overall health and safety of the public, which is why these organizations agree that permanent Standard Time is a better fit for our circadian rhythms and the better choice for health and well-being.

• NSF ST Position Statement

• National Sleep Foundation’s 2021 Sleep in America® Poll Shows Gaps Between Public Sentiment and the Effects of Clock Change

• Gu et al. article on cancer rates by location within a time zone

• Sleep experts say Senate has it wrong: Standard time, not daylight saving, should be permanent

• When the US tried Permanent Daylight Saving Time in 1974

• American Academy of Sleep Medicine Statement

• Sleep research society position statement

Society for Research on Biological Rhythms Position Statement and Press Kit

• Why Should We Abolish Daylight Saving Time?

• Sunrise time charts, educational material and videos and advocacy information at Savestandardtime.com

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