Metabolic rate is a dynamic process influenced by time of day, ambient conditions, meal type, and individual differences. Using fixed met values for specific activities oversimplifies this complexity and limits accurate metabolic rate estimation. This study challenges the applicability of standard met values in indoor environment design and thermal sensation predictions through PMV models. Controlled experiments were conducted in a climatic chamber with 12 female participants across two days, with room temperatures set at 24 °C on Day 1 and 18 °C on Day 2. Each day included four sessions involving sitting, standing, and walking distributed across the morning and afternoon, with only sitting and standing performed in two sessions. The findings re-vealed substantial interindividual differences in metabolic rate across activities, with ISO values significantly overestimating metabolic rate for standing (up to 0.86 met or 59.1%) and underestimating metabolic rate for walking (up to 1.13 met or 37.3%). Additionally, intraindividual fluctuations in metabolic rate for the same activity were observed, influenced by time of day and room temperature. These variations, over-looked by standard met values, impact thermal comfort predictions. Incorporating these insights into dynamic, personalized thermal comfort systems could improve both occupant comfort and energy efficiency in buildings.