Sleep Restriction and Heat Stress as Compounded Risks to Military Cognitive Readiness

Operational forces increasingly face concurrent sleep restrictions and thermal strain, yet mission planning lacks field-relevant guidance on their combined cognitive effects and management. This critical narrative review synthesizes laboratory, field, and military-relevant evidence on how sleep loss and heat stress affect sustained attention, drowsiness, reaction time, executive control, and feedback-guided decision making. Sleep deprivation produces dose-responsive vigilance instability across total sleep deprivation and chronic partial restriction, with lapses and response-time variability providing sensitive monitoring endpoints. Heat stress produces task-dependent cognitive costs. Simple reaction time may remain relatively preserved during modest strain, whereas executive control, working memory, inhibition, and cognitive flexibility appear more vulnerable as physiological reserve narrows, particularly during dehydration, exertion, and protective equipment use. Military multistressor studies show broad cognitive deterioration when sleep restriction co-occurs with heat, dehydration, undernutrition, and sustained workload. However, bundled designs cannot determine whether sleep and heat act additively, synergistically, or through threshold-dependent interactions. Key gaps include factorial sleep × heat trials, standardized cognitive batteries, mechanistic telemetry, and prospective modeling of moderators such as trait-like sleep-loss vulnerability, baseline sleep debt, chronotype, heat acclimation, hydration practices, protective-equipment burden, and sex. Commanders and medical planners should treat concurrent sleep restriction and thermal strain as a compounded operational risk state and apply layered controls, including protected sleep, scheduling, cooling, hydration, work-rest cycles, objective vigilance screening, task rotation, and supervisory cross-checks for high-consequence decisions.