0248 The Influence of Circadian Shifts on Post Mission Event Subjective Fatigue during a 72H Live-Fire Mission Simulation

Abstract

Abstract Introduction Circadian misalignment from sustained 24/7 operations and sleep loss are common occurrences in military operations, and operational demands can often lead to high levels of fatigue. Although the relationship between the circadian system and fatigue is well characterized, the relationship between magnitude of the circadian shift and fatigue during operational mission events has not been widely explored. Methods Twenty-one male participants aged 23.3±3.9 years (mean±SD) were recruited from a sample of active-duty Soldiers. The study consisted of a single day baseline data collection and then a month later the Soldiers participated in a sustained live-fire mission simulation consisting of a pre-mission day, the 72-hour live-fire exercise, and post-mission day. During the simulation, Soldiers completed the following mission events: Tactical Stress Marksmanship Assessment (TSMA), Individual Shooter Scenario (ISS), Small Unit Performance Analytics (SUPRA), Reconnaissance (RECON), infiltration and extraction ruck-march (INFIL/EXFIL RUCK), and RAID, to model actual combat or operational activities. Subjective fatigue was assessed using a Visual Analog Scale (VAS) after each mission event. Circadian phase was measured by salivary dim light melatonin onset (DLMO) at baseline and then again following the post-mission day. Half-hourly saliva samples were collected under dim light (<10lux) sedentary conditions. The circadian phase shift was calculated as the difference between the two DLMO collections. Results Twenty of the twenty-one participants advanced their circadian rhythm with a total mean shift of 0.91±0.88 SD hours. Of the six mission events, significant correlations were found between the magnitude of the phase shift and subjective fatigue for three of the events. The magnitude of the circadian phase shift was negatively correlated with fatigue for RECON r=-0.59 (p=0.008), RAID r=-0.55 (p=0.01), and EXFIL RUCK r=-0.44 (p=0.046). The amount of sleep in the 24 hours prior to each of these events had no significant correlation. Conclusion For the RECON, RAID, and RUCK, we found that the greater the magnitude of the phase shift, the less fatigue individuals endorsed. Yet, there was no significant relationship with sleep prior to the events. This suggests a potential greater need to consider the impact of the circadian system on military operations, especially with regard to circadian effects on fatigue. Support (If Any) Department of Defense Military Operational Medicine Research Program (MOMRP); DEVCOM SC