Why daylight saving time is worse for your body than standard time

Within days, 48 states and the District of Columbia will reset their clocks and fall back into standard time. From a health standpoint, most sleep and circadian experts say we should stay there.

Experts say early morning sunlight is key to maintaining our circadian rhythms, sleep-wake cycles and overall health. Phyllis Zee, a neurologist and chief of sleep medicine at Northwestern University Feinberg School of Medicine, said without that sunlight, we can slip into circadian misalignment — "when your internal body clocks fall out of sync with that of the sun clock and your social clocks."

The concern with adopting a permanent change to daylight saving time, which the Senate has voted to do, is that it may chronically throw our bodies out of sync with the sun and lead to a variety of health problems, sleep experts say.

"We would be misaligned all year long," said Beth Malow, professor of neurology and pediatrics and the director of Vanderbilt University's sleep division.

How is it that one hour can have such a significant impact? Here's how your brain and health are affected by time changes.

Your internal clock

Your brain has an internal clock. It helps keep time each day, making sure the right things are happening at the right moments inside our bodies, such as cardiac functions, metabolic processes, hormone fluctuations and sleep.

But for most of us, that internal clock runs slightly longer than 24 hours.

Luckily, the sun steps in to help us by sending signals to specialized receptors in our eyes.

Each morning, sunlight resets our internal clock by "pulling" us back into sync with a 24-hour day. Then, after the sun sets, the lack of sunlight allows our bodies to produce hormones such as melatonin, which promotes sleep.

Similar to how sunlight in the morning can pull our internal clock earlier, light from any source too late into the evening can do the opposite and "push" our internal clock later. This can interfere with our ability to sleep.

Why does changing the clock affect us?

With the time change each spring to daylight saving time, we abruptly disrupt this important relationship between our brains and the sun. Our internal clock evolved to align with the sunrise, sunset and a 24-hour day. Likewise, the clocks we developed to schedule our days were conceived to align with the 24-hour solar day. When we are observing standard time, noon is when the sun is generally at its highest point. Let's call it solar noon.

For most of us, our watches and phone clocks dictate how we live large parts of our lives — when to wake up, be at work and go to bed. Each March, when we switch to daylight saving time, our daily schedules shift by an hour. Notice how now we wake up in the dark and leave work with more hours of sunlight left in the day.

But the sun doesn't move. Solar noon, which is synced with your internal clock, is still at the same time as the previous day. Now your daily schedule and solar noon are misaligned. Unfortunately, the internal clock in your brain can't adjust as fast as your watch. It takes your internal clock at least one day to adjust to one hour in time change. And for some people, it can take longer than that.

One of the major problems with changing to daylight saving time is that it disrupts sleep. Your body isn't yet ready to fall asleep at your usual bedtime, which is now an hour earlier. And in the morning, your body wants its old wake time, too. But your alarm clock wakes you an hour earlier, so you can get to work on time. Now you're cutting into your sleep at both ends.

This sudden one-hour shift each spring has been associated with more heart attacks and strokes. Automobile and other types of accidents also increase. There is also a cumulative toll of circadian misalignment.

What would happen if we had daylight saving time all year long?

The summer solstice, around June 21 in the Northern Hemisphere, is the longest day of the year. In some locations, such as Washington, D.C., there may be nearly 15 hours of daylight. That extra hour of sunlight after work may feel nice, but having a later sunset close to bedtime can delay the onset of those sleep hormones, pushing your internal clock later. Even though you may end up losing some sleep, during the summer you would still have early morning sun to "pull" you back a little bit.

But let's imagine if daylight saving time becomes permanent, as has been proposed. Six months later, during the shortest day of the year, on Dec. 21, you would have a different problem. For instance, there would be a bit over nine hours of sunlight in Washington, D.C., during the day. You would wake up in darkness, and the sun wouldn't rise until you were well awake and already at work.

Because you already would have been awake for hours before the sun rose, it would be less capable of "pulling" your clock back into alignment. If your internal clock doesn't get "pulled" enough, it can't reset itself. So, your internal clock may remain slightly longer than 24 hours, causing you to drift out of sync with the solar day.

With each passing day, those extra minutes can accumulate. Over time, your internal clock and your real-world clocks move further apart.

This results in circadian misalignment, a mismatch between our schedules that tell us when to wake, "and what our outside environment wants us to do, which is to stay in bed and sleep because it's dark outside," Malow said.

As a result, staying in daylight saving time year round could put people at greater risk for seasonal depression in the winter months, experts say. Experts also say circadian misalignment can affect the production of important hormones such as melatonin.

Living chronically out of sync with our internal clock puts us at an increased risk for sleep loss, obesity, diabetes, heart disease, mood disorders and even certain types of cancer. Melatonin has oncostatic properties, meaning it can slow the spread of cancer, said Charles Czeisler, chief of the Division of Sleep and Circadian Disorders at Brigham and Women's Hospital.

But what would happen if we adopted standard time year round?

Experts say we would eliminate the short-term health problems (and frustration) created by adjusting our clocks twice per year. More importantly, our internal clock would stay in sync with the solar day. With better circadian alignment, we may reduce the risk for many long-term health issues.

Circadian and sleep experts say there is a powerful biological case for staying in standard time year round. Changing the clocks doesn't change the overall duration of sunlight in a day. With permanent standard time, at least you would be getting that light at the right time of day for optimal health.