The human internal clock fails to adapt to non-24-hour days and that fact takes its toll on astronauts, international travelers and shift workers. “Due to the shuttle orbit, astronauts often experience days that are less than 24 hours,” said Dr. Kenneth Wright, a researcher on the National Space Biomedical Research Institute’s human performance team. “Many experience sleep difficulties, averaging only about six hours of sleep a day in contrast to the seven or eight hours they get on the ground. This can lead to increased risk of accidents due to fatigue and sleepiness.” In a study funded by the NSBRI and NASA, Wright and colleagues evaluated how people’s internal clocks were affected by exposure to 23.5-, 24- and 24.6-hour days. Shuttle missions typically operate on 23.5-hour days, and astronauts exploring Mars would experience a 24.65-hour day.