Evaluation of the Validity of Bio-Mathematical Models in Predicting Fatigue in an Operational Environment

During long-duration spaceflight missions, crewmembers and ground-support staff experience irregular sleep schedules, erratic natural light patterns, and high workload due to mission demands. Such conditions can cause circadian misalignment and sleep loss, which in turn cause deficits in cognitive performance. While bio-mathematical models have been implemented within workplace settings to predict fatigue profiles, the accuracy of sleep-wake models under conditions of non-traditional shiftwork is little known. Thus, the present study aims to evaluate the validity of four sleep-wake models (e.g., SAFTE-FAST, the Unified Model of Performance, the Adenosine-Circadian Model, and the State-Space Model) designed to predict human performance and fatigue levels against objective measures of performance in a spaceflight analog. To accomplish this aim, we will collect Psychomotor Vigilance Task (PVT) data from four crews (n=16) in the Human Exploration Research Analog (HERA) over 45 days. HERA is a closed, 3-story habitat at Johnson Space Center where inhabitants are exposed to extreme space exploration scenarios under varying sleep-wake conditions. The PVT is a simple reaction time test that involves minimal learning, making it sensitive to the effects of sleep loss and circadian misalignment. Findings from this study will help inform work scheduling and implementation of effective countermeasures (e.g., caffeine, lighting) to improve work efficiency and combat fatigue, as well as offer valuable insight into the applicability of bio-mathematical fatigue models in future space exploration missions

Changes in performance and bio-mathematical model performance predictions during 45 days of sleep restriction in a simulated space mission

Lunar habitation and exploration of space beyond low-Earth orbit will require small crews to live in isolation and confinement while maintaining a high level of performance with limited support from mission control. Astronauts only achieve approximately 6 h of sleep per night, but few studies have linked sleep deficiency in space to performance impairment. We studied crewmembers over 45 days during a simulated space mission that included 5 h of sleep opportunity on weekdays and 8 h of sleep on weekends to characterize changes in performance on the psychomotor vigilance task (PVT) and subjective fatigue ratings. We further evaluated how well bio-mathematical models designed to predict performance changes due to sleep loss compared to objective performance. We studied 20 individuals during five missions and found that objective performance, but not subjective fatigue, declined from the beginning to the end of the mission. We found that bio-mathematical models were able to predict average changes across the mission but were less sensitive at predicting individual-level performance. Our findings suggest that sleep should be prioritized in lunar crews to minimize the potential for performance errors. Bio-mathematical models may be useful for aiding crews in schedule design but not for individual-level fitness-for-duty decisions

The Effect of a Dynamic Lighting Schedule on Neurobehavioral Performance During a 45-Day Simulated Space Mission

Study Objective: We previously reported that during a 45-day simulated space mission, a dynamic lighting schedule (DLS) improved circadian phase alignment and performance assessed once on selected days. This study aimed to evaluate how DLS affected performance on a 5-minute psychomotor vigilance task (PVT) administered multiple times per day on selected days. Methods: Sixteen crewmembers (37.4 ± 6.7 years; 5F) underwent six cycles of 2 × 8-hour/night followed by 5 × 5-hour/night sleep opportunities. During the DLS (n = 8), daytime white light exposure was blue-enriched (∼6000 K; Level 1: 1079, Level 2: 76 melanopic equivalent daytime illuminance (melEDI) lux) and blue-depleted (∼3000-4000 K; L1: 21, L2: 2 melEDI lux) 3 hours before bed. In the standard lighting schedule (SLS; n = 8), lighting remained constant (∼4500K; L1: 284, L2 62 melEDI lux). Effects of lighting condition (DLS/SLS), sleep condition (5/8 hours), time into mission, and their interactions, and time awake on PVT performance were analyzed using generalized linear mixed models. Results: The DLS was associated with fewer attentional lapses (reaction time [RT] \u3e 500 milliseconds) compared to SLS. Lapses, mean RT, and 10% fastest/slowest RTs were worse following 5 compared to 8 hours of sleep but not between lighting conditions. There was an effect of time into mission on RTs, likely due to sleep loss. Overall performance differed by time of day, with longer RTs at the beginning and end of the day. There were more lapses and slower RTs in the afternoon in the SLS compared to the DLS condition. Results: The DLS was associated with fewer attentional lapses (reaction time [RT] \u3e 500 milliseconds) compared to SLS. Lapses, mean RT, and 10% fastest/slowest RTs were worse following 5 compared to 8 hours of sleep but not between lighting conditions. There was an effect of time into mission on RTs, likely due to sleep loss. Overall performance differed by time of day, with longer RTs at the beginning and end of the day. There were more lapses and slower RTs in the afternoon in the SLS compared to the DLS condition. Conclusions: Future missions should incorporate DLS to enhance circadian alignment and performance. This paper is part of the Sleep and Circadian Rhythms: Management of Fatigue in Occupational Settings Collection

