It’s tough fitting one’s busy schedule into the confines of a 24-hour day. If only it could be extended to 25 hours.Researchers may have found a way to do just that, at least as far as the brain’s circadian rhythms are concerned.The internal clock that controls our circadian (from the Latin “circa diem,” meaning “for about a day”) rhythms is, for most people, synchronized pretty closely to the rotation of the Earth — a 24-hour day — by the light-dark cycle. But this biological regulator sometimes gets thrown off by atypical work hours or by exposure to unusual conditions such as space flight, jet lag or shift work. People who are inappropriately synchronized suffer from sleep, endocrine and neurobehavioral disorders. Controlled doses of light may provide relief. A study has found evidence that exposure to certain levels of light may recalibrate someone’s cerebral clock, shifting the brain’s accustomed 24-hour cycle to a slightly longer, or shorter, version.The evidence suggests that the circadian rhythms can be tweaked by as much as an hour with repeated exposure to certain intensities of light, according to the researchers, who were led by Claude Gronfier of the Institut National de la Santé et de la Recherche Médicale in Lyon, France. Their results were reported in the May 22 issue of PNAS.A dozen test subjects spent 65 days in individual suites at Harvard Medical School’s Division of Sleep Medicine in Boston. All were isolated and had no access to the time of day. A computer system controlled the duration and intensity of cool-white fluorescent light to which subjects were exposed. Lighting conditions ranged from complete darkness (during sleep) to very bright conditions.The researchers found that lighting levels of ~25 lx were insufficient to modify the rhythms of all the test subjects, whereas daily exposure to brief pulses of bright light (~10,000 lx) was sufficient to reset the internal human clock to a day that is longer than 24 hours. Gronfier said that the results suggest that appropriately timed light exposure can be an effective means of maintaining a person’s circadian clock in synchrony with a rest-activity schedule different from 24 hours or one with insufficient lighting conditions.The method could, for instance, be used to help future astronauts adjust to life on Mars and the 24.65-hour Martian day. Closer to home, it could be used to treat circadian rhythm sleep disorders, jet lag and seasonal affective disorder.