You may welcome the longer spring evenings that daylight saving time ushers in — or you may be an early riser who curses the darker spring mornings. Nobody likes the inevitable jet lag that results from turning the clock forward — or backward, for that matter.
Now, mounting research shows that the biannual clock change also takes a toll on our health, increasing the risk of everything from cardiovascular disease to digestive problems to mental illness to accidents.
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Why We Began Changing Our Clocks
Daylight saving time was first enacted in 1918 during World War I as an energy-saving measure, as pushing daylight later into the evening resulted in lower electricity use. It was repealed the following year, and was enacted and revoked various times before 1966, when Congress made it a federal law. Recently, policy makers in several states have proposed legislation that would keep daylight saving time permanent rather than changing the clocks twice a year, but sleep experts believe permanent standard daylight time is healthier.
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The Link Between Obesity and Daylight Saving Time
A new study from Stanford University relying on computer simulation modeling projected that switching biannually between standard time and daylight saving time increases the risk of both obesity and stroke. Using a model to simulate sunlight exposure, sleep biology, and health data across the United States, researchers calculated and compared the impact of standard time, daylight saving time, and biannual switching on health outcomes.
The study’s authors projected that the number of people with obesity would decrease by about 0.78% if we stopped clock-changing and stuck with standard daylight time permanently. Though this may seem like a negligible number, it amounts to about 2.6 million fewer people with obesity. The researchers also projected a reduction in the prevalence of stroke, though to a lesser extent. A relatively smaller decrease in obesity and stroke is projected if we switched permanently to daylight saving time.
Fueled by this research, medical associations including the American Medical Association, the American Academy of Sleep Medicine, and the National Sleep Foundation are advocating to end the practice of clock changing.
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Understanding “Circadian Burden”
To understand why clock-changing matters so much, it helps to understand how your circadian rhythm works. Your circadian clock is a 24-hour biological clock that governs when you feel alert, hungry, sleepy, and everything in between. It’s largely regulated by the rising and setting of the sun; exposure to morning sunlight wakes us up and the evening’s darkness makes us sleepy.
But here’s the rub. Our internal circadian clock is actually about 12 minutes longer than the 24-hour day. The circadian clock is reset every morning when we’re exposed to sunlight. To stay aligned with the 24-hour day, we rely on early morning sun exposure to nudge it forward.
Daylight saving time, however, delays the morning wake-up light and extends evening light, which, put together, makes our bodies strain to align with the 24-hour day. This strain is referred to as a circadian burden. When we notice it, it feels like jetlag, causing fatigue, grogginess, or a lack of clear thinking. But the circadian burden also causes stressors on our bodies that can contribute to chronic health problems.
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Short-Term Health Costs of Clock Changing
The circadian burden is increased in people who work night shifts as well as those with delayed sleep disorders. It also emerges when we travel across time zones and when we switch back and forth to daylight saving time. Some of the health consequences occur in the days following the time change, but some have longer-term implications.
One immediate burden is activation of the sympathetic nervous system, which leads to increased heart rate, blood pressure, constriction of coronary blood vessels, and inflammation. Because our metabolism is aligned with the 24-hour clock, daylight saving time can also throw off our eating cues, gut hormones, and how we metabolize food.
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Long-Term Health Implications of Daylight Saving Time
Research has linked daylight saving time to longer-term implications, including:
- Daytime fatigue and brain fog
- Impaired cognitive performance
- Headache
- Increased risk of car accidents
- Higher blood pressure
- Increased risk of heart attack and stroke
- Diabetes
- Obesity
- Worsened menstrual symptoms
- Infertility and menstrual irregularities
- Fatty liver disease
- Gut inflammation
- Mood disorders
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How to Help Your Body Adjust
The goal is to reduce the circadian burden, which you can do by gradually acclimating yourself to the change in time. The American Academy of Sleep Medicine recommends these steps:
- Get enough sleep. It’s always important to consistently get at least seven hours of sleep, and it’s even more important in the weeks leading up to daylight saving time.
- Shift your bedtime. A few nights before daylight saving time, gradually shift your bedtime and wake-up time earlier by 15 to 20 minutes each night.
- Get morning sunlight. Go outside in the morning the week after the time change. The bright light helps align your internal clock to the new time.
