Light-emitting diode phototherapy treatment for jaundice of the preterm infant presents adverse effects, such as discomfort, changes in metabolism, and overheating. This study quantified the body heat exchanges between the environment and a simulated preterm infant requiring phototherapy treatment in a closed incubator. Phototherapy treatment increased the mean incubator roof temperature by 2.9 °C (p < 0.001) and the incubator air temperature by 1 °C (p < 0.001). Analytical calorimetry was used to calculate the additional energy received during phototherapy and thus deduce the optimal incubator air temperature for ensuring thermoneutrality and preventing hyperthermia. The optimal air temperature settings inside the incubator during phototherapy were – 0.51 to – 1.25 °C lower than references, for preterm infant weighing 500 to 2000 g.Conclusion: Phototherapy treatment for jaundice of the preterm infant increased the incubator air temperature. To prevent overheating in the preterm during phototherapy, new curves for optimal air temperature settings inside the incubator were calculated with analytic calorimetry. What is Known •Phototherapy treatment is the first-line treatment for jaundice in the preterm infant. •Phototherapy treatment increases the risk of overheating. What is New •The heat transfers and risk of overheating were quantified using a thermal manikin during phototherapy treatment. •Phototherapy treatment increased the incubator roof temperature and heat transfers. •New incubator’s air temperature settings during phototherapy treatment were calculated with analytical calorimetry for preterm infant weighing 500-2000 g.