Cognitive activity imposes measurable metabolic demands, yet most models assume fixed metabolic rates for sedentary indoor environments. This study investigates how metabolic rate, thermal sensation, and nutritional state jointly influence cognitive performance across four domains—alertness, reasoning, memory, and concentration—under controlled experimental conditions. Five participants (3M, 2F) completed cognitive tasks across three experimental scenarios: neutral-fed, cold-fed, and neutral-fasted. Results revealed a significant inverse relationship between metabolic rate (MR) and alertness performance in postprandial states (slope = –109.20·MR), suggesting metabolic diversion to digestion. Under fasting, the trend reversed (slope = +134.29·MR), supporting the hypothesis that mental effort elevates MR. Thermal sensation also influenced MR: males exhibited higher MR in cold environment (1.08 ± 0.06 met) than females (0.85 ± 0.07 met, p ˂ 0.05). Cognitive performance patterns varied by sex, with males performing better in cooler conditions and females in warmer ones. These findings suggest that cognitive load, particularly alertness, may serve as a real-time proxy for metabolic variation in the absence of physical activity. Integrating such feedback into indoor environmental models could improve thermal comfort, cognitive support, and energy efficiency in sedentary settings.