Human circadian clocks have evolved to resonate with Earth’s 24-hour day. So what would happen if astronauts were to travel to, say, Mars, where a day lasts 24 hours and 39 minutes? Can our circadian clock be reset to synchronize to a longer-than-24-hour day?
That was the question NASA’s National Space Biomedical Research Institute put to circadian researchers led by Charles Czeisler of Harvard and Claude Gronfier of INSERM in France. Previous research found that putting healthy people in an experimental environment matching the longer Martian day, with light intensity mimicking that of the space shuttle or the international space station, “desynchronizes” the circadian system. The result: disrupted sleep, cognitive impairment, and metabolic problems—not the kind of problems you want to see in astronauts who are at least 36 million miles from home.
“People think 39 minutes longer is not a big deal,” Gronfier says. “It is a big deal.”
Because the biological clock is sensitive to light, Czeisler’s group reasoned that carefully timed exposure to light might delay the clock, effectively stretching it beyond its evolutionary programming. The researchers used two 45-minute pulses of bright light—roughly the level available at sunset or sunrise—delivered one hour apart at the end of the day. Two control groups were exposed to lower intensities of light.
At the end of 30 days, subjects exposed to the brightest light pulses had successfully adapted to the longer day, showing normal behavior and physiological functioning. Subjects in the control groups did not adapt successfully.
They experienced symptoms generally associated with circadian rhythm sleep disorders, such as poor sleep, daytime drowsiness, inappropriate functioning and cognitive deficits. The results were published May 14 in the Proceedings of the National Academy of Sciences’ early online edition.
Gronfier envisions a day when precisely scheduled pulses of colored or white light might be offered on international flights to help travelers avoid jet lag, or delivered to shift workers to readjust their rhythms. Such approaches, he points out, are not that far removed from “light therapy” devices long used to effectively treat circadian rhythm sleep disorders and Seasonal Affective Disorder.