00:00Every year, Americans adjust their clocks twice. In spring, clocks are moved forward by one hour to start daylight saving time and in fall, Americans turn the clocks back an hour to standard time.
00:12Now, a new research from Stanford scientists shows their biannual clock changing does more than inconvenience. It's actually bad for their health.
00:21The study reveals that these twice yearly time shifts can contribute to serious long term health problems like strokes and obesity across the country.
00:31Researchers examined three time policies. First, keeping permanent standard time, meaning clocks stay on the usual standard time year round without shifting forward or backward.
00:43Second, adopting permanent daylight saving time, which means setting clocks one hour ahead permanently without reverting them in autumn.
00:52And third, continuing the current system of switching clocks twice a year forward in spring and back in fall.
01:00They specifically looked at how these choices affect our body's circadian rhythms, our internal biological clocks that follow a roughly 24 hour cycle.
01:09What they found is that the current system Americans use, which is switching clocks twice a year, is the worst choice for their health.
01:17Either staying on permanent standard time or permanent daylight saving time would be better for them with permanent standard time benefiting the most people.
01:26But how big an impact are we really talking about?
01:29The scientists estimate that if the US switched to permanent standard time, we could prevent around 300,000 stroke cases every year.
01:38That's a huge number of lives potentially saved.
01:41Plus, there could be a 2.6 million fewer people struggling with obesity nationwide.
01:47Permanent daylight saving time would also help, but to a lesser extent, preventing about two thirds of those health impacts.
01:54So why is permanent standard time healthier?
01:57It all comes down to how exposure shifts our circadian rhythm.
02:02A biological clock isn't exactly 24 hours. It's actually usually about 12 minutes longer.
02:07Morning light helps speed up the clock, keeping it aligned with the day.
02:11Evening light, on the other hand, slows it down.
02:13To stay well synchronized with a 24 hour day, we generally need more morning light and less evening light.
02:20The scientists modeled how sunlight at different times of the day, depending on which time policy is in place, impacts this synchronization.
02:28They found that permanent standard time gives people better access to morning light.
02:33This better alignment helps reduce what they call circadian burden, which is how much of the internal clock has to adjust to stay in sync.
02:43Interestingly, people who are naturally morning larks with a shorter circadian cycle might actually experience less burden under permanent daylight saving time.
02:53They benefit from the extended evening light, which brings their clock closer to 24 hours.
02:58The study also considered real world health data tracking eight conditions tied to circadian health, such as heart disease, diabetes, depression and stroke.
03:09Their calculations showed that shifts in time policy could meaningfully reduce the rates of stroke and obesity to major public health concerns.
03:20Of course, this research isn't the final word.
03:24Factors like geography, weather, human behavior, such as how much time people actually spend outdoors also play a role.
03:32In fact, many people spend less than 5% of their day outside, which might worsen circadian misalignment.
03:40For decades, debates have gone back and forth between supporters of permanent daylight saving time and advocates for permanent standard time.
03:48But until now, there hasn't been solid data on which is healthier.
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