Effects of Circadian Chronotype on PVT Performance in a Chronically Sleep Deprived Condition

Morning and evening chronotypes have been used to classify individuals into categories that represent differences in endogenous circadian preferences and periods of peak performance. This study investigated the influence circadian chronotype has on psychomotor vigilance test (PVT) performance during a 45-day spaceflight analog sleep restriction experiment. Twenty healthy participants (ages 29-55; 11 males) were restricted to five hours of sleep during weekdays and eight hours of sleep on weekends. Every third day, participants completed five PVT sessions throughout the day. PVT data of interest were compared from the first and last mission day of each sleep condition. PVT measures analyzed were mean response time (RT), mean speed (1/RT), and mean lapses. Mean RTs were faster overall for evening chronotypes regardless of the mission day or sleep condition. Mean speed was faster and mean lapses were fewer for evening chronotypes. A mixed effects ANOVA determined that there was no significant three-way interaction between circadian chronotype, sleep condition, and mission day. There was a significant difference between performance on the first and the last mission day of a sleep condition. For all chronotypes, performance was poorer on the last mission day after having experienced many days of chronic sleep deprivation. These results suggest that circadian chronotype does not have a large effect on individual performance, but confirms that chronic sleep deprivation over time impairs cognitive performance

Individual Differences in Response to Sleep Deprivation in a Space Analog Environment

Adverse conditions in space severely affect astronauts' sleep. Importantly, previous research yielded evidence that even minor sleep deprivation has the potential to massively influence individual performance. Thus, considering the high risk operational working environment of space missions, it is essential to study potential influences on the effects of sleep deprivation. As part of a larger NASA project, our study aimed at investigating whether individual characteristics such as personality traits predict vulnerability to sleep deprivation with respect to individuals' performance and perceived fatigue. The study was conducted in the NASA Human Exploration Research Analog (HERA), a ground-based habitat. Data were collected from four four-person crews, who volunteered to partake in confined isolation for 45 days each. Before participating, individual characteristics such as personality traits were assessed. During confinement, the crew members were instructed to maintain a sleep rhythm of eight hours per night during weekends and five hours per night during weekdays (i.e., inducing sleep deprivation). Moreover, repeatedly during the mission, participants' performance was measured by means of the psychomotor vigilance task (PVT) and, additionally, several rating scales were administered (e.g., perceived fatigue). In contrast to prior research, extraversion did not predict differential impairment in PVT performance or perceived fatigue as a result of sleep deprivation in the present study. However, we found several other individual characteristics such as the personality trait aggressiveness to be significantly associated with vulnerability to the effects of sleep deprivation. In addition to our main analyses, we also explored the time courses of three basic dimensions of mood during the mission (i.e., valence, calmness, energetic arousal). In sum, the present study made use of an ecologically valid space analog to shed further light on the relationship between individual characteristics such as personality traits and vulnerability to the effects of sleep deprivation

