Switching the Clocks May Be Harmful to Health

• New research suggests the twice-yearly practice of changing the clocks does more than disrupt sleep — it’s also hazardous to health.
• Predictive models indicate that a permanent switch to either standard time or daylight time would lead to significant declines in obesity rates and stroke cases.
• Circadian rhythm disruption has direct implications for cardiovascular health, and those with certain health conditions should take extra precautions to prioritize quality sleep.

Daylight Saving Time ends on Sunday, November 2, at 2:00 a.m. Eastern Time.

As Americans “fall back” to Standard time in preparation for the shorter, darker days to come, the debate over whether there’s any real benefit to changing the clocks continues.

While most countries do not observe daylight saving time, many parts of North America and Europe continue the twice-yearly ritual that was implemented in Germany during World War I to prolong daylight and conserve energy in the summer months.

However, today, the century-old practice has many scientists, health experts, and even politicians debating potential safety concerns and health risks.

Now, a recent study from Stanford Medicine researchers suggests that this biannual shifting of the clocks not only disrupts circadian rhythms but also the physiological processes they regulate, which in turn impact overall health outcomes.

By modeling light exposure, the researchers compared the health effects of switching to either permanent standard time or permanent daylight time, as well as the usual biannual clock shifts.

They estimate that permanent standard time had the most benefits, preventing some 300,000 stroke cases per year and resulting in 2.6 million fewer people with obesity. Permanent daylight saving time, they found, would yield around two-thirds of the same positive effects.

“We found that staying in standard time or staying in daylight saving time is definitely better than switching twice a year,” Jamie Zeitzer, PhD, professor of psychiatry and behavioral sciences at Stanford and senior author, said in a news release.

“Standard time increases exposure to light earlier in the daytime, which is helpful to keep the output of the circadian clock robust,” Zeitzer told Healthline.

To estimate the physiological effects of circadian rhythm disruption, the researchers examined county-specific data from the Centers for Disease Control and Prevention (CDC).

The looked at the prevalence of health outcomes, including:

Predictive models indicate that permanent standard time would lower the nationwide obesity rate by 0.78% and the prevalence of stroke by 0.09%, two health conditions that are influenced by the circadian rhythm.

This led to 2.6 million fewer people with obesity in the United States and 300,000 fewer stroke cases.

If permanent daylight time were implemented, 1.7 million fewer people would have obesity, and there would be 220,000 fewer cases of stroke.

The models did not show a difference in health outcomes, such as arthritis, that are not linked to circadian rhythms.

“The circadian clock acts as a conductor of an orchestra, with the various organs of the body being synchronized together through a strong central circadian clock,” Zeitzer explained.

“A stronger clock, generated through more regular exposure to early morning light, leads to more optimal function of organ systems, including those involved in metabolism and cardiovascular function.”

Mir Ali, MD, board certified general surgeon, bariatric surgeon, and medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, CA, said he was surprised by the findings, particularly the overall health impacts of changing the clocks by just one hour. Ali wasn’t involved in the study.

“Other studies have demonstrated adverse health effects on night shift workers, and this study supports the benefit of a consistent daylight schedule. The effects appear to be modest, and further research would help define this more clearly,” Ali told Healthline.

Ali echoed the study’s findings, noting that circadian rhythm disruption does much more than affect sleep, with effects on metabolic and hormonal regulation, as well as immune system suppression, which can impact multiple organ systems.

“The change in time, even by one hour, affects sunlight exposure, thus altering the circadian rhythm. This alteration in the circadian rhythm leads to the problems noted above,” Ali said.

Cheng-Han Chen, MD, board certified interventional cardiologist and medical director of the Structural Heart Program at MemorialCare Saddleback Medical Center in Laguna Hills, CA, explained the cardiovascular effects of circadian rhythm disruption. Chen was likewise not involved in the study.

“We know that our body’s natural circadian clock regulates many important aspects of our cardiovascular system, such as blood pressure, heart rate, and metabolism, and that disruptions to this rhythm can have adverse effects on our heart health,” he told Healthline.

“Many aspects of our cardiovascular function, such as blood pressure, heart rate, and metabolism, will vary throughout the day, governed by our internal circadian clock. Much of this is due to specific fluctuations in cortisol and adrenaline,” Chen continued.

He explained that these variations are essential in promoting healthy cardiovascular function.

Disruptions to circadian rhythm, such as irregular work hours, shifts, or sleep patterns, have been linked to an increased risk of high blood pressure, obesity, and arrhythmia.

“This then can lead to increased risk of cardiovascular diseases such as heart attack, heart failure, or stroke,” Chen said.

Recent political efforts to abolish daylight saving time have been abandoned, which means Americans will continue to adjust their clocks in the fall and spring for the foreseeable future.

“The next step in the work will be to get the various expert stakeholders together to understand the totality of the changes that the different time policies lead to,” Zeitzer said. “That is, people who understand the impact of time policy on exercise, diet, safety, economics, etc.”

Until more research is conducted into the potential health hazards of this seasonal waffling, individuals with cardiovascular conditions may wish to take extra precautions.

“The ‘spring forward’ loss of an hour of sleep causes a significant disruption in the body’s circadian clock. For patients with underlying chronic health conditions, this can be enough to result in a major cardiovascular event such as heart attack or stroke,” Chen said.

To adjust to the new sleep-wake schedule this fall, increasing exposure to natural light during the day can help promote quality sleep and prevent disruptions to circadian rhythms.

If you live with a chronic health condition and have concerns about how your sleep is affecting your health, talk with your doctor or cardiologist.

“This study provides useful information on the potential benefit that eliminating the yearly daylight saving time switches can have on our nation’s health,” Chen said.

“Even small percentage improvements in health outcomes can have a positive impact on possibly millions of people.”