Purpose Individuals exposed to total sleep deprivation may experience an increased risk of impaired thermoregulation and physiological strain during prolonged physical activity in the heat. However, little is known of the impact of more relevant partial sleep deprivation (PSD). This randomized counterbalanced study investigated the effect of PSD on thermal strain during an exercise-heat stress. Methods Ten healthy individuals performed two stress tests (45 min running, 70 % _V O2max 33 C, 40 % RH). Each trial followed three nights of controlled sleep: normal [479 (SD 2) min sleep night-1; Norm] and PSD [116 (SD 4) min sleep night-1]. Energy balance and hydration state were controlled throughout the trials. Rectal temperatures (Tre), mean skin temperature (Tsk), heart rate (HR), RPE, and thermal sensations (TS) were measured at regular intervals during each heat stress trial. Results There was a significant main effect of time (P\0.05) for all of these variables. However, no differences (P[0.05) were observed between PSD and Norm, respectively, for Tre [39.0 (0.5) vs. 39.1 (0.5) C], ;Tsk, [36.1 (0.6) vs. 36.0 (0.7) C] and HR [181 (13) vs. 182 (13) beats min-1)] at the end of exercise-heat stress. There were no differences (P[0.05) in Tsk, PSI, RPE, TS and whole-body sweat rate between PSD versus Norm. Conclusion Since greater physiological strain during exercise-heat stress did not follow three nights of PSD, it appears that sleep loss may have minimal impact upon thermal strain during exercise in the heat, at least as evaluated within this experiment.