## 4 Discussion  
In this study, we developed a single summary score for 14 items of the Alertness and Mood survey that precedes NASA's Cognition test battery. The summary score is not a simple but a weighted average across items. The weights were informed by a PC analysis performed by 19 SMEs. A PC can be used to rank a large number of items relative to their importance or relevance. It breaks down the ranking problem into n \(\times\) (n \(-\) 1)/2 pairwise comparisons, where the rater has to decide which of the two items is more relevant for the question at hand. The proportion of comparisons where an item was considered more relevant than the other items ranges from 0 to 1 and reflects its relevance or rank relative to the other items. This is an advantage of the PC as its results can be considered ratio-scaled as opposed to only interval-scaled results that a classical ranking would produce. The PC proportions were used as weights for generating the AMS summary score. As the importance of each item may depend on space mission duration, SMEs were asked to perform the PC twice, for short- (up to 6 months) and long-duration (>6 months) space missions. Averaging across short- and long-duration PCs, AMS items considered the least relevant (chosen <40% of comparisons) were boredom, monotony, sleepiness and workload. Tiredness, happiness, loneliness, energy level, sleep quality, mental status, and stress were considered somewhat relevant (chosen in 40-70% of comparisons), while  
Figure 4: Change in survey responses during recovery relative to pre-head-down tilt bed rest (HDBR) baseline for the control group (black circles), continuous artificial gravity group (white squares), and intermittent artificial gravity group (white triangles). Estimates reflect points on an 11-point scale. For each variable, the negative response anchor is shown (e.g., “unhappy”, “sleepy”). Positive scores reflect more negative assessments relative to baseline (graphs on the **left**) or control (graphs on the **right**). Error bars reflect unadjusted 95% confidence intervals. Adapted from [PERSON] et al. [16] with permission. * adjusted \(p\) < 0.05; ** adjusted \(p\) < 0.01; *** adjusted \(p\) < 0.001; *** adjusted \(p\) < 0.0001.  
health, depression and crew conflicts were considered most relevant (chosen \(>\)70% of comparisons). These results highlight the importance of crew cohesion [19; 20], on the one hand, and mental health, on the other hand, for mission success. They corroborate the aforementioned fact that adverse cognitive or behavioral conditions and psychiatric disorders are considered one of the greatest unmitigated risks of space exploration [7]. The SMEs considered sleepiness, tiredness, energy level and mental status significantly more relevant for short-duration compared to long-duration missions, while they considered monotony and loneliness significantly more relevant for long-duration compared to short-duration missions. This makes sense in light of the fact that the workload on shuttle and ISS missions has typically been high and considered one of the reasons for insufficient sleep [21; 22]. In contrast, one of the main challenges of exploration-class missions to Mars and beyond will be to provide the crew with meaningful work while en route to their destination [23] and to cope with prolonged periods of isolation and confinement with the aggravating factor of communication delays [14]. Given the importance of sleep for cognitive performance, mood and health [24; 25], we were surprised to see the low relevance scores for sleepiness, especially for long-duration missions. The SMEs may have been unaware of the fact that several of the states identified as highly relevant can result from acutely or chronically insufficient sleep (e.g., stress, aggressive behavior, depression), and that sleep loss and circadian misalignment have been shown to be prevalent in spaceflight [21; 22] and to impair cognitive performance [5; 21].