Operative versus Nonoperative Treatment of Acute Achilles Tendon Ruptures

BACKGROUND: To date, studies directly comparing the rerupture rate in patients with an Achilles tendon rupture who are treated with surgical repair with the rate in patients treated nonoperatively have been inconclusive but the pooled relative risk of rerupture favored surgical repair. In all but one study, the limb was immobilized for six to eight weeks. Published studies of animals and humans have shown a benefit of early functional stimulus to healing tendons. The purpose of the present study was to compare the outcomes of patients with an acute Achilles tendon rupture treated with operative repair and accelerated functional rehabilitation with the outcomes of similar patients treated with accelerated functional rehabilitation alone. METHODS: Patients were randomized to operative or nonoperative treatment for acute Achilles tendon rupture. All patients underwent an accelerated rehabilitation protocol that featured early weight-bearing and early range of motion. The primary outcome was the rerupture rate as demonstrated by a positive Thompson squeeze test, the presence of a palpable gap, and loss of plantar flexion strength. Secondary outcomes included isokinetic strength, the Leppilahti score, range of motion, and calf circumference measured at three, six, twelve, and twenty-four months after injury. RESULTS: A total of 144 patients (seventy-two treated operatively and seventy-two treated nonoperatively) were randomized. There were 118 males and twenty-six females, and the mean age (and standard deviation) was 40.4 +/- 8.8 years. Rerupture occurred in two patients in the operative group and in three patients in the nonoperative group. There was no clinically important difference between groups with regard to strength, range of motion, calf circumference, or Leppilahti score. There were thirteen complications in the operative group and six in the nonoperative group, with the main difference being the greater number of soft-tissue-related complications in the operative group. CONCLUSIONS: This study supports accelerated functional rehabilitation and nonoperative treatment for acute Achilles tendon ruptures. All measured outcomes of nonoperative treatment were acceptable and were clinically similar to those for operative treatment. In addition, this study suggests that the application of an accelerated-rehabilitation nonoperative protocol avoids serious complications related to surgical management

Changes in performance and bio-mathematical model performance predictions during 45 days of sleep restriction in a simulated space mission

Lunar habitation and exploration of space beyond low-Earth orbit will require small crews to live in isolation and confinement while maintaining a high level of performance with limited support from mission control. Astronauts only achieve approximately 6 h of sleep per night, but few studies have linked sleep deficiency in space to performance impairment. We studied crewmembers over 45 days during a simulated space mission that included 5 h of sleep opportunity on weekdays and 8 h of sleep on weekends to characterize changes in performance on the psychomotor vigilance task (PVT) and subjective fatigue ratings. We further evaluated how well bio-mathematical models designed to predict performance changes due to sleep loss compared to objective performance. We studied 20 individuals during five missions and found that objective performance, but not subjective fatigue, declined from the beginning to the end of the mission. We found that bio-mathematical models were able to predict average changes across the mission but were less sensitive at predicting individual-level performance. Our findings suggest that sleep should be prioritized in lunar crews to minimize the potential for performance errors. Bio-mathematical models may be useful for aiding crews in schedule design but not for individual-level fitness-for-duty decisions

Operative versus nonoperative treatment of acute Achilles tendon ruptures : a multicenter randomized trial using accelerated functional rehabilitation

BACKGROUND: To date, studies directly comparing the rerupture rate in patients with an Achilles tendon rupture who are treated with surgical repair with the rate in patients treated nonoperatively have been inconclusive but the pooled relative risk of rerupture favored surgical repair. In all but one study, the limb was immobilized for six to eight weeks. Published studies of animals and humans have shown a benefit of early functional stimulus to healing tendons. The purpose of the present study was to compare the outcomes of patients with an acute Achilles tendon rupture treated with operative repair and accelerated functional rehabilitation with the outcomes of similar patients treated with accelerated functional rehabilitation alone. METHODS: Patients were randomized to operative or nonoperative treatment for acute Achilles tendon rupture. All patients underwent an accelerated rehabilitation protocol that featured early weight-bearing and early range of motion. The primary outcome was the rerupture rate as demonstrated by a positive Thompson squeeze test, the presence of a palpable gap, and loss of plantar flexion strength. Secondary outcomes included isokinetic strength, the Leppilahti score, range of motion, and calf circumference measured at three, six, twelve, and twenty-four months after injury. RESULTS: A total of 144 patients (seventy-two treated operatively and seventy-two treated nonoperatively) were randomized. There were 118 males and twenty-six females, and the mean age (and standard deviation) was 40.4 +/- 8.8 years. Rerupture occurred in two patients in the operative group and in three patients in the nonoperative group. There was no clinically important difference between groups with regard to strength, range of motion, calf circumference, or Leppilahti score. There were thirteen complications in the operative group and six in the nonoperative group, with the main difference being the greater number of soft-tissue-related complications in the operative group. CONCLUSIONS: This study supports accelerated functional rehabilitation and nonoperative treatment for acute Achilles tendon ruptures. All measured outcomes of nonoperative treatment were acceptable and were clinically similar to those for operative treatment. In addition, this study suggests that the application of an accelerated-rehabilitation nonoperative protocol avoids serious complications related to surgical management