| The success of long-duration missions depends on the ability of crewmembers and mission support specialists to be alert and maintain high levels of cognitive function while operating complex, technical equipment. Optimal human health, performance and safety during spaceflight requires sufficient sleep and synchrony between the circadian pacemaker - which regulates the timing of sleep, endocrine function, alertness, and performance - and the timing of the imposed sleep-wake schedule.
Crewmembers of the 105-day experiment were required to work one night shift every sixth night. We hypothesized that this schedule would likely result in sleep loss and circadian misalignment, especially when lighting conditions are similar to those experienced during spaceflight. Mission controllers supporting the 105-day study were required to work 24-hour shifts. We hypothesized that this schedule, too, would result in both sleep loss and circadian misalignment. It has been well documented in laboratory and field studies that both working the night shift and working extended-duration shifts result in negative effects on alertness, performance, and mood.
Light has been successfully used as a countermeasure for circadian misalignment and to acutely increase alertness. Recently, shorter wavelength light (480-500 nm) has been reported to be more effective for these purposes than longer wavelength light. The goals of the study were to test the operational feasibility of sleep and circadian assessments and test a lighting countermeasure to improve alertness and performance during night-shift work occurring during a long-duration analog space mission.
1) Evaluate the feasibility of monitoring sleep and circadian neuroendocrine rhythms in a high fidelity operational simulation
2) Test the hypothesis that sleep, alertness, performance, and mood will be impaired during night shift work operations, in both crewmembers and external mission controllers
3) Test the hypothesis that alertness, performance, and mood of crewmembers and external mission controllers exposed to shorter wavelength light (with a peak wavelength between 485 to 525 nm) during the night shift will be significantly better than the alertness, performance, and mood of those same crewmembers when they are exposed to intermediate wavelength light (with a peak wavelength of either 545 nm to 555 nm) or longer wavelength light (620 nm to 690 nm) during the night shift
Throughout the 105-day experiment, measurements were obtained to assess sleep, performance, alignment of the circadian system, and nighttime melatonin levels. We collected 349 days of actigraphy from 6 crewmembers. Crewmembers slept significantly less during the 24 hour day that included a night shift and significantly more in the 24 hours following the night shift. Crewmembers reported using caffeine and naps to counter fatigue.
Inspection of the night shift room after the completion of the mission revealed supplemental polychromatic lighting had been added to the room during the mission that further increased the intensity of light exposure on the night shift. Consequently, there was no difference in light intensity, as measured by wrist-worn actigraphy, or in melatonin suppression between the three lighting conditions.
Crewmembers completed 48-hour urine collections approximately every two weeks for analysis of 6-sulphaxtoxymelatonin and free cortisol rhythms to estimate the phase of the circadian pacemaker. Some crewmembers maintained stable circadian phase and other had considerable phase misalignment.
Eighteen mission controllers reported working 358 24-hour shifts. Mission controllers slept <4 hours on their 24-hour extended duration work shifts.
As expected, learning was observed across the study. Nonetheless, performance, alertness, sleepiness, and mood of crewmembers and mission controllers deteriorated during night work across the study, indicating little adaptation to 24h work operations. No significant differences in performance were seen between light conditions.
These data demonstrate that it is feasible to monitor sleep, circadian rhythms, and performance in an analog spaceflight environment. Cognitive learning across the 105-day mission in both crewmembers and mission controllers was consistent with subjects in ground based studies who have appropriate circadian alignment. Deterioration of performance, alertness, and mood were evident during 24-hour extended duration overnight work shifts despite countermeasure use. Some crewmembers had considerable circadian phase misalignment. Whether this misalignment was due to failure to entrain or a consequence of the recurrent night shifts is a subject for future research.
There were no significant differences in alertness or performance across lighting conditions. Factors in the protocol over which the experimenter had no control (e.g., additional lighting used in the night shift room beyond that currently possible during spaceflight) may account for this latter finding and necessitate further exploration of this aspect of the study.