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Changing teens’ biological clocks to wake them up earlier may be easier than changing high school start times, said a Stanford professor who published a paper on Monday suggesting a methodology to do just that.

“I just don’t think it’s possible to revert to a simpler time when we’re all getting up with the sun and going to sleep with the sun,” said Jamie Zeitzer, an assistant professor of psychiatry and behavioral science at Stanford.

Instead, Zeitzer is focusing on a way to modify people’s circadian clocks  the cycle that governs our body’s day-night rhythms, and which can get out of whack in jet lag or overnight work. This cycle is sensitive to changes in many factors, such as diet and light, but there’s no method of shifting the cycle that works for everyone.

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In previous work, Zeitzer had demonstrated that he could modify individuals’ internal clocks by flashing light on their closed eyelids while they were asleep. In this follow-up study, Zeitzer wanted to learn more about what types of flashes are most effective.

So, he woke people up in the middle of the night for an hour and exposed them to bursts of light akin to camera flashes. The flashes lasted 2 milliseconds, with gaps in between ranging from 2.5 seconds to 4 minutes. Subjects also periodically gave saliva samples, from which researchers could measure melatonin levels, a hormone that promotes sleepiness.

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The rhythm that worked best at shifting subjects’ circadian clocks, Zeitzer found, was a flash somewhere between every 2 and 8 seconds. A couple of individuals in this group saw a shift in their internal clock of up to three hours.

By contrast, individuals exposed to continuous light for the hour experienced an average change to their circadian clocks of only about 30 minutes, researchers reported in the Journal of Clinical Investigation.

Zeitzer said he thinks the reason for this relates to the physics of the retina. Allowing certain light-sensitive eye cells to recover from a flash resensitizes them to light. That lets the cells send more signals to the circadian clock, leading to a larger change.

Mark Rea, director of the Lighting Research Center at Rensselaer Polytechnic Institute, said that Zeitzer’s results are “plausible” but “ambitious.” Rea pointed out that only two individuals saw a change to their internal clocks of three hours.

Frank Scheer, director of the medical chronobiology program at Brigham and Women’s Hospital, said that these results were provocative, and that it would be important to replicate them in the future.

Zeitzer said that work like this could be used to improve the lives of people who work night shifts. On the weekends, these people want to be awake during normal hours to spend time with their family, but their body wants them to sleep. Similarly, it may help international travelers overcome jet lag, Zeitzer wrote in the paper. This protocol might one day develop into a therapy to allow people to quickly change their biological clock as circumstances demand.

However, Zeitzer pointed out, work schedules that conflict with the natural rhythms of our bodies put people at risk for a variety of diseases, including cancer. So far, his studies have not tested whether the light-flashing technique would change long-term health.

“Convincing people of that becomes exceedingly difficult,” Zeitzer said. “If we can’t convince them to change, then the question is: How do we help them adapt to this choice?”

Some of those who are hardest to convince are school administrators. Later high school start times aren’t yet being widely accepted, so Zeitzer is researching how to shift teens’ sleep cycles earlier to line up with school schedules.

His work so far has been a success, with one caveat: Zeitzer was able to change the teens’ biology, but not their motivation. “They weren’t going to sleep early because they didn’t want to,” Zeitzer said.

The next step is behavioral therapy. He hopes to be finished with that project by the summer.