HOUSTON – 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.
"Adapting to these different day lengths is critical to mission success," said Wright, an instructor in medicine at Harvard Medical School and associate neuroscientist and director of the Fatigue Countermeasures Research Program in the Division of Sleep Medicine at Brigham and Women’s Hospital (BWH) in Boston, where the research was conducted.
Since spacecraft are dimly lit, study participants at BWH were exposed to low-level daytime lighting equivalent to candlelight. All groups were placed on a fixed work/rest schedule. Melatonin levels were evaluated to determine how participants adapted to the various day lengths. The findings appear in the Nov. 20 edition of the Proceedings of the National Academy of Science.
Melatonin, a hormone, regulates the body’s sleep activities. In a normal day/night sleep cycle, melatonin levels will rise about two hours before sleep to signal the body to prepare for sleep. The levels are high during sleep and low during the day.
"Light exposure is the strongest cue for our internal clock. However, participants on the 24-hour day fixed work/rest schedule were able to maintain the appropriate melatonin cycles for sleep even in the dim light," Wright said. "Their internal clocks kept time with the day. The groups experiencing the shorter or longer days did not adapt."
In the other groups, melatonin levels lost the normal cycle. Levels were high when the participants were awake and low when they were trying to sleep. This factor made it difficult to sleep at the scheduled time.
"This problem with the melatonin cycle occurs during jet lag and in people working on night shifts," Wright said. "In effect, astronauts on shortened days are experiencing jet lag in space."
The NSBRI human performance factors team is working to develop a remedy, or countermeasure, to help people adapt to various day lengths. While this solution is essential to prepare astronauts for exploration missions beyond low-Earth orbit, it will also have practical implications on Earth. In addition to jet lag and shift-work adaptation, it will be useful for certain sleep rhythm disorders.
"People with advanced sleep phase syndrome typically have difficulty staying awake after 5 p.m. and will wake up in the early hours. Another disorder impacts people the opposite way, leaving them unable to sleep until early morning and making it hard to be awake during traditional work hours," Wright said. "For these patients, a treatment to adjust the internal clock would make a big difference in work and home life."
The study is part of the NSBRI’s research examining the health issues related to long-duration space flight. Benefits to similar conditions on Earth – muscle wasting, bone loss, radiation exposure – are also being pursued.
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The NSBRI’s consortium members include Baylor College of Medicine, Brookhaven National Laboratory, Harvard, The Johns Hopkins University, Massachusetts Institute of Technology, Morehouse School of Medicine, Mount Sinai School of Medicine, Rice University, Texas A&M University, University of Arkansas for Medical Sciences, University of Pennsylvania Health System and University of Washington.