Why sleep experts say permanent daylight saving time is a bad idea

Tired of changing the clock on your oven twice a year when the time changes? You are not alone.

Only 12% of Americans are satisfied with the nation’s current semiannual system of shifting clocks. 

Congress is considering a bill to get rid of the changes. 

On July 14, the U.S. House passed the Sunshine Protection Act to put the nation’s clocks on permanent daylight saving time, meaning darker mornings and more daylight in the evenings through the winter. It has not yet passed the Senate. 

Getting rid of clock changes is popular, but people are divided over whether year-round daylight saving time or standard time is best. 

Health and sleep experts, meanwhile, aren’t. 

The American Medical Association, the American Academy of Sleep Medicine and the National Sleep Foundation support getting rid of our clock changes but agree that permanent standard time — the opposite of what Congress is advocating for — would be best for the nation’s health. 

Darker mornings and bright evenings that come with daylight saving time could mess with the body’s internal clock, disrupt sleep, and harm long-term health. 

Here’s why. 

The sun regulates the body’s internal clock 

We may not “tick” like the clock-swallowing crocodile in Peter Pan, but we all have our own internal clocks, called circadian rhythms.

These internal clocks keep our bodies on a schedule: spikes of cortisol, the stress hormone, help wake us up in the morning. Melatonin surges in the evening and makes us sleepy. We get a surge of hormones stimulating hunger before expected mealtimes, and appetite-suppressing hormones while we sleep.

It’s not just hormones that run on time. Research shows that blood pressure, metabolism, body temperature, digestion and the immune system all fluctuate throughout the day according to our internal clocks. 

And that clock’s most important signal? Light. 

“That’s why we get so concerned with discussion about anything that could disrupt that alignment,” said Dr. Anita Shelgikar, professor of neurology at the University of Michigan.

Morning light, with its shorter wavelengths of blue-green light, is especially important to training circadian rhythms. It enhances our mood and alertness, said Dr. Phyllis C. Zee, a specialist in sleep medicine at Northwestern University. Late-afternoon and evening light is composed of longer orange-red wavelengths that are more relaxing and less activating.

We can’t change how much the sun shines — days will be longer in the summer, shorter in the winter, and just how long depends on where you live. What we can change is how well our literal clocks — the ones that generally guide our sleep-wake schedules — line up with those external light patterns.

Changing clocks twice a year affects circadian rhythms

The U.S. in 1966 codified the current model of switching between standard time and daylight saving time. It aims to give people the best of both worlds — bright mornings in winter, sunny evenings in the summer. Under the current system, standard time runs from the first Sunday in Nov. to mid-March and daylight saving time kicks in the rest of the year.

But sleep experts warn that the current pattern comes with a cost.

Heart attacks, strokes, traffic accidents, workplace injuries, sleep loss and other cardiovascular events increase in the days after the time change, some research shows. Other studies have found no effect.  

The negative effects are most pronounced in the spring, when the time change can cause people to lose an hour of sleep. 

“The risk to any individual is small,” Jamie Zeitzer, professor and co-director of the Center for Sleep and Circadian Sciences at Stanford University, said via email. After all, there are many days when people wake up earlier than they would like to without major consequences. “But when 300+ million people do this on the same day, you can see the effect.” 

Eliminating the switch would mean sacrificing sunny summer evenings or bright winter mornings.

Why experts say permanent standard time is better for our health

Following standard time is the best way to sync the body’s internal clock with light from the sun, sleep experts say. Our bodies are designed to wake up to light in the morning, and our sleep patterns embrace having darkness in the evening. 

“Bright light in the morning actually helps us fall asleep at night,” said Dr. Beth Malow, a neurologist and sleep researcher at Vanderbilt University. “It seems kind of paradoxical, but by resetting our clocks, we’re ready to go to sleep at night.” 

Shifting to daylight saving time in those winter months would mean many Americans wouldn’t experience sunrises until after 8 a.m. Waking up in darkness leaves your body without the light signal it needs to start its daytime routine, whereas bright evenings suppress melatonin production and make it harder to fall asleep when bedtime arrives. 

These circadian disruptions throw our hormones, appetite and mood out of whack, and could do damage over the long term. 

It also leads to chronically less sleep, which is already linked to negative health outcomes, including dementia, heart disease, type 2 diabetes and some cancers. 

Researchers found that within a given time zone, those who live further west and experience a later sunrise and sunset get less sleep and have worse health outcomes than people on the eastern side of the time zone. 

Hawaii, Arizona, and other U.S. territories like Puerto Rico and Guam have already shifted to year-round standard time.  

If it’s bad for human health, why do people want it? 

The short answer is health is not all we care about. 

Many industries centered around shopping, outdoor recreation and tourism love that extra summer hour of sunshine. We have memories and traditions associated with long summer days. 

Yet the science points the other way. 

“It would be wonderful if we could get to a point where we don’t look at these things as mutually exclusive. Like I have to choose between my health and making great memories or socializing with people I care about,” Dr. Shelgikar said. “Both can happen. And actually, if we are better rested and get the amount of sleep that we need, then maybe those things can become even more enjoyable.” 

The last time the U.S. chose to move to permanent daylight saving, Congress scrapped the schedule within months amid popular outcry.