With the advent of large-scale combat operations (LSCO) and contemporary threat groups’ use of underground tunnels, it is essential to understand the impact of subterranean military operations on human health and performance. Subterranean operations are not rapid in execution. Rather, warfighters can expect to spend days, weeks, and possibly months operating underground with limited access to sunlight, potable water, food, medical evacuation, and the resupply of rations and other military equipment. In brief, subterranean operations directly challenge human endurance, physiologically and psychologically. While subterranean operations are predicted to acutely strain and chronically suppress most, if not all, physiological systems of the body, this article focuses on the fatigue-hormone-mood triad.
 
The fatigue-hormone-mood triad referenced here characterizes the interconnected nature of physiological symptoms experienced by humans enduring prolonged missions in subterranean conditions. Subterranean environments induce variations in cortisol, free-floating testosterone, and other hormones that affect the entrainment of biological rhythms that regulate sleep, core body temperature, digestion, inflammation, and many other physiological processes of health and performance. In turn, those effects on biological processes alter waking behaviors such as mood and emotional well-being, which increase self-perceived and objective fatigue and fatiguability and ultimately result in a cyclical disruption of hormonal regulation, sleep/circadian processes, and waking performance.
 
But this desire to understand the real-world and ecologically relevant impacts of subterranean living on human health and performance is met with an increasing gap in knowledge on the subject matter. In fact, our best understanding, at present, dates to 1938. In 1938, a professor from the University of Chicago, Dr. Nathaniel Kleitman, determined that it was essential to conduct a human experiment inside Mammoth Caves, Kentucky. His intent was to examine the impact of constant environmental conditions (termed constant routine) on human sleep physiology, temperature rhythms, and waking behavior over a period of 28 days.
 
It would have been opportune for the team to investigate the impact of subterranean living on human molecular processes – which we can surmise today from studies of simulated shift work and constant routine.⁰¹  It would be another 20 years before Dr. James Watson and Dr. Francis Crick would discover the holy grail of human nature: DNA. But, in 1938, Dr. Kleitman only had suitable enough technology to monitor patterns and rhythms of human sleep, core body temperature, digestion, mood, and general waking behavior during subterranean inhabitation. After a month at Mammoth Caves, the research team discovered something unique about human physiology: the human biological clock controlling all daily physiological and behavioral processes “ticks” at a speed greater than 24 hours (24.6 hours). Broadly, Dr. Kleitman’s and Dr. Askerinsky’s research confirmed a prevailing hypothesis that humans must rely on seasonal variations from sunlight and social cues to properly entrain and optimize biological rhythms of sleep, core body temperature, digestion, mood, and general waking behavior.⁰²
 
Since 1938, our best hypotheses and predictions on the impact of subterranean operations on human health and behavior derive from: (i) clinical studies of simulated shift work and constant routine⁰¹; (ii) epidemiological studies of actual shift workers⁰¹; (iii) and a handful of field studies in military personnel performing shift work and/or stationed in polar climates⁰³. To this end, in 2022, leading experts in the field of circadian biology in civilian medicine published a position piece in the flagship medical journal, Clinical and Translational Medicine, on the mechanisms through which night shift work, rotating shift work, and non-optimal lighting conditions increase human morbidity (i.e., risk for poor health outcome/disease state) and mortality (e.g., risk of death). In 2020, leading experts in the field of circadian biology for the military published a similar report highlighting that chronic sleep loss compounded by rotating shift work and non-optimal lighting conditions compromises medical readiness, morale, health, and welfare; increasing risks for high blood pressure, ulcers, diabetes, substance abuse, traumatic stress, mood disorders, and suicide.⁰⁴ Very recently, reverse-cycle military operations have been known to compromise the circadian-driven release of endocrine factors critical for health, recovery, and repair (e.g., free-floating testosterone). For example, transitions from day to night operations in U.S. Army Rangers acutely arrest the release of circadian-driven factors, presenting an increased risk for injury and burnout.⁰⁵    
 
From the perspective of performance, warfighters — under limited acuity — must rapidly respond and react to unpredictable stimuli, traverse across challenging terrain, and swiftly and strategically execute command and control of movement and maneuver. Subterranean operations present an additional set of challenges to include: (i) time to train; (ii) time to acclimate; (iii) the ability to monitor health and performance in real-time for purposes of sustainment and survival; (iv) and even understanding redeployment impacts on psychological health. In order to mitigate risk for human health and performance in subterranean environments, the medical acquisitions community must rely on technological advancements suited for sustainment and survivability to include but not limited to: (i) augmented night-vision (scotopic) capabilities combined with technologies designed to preserve sensitivity to light (photic); (ii) adhesive transcutaneous patches that time-release hormones and nutritional supplements; (iii) precision medicine-tailored MREs; and (iv) human-machine interfaces (i.e., wearables, monitors, and devices) that can entrain rhythms of human physiology and behavior under conditions of constant routine.
 
In order to develop these technologies, it is imperative to understand how a physiological attribute required for mission success is negatively impacted by a subterranean environment and also what science and technology innovations can be leveraged to fill this capability gap. To this end, Table 1 summarizes the first, second, and third-order impacts of subterranean operations on fatigue, hormones, and mood. The “materiel solutions” column serves to identify potential solutions or areas of interest for further research and development.



To conclude, the current literature can guide our knowledge and understanding of the physiological challenges that warfighters will face in subterranean environments, but it does not offer a complete picture of how sustainment and survivability will be possible. Essentially, if we want to maintain battlefield overmatch and win our nation’s war, we must extend our knowledge to aggressive research, development, testing, and evaluation of materiel solutions. The contemporary use of complex underground tunnels (e.g. Hamas, cartels, etc.) may very well be a prelude to the next generation of irregular warfare. 
 
About the Author: Maj. Allison Brager previously served as a research psychologist at the U.S. Army John F. Kennedy Special Warfare Center and School. She now serves as an assistant professor in the Department of Behavioral Sciences and Leadership at the United States Military Academy, West Point, New York.
 
References:
01  Klerman B, et al. Keeping an eye on circadian time in clinical research and medicine. Clinical and Translational Medicine. 2022;12:e1131
02  Kleitman N and Kleitman, E. Effect of non-twenty-four-hour routines of living on oral temperature and heart rate. Journal of Applied Physiology 1953; 6: 283-291.
03  Folgueira A, et al. Sleep, napping, and alertness during an overwintering mission at Belgrano II Argentine Antarctic station. Sci Rep. 2019, 9(1): 10875.
04  Good C, et al.: Sleep in the United States Military. Neuropsychopharmacology. 2020; 45: 176–191.
05  Mantua J, et al. Sleep Loss during Military Training Reduces Testosterone in U.S. Army Rangers: A Two-Study Series. Int J Sports Exerc Med. 2020; 6: 6