What does the science say about Daylight Saving Time?

Health Policy

Q: What does the science say about Daylight Saving Time?

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), they prefer permanent Standard Time (which has more light at the beginning of the day).

The United States Senate recently voted in support of permanent Daylight Saving Time. If this bill is approved by the House and signed by President Biden, the US would abolish the disruptive twice yearly clock change in 2023. 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.

The more important conversation is whether clocks should shift to permanent Daylight Saving Time (as proposed by the Senate) 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 one may get more hours per day of sunlight, but in reality, the plan just shifts total bright hours from when the body and brain need them in the morning to later in the afternoon and early evening. It is both less healthy and less safe to shift our daylight hours from the morning 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 the body out of sync with the environmental clock.

• 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 consistent year-round time system is agreed upon by the National Sleep Foundation, American Academy of Sleep Medicine, National Safety Council, 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 sleep, and the better choice for overall health and well-being.



The National Sleep Foundation Standard Time Position Statement

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

Washington Post coverage of Daylight Saving bill