Circadian Entrainment, Sleep-Wake Regulation and Neurobehavioral Performance During Extended-Duration Spaceflight
Charles A. Czeisler, Ph.D., M.D.
Harvard - Brigham and Women's Hospital
Crew members are provided, and presently use, long acting benzodiazepine hypnotics on board the current, relatively brief space shuttle missions to counteract such sleep disruption, a situation that is only likely to worsen during extended duration missions. Given the known carry over effects of such compounds on daytime performance, together with the reduction in emergency readiness associated with their use at night, NASA has recognized the need to develop effective but safe countermeasures to allow crew members to obtain an adequate amount of sleep. Over the past nine years, we have successfully implemented a new technology for shuttle crew members involving bright light exposure during the pre launch period to facilitate adaptation of the circadian timing system to the inversions of the sleep wake schedule often required during dual shift missions. However for long duration space station missions it will be necessary to develop effective and attainable countermeasures that can be used chronically to optimize circadian entrainment.
Our current research effort is to study the effects of light-dark cycles with reduced zeitgeber strength, such as are anticipated during exploration class space missions, on the entrainment of the endogenous circadian timing system and to study the effects of a countermeasure that consists of scheduled brief exposures to bright light on the human circadian timing system. The studies are designed to address the following Specific Aims:
1) test the hypothesis that synchronization of the human circadian pacemaker will be disturbed in men and women by the reduction in LD cycle strength.
2) test the hypothesis that this disturbed circadian synchronization will result in the secretion of the sleep-promoting hormone melatonin during the waking day, disturbed sleep, reduced growth hormone secretion, and impaired performance and daytime alertness;
Results suggest that a strictly scheduled wake-sleep cycle with dim light levels, similar to that which astronauts are currently exposed on space shuttle missions, is sufficient to maintain entrainment of the human circadian pacemaker to the 24.0 hour day for most but not all subjects tested. No subjects entrained to a longer-than-24-hour day under similar conditions. Circadian misalignment resulted in disturbed sleep, impaired alertness and performance, secretion of melatonin during the waking day and reduced nocturnal growth hormone secretion. Preliminary studies suggest that stronger synchronizers, such as brighter light, will be necessary to entrain the longer-than-24-hour intrinsic circadian period of all humans to the 24.0 day and other day lengths such as the ~24.65 solar day of Mars.
The results of the current research may have important implications for the treatment of circadian rhythm sleep disorders, such as delayed sleep phase syndrome and shift-work dyssomnia, which are anticipated to have a high incidence and prevalence during extended duration spaceflight such as planned for the International Space Station and astronaut missions to Mars.