Earlier this month, the Senate unanimously passed the Sunshine Protection Act. The legislation proposes making daylight saving time (DST) permanent in the U.S. One motivator is to stop the unpopular practice of switching back and forth from standard time (ST) to DST each year. The springtime switch from ST to DST is most annoying because we lose sleep to wake up an hour earlier. This “spring forward” is also associated with an increase in traffic accidents, cardiovascular events, and other health hazards.
Although there is strong support from the public and research communities for ending the biannual time changes, the question is, what’s the best alternative: permanent ST or DST? Unfortunately, the Sunshine Protection Act mandates permanent DST. From the scientific and medical point of view, this is the worse alternative. The optimal choice is permanent standard time.
Why is permanent ST better for human health and wellbeing? We need a little background: Our 24-hour day is determined by the rotation of earth that leads to a day-night energetic cycle driven by the sun. Life on earth has evolved under this daily cycle for millennia, and virtually all living systems have internal 24-hour or “circadian” clocks that anticipate daily challenges such as food and shelter. Having our body clocks in tune with the sun clock aligns just about every aspect of our physiology and behavior. Humans synchronize to local time primarily by light. While ST was designed so the middle of each time zone has the sun directly overhead at noon, DST is a social and political construct that advances our social schedule by 1 hour without changing environmental light cycles. During permanent DST, increased exposure to evening light and decreased morning light exposure tells the circadian system to wake up later the next morning, misaligning our daily rhythms with local time and making it tougher to live without an alarm clock.
Even a 1-hour deviation creates a chronic internal desynchrony between our body clocks and the sun. Why would we go against the tuning of our body clock to the sun? Does it improve mental health? Does it decrease accidents? Does it save energy? Does it reduce crime? The answer to all of these is no.
Research over the last 25 years shows we have 24-hour biological rhythms that are generated by a set of genes that turn on and turn off once each day. Surprisingly, this research showed that this circadian clock is found in almost every cell in our bodies. This cellular circadian clock not only controls our daily behaviors, such as our sleep-wake and fasting-eating cycles, but also our metabolism, immune function, and cognitive ability. The circadian clocks in our bodies are normally aligned and kept in sync by the environmental day-night cycle. Until the advent of DST in 1916, the sun was the primary synchronizer of our circadian clocks. The normal alignment of our circadian clocks by the sun is in register with Standard Time. DST causes a 1-hour misalignment of our clocks with the natural day-night cycle. Permanent DST would lead to a permanent misalignment of our clocks with the solar day.
While a 1-hour misalignment may sound trivial, consider that there would be 75 to 100 more dark morning commutes during permanent DST compared to permanent ST. Many lines of evidence show significant consequences of circadian misalignment. Laboratory and field experiments in humans and rodents have shown that circadian misalignment can lead to cognitive impairment, mood dysregulation, a reduction in glucose regulation and insulin sensitivity, and changes in satiety hormones such as leptin that can lead to increased food consumption and weight gain.
Most concerning regarding permanent DST is epidemiological evidence showing a significant increase in the rates of cancers on the western border of each U.S. time zone compared to the eastern border. There is an overall 3 to 4% increase in cancer risk ratio for every 5 degrees west within a time zone, which is on average about 15 degrees in longitude. This was found in each of the four U.S. time zones. The border separating time zones is an arbitrary, politically drawn line, making it both surprising and notable that cancer rates can vary on either side of each time zone. This suggests the cause of this association with cancer is the greater circadian misalignment for those living on the western edge of their time zone.
Now the bad news: being on permanent DST is equivalent to being on the extreme western border of each time zone, suggesting this may increase cancer rates (as a consequence of permanent circadian misalignment). Studies also find increases in obesity and heart attacks on western edges of time zones. Congress must seriously consider these consequences of permanent DST.
Countering Permanent DST Proponents
Proponents of permanent DST argue that 1 hour more of light at the end of the day will enable people to have more time to enjoy sunshine after work. This is the “extend the day” argument. But extending the day by 1 hour of light in the evening can only occur at the expense of a loss of 1 hour of light in the morning. As a wise old quote goes, only a fool would believe you could cut a foot off the top of a blanket and sew it to the bottom and have a longer blanket. Some argue 1 hour of light in the evening is more valuable than 1 hour of dark in the morning — they say businesses will have more customers, and there will be less crime and fewer traffic accidents at night. Thus, there is strong motivation and strong lobbies from retail and leisure businesses to adopt permanent DST. The evidence for improved public safety is, however, misrepresented and even contradicted by other studies.
Others, such as individuals with seasonal affective disorder (SAD) — which is triggered by the short days of winter — argue permanent DST is better because extra light in the evening makes them feel better. However, there is not an “extra” hour of light; rather that hour of light is moved in our social schedules from the morning to the evening. Research on bright light therapy for SAD has shown morning light is more effective in alleviating the symptoms of SAD than evening light. In addition, researchers find that light in the morning (e.g., during the commute to school or work) is associated with improved health, safety, and economics. In contrast, dark mornings and light afternoons (resulting from switching clocks to DST) is associated with poorer outcomes.
We should also pay attention to history — this wouldn’t be the first time we tried switch to permanent DST. In the 1970s, the U.S. adopted permanent DST in response to the oil embargo in order to save energy. It was reversed within a year primarily because of concerns when children, sadly, were killed waiting in the dark for their school buses. The idea of saving energy stems from “war time,” which was adopted during World War I and II with the thought of saving energy and increasing productivity. However, in each of the cases when permanent DST was adopted, there was a cry to go back to standard time.
It turns out that people don’t like permanent DST in the winter time. Sunrise will occur 1 hour later, and the afternoons will still be dark because of the short days of winter in northern latitudes. So, there is not much extra evening light in the winter because sunset occurs near the end of the work day. Thus, we predict most people will not be happy with DST in the winter, and history will likely repeat itself and we will again abolish DST.
Another fallacy of DST is that farmers like it. This is not true. Farmers generally dislike the current switching between ST and DST because farm animals (and in reality all animals) are not paying attention to our social time zones, but rather are waking and sleeping with the sun. Thus, animals are not going to switch to permanent DST because they will remain on solar time, and farmers (if we adopt DST) will be permanently misaligned with the natural cycles of their animals.
Finally, many medical organizations have endorsed permanent ST. None, to our knowledge, advocate for permanent DST. Therefore, for scientific and medical reasons, Congress should abolish switching to DST and adopt permanent standard time.
Joseph S. Takahashi, PhD, is an Investigator in the Howard Hughes Medical Institute in Chevy Chase, Maryland, and a member of the National Academy of Science and National Academy of Medicine, and recipient of the Gruber Neuroscience Prize. He is known for his discovery of the CLOCK gene in mice and humans, which led to the description of the circadian clock mechanism in mammals. Erik Herzog, PhD, is the Viktor Hamburger Distinguished Professor of Arts and Sciences at Washington University in St. Louis. His laboratory studies the molecules, cells, and circuits that underlie daily rhythms in physiology and behavior.
Takahashi is a co-founder and Science Advisory Board member of Synchronicity Pharma.