There is limited and inconclusive evidence surrounding the physiological and perceptual responses to heat stress while sleep deprived, especially for females. This study aimed to quantify the effect of 24-hrs sleep deprivation on physiological strain and perceptual markers of heat-related illness in females. Method: Nine females completed two 30 min heat stress tests (HST) separated by 48 hrs in 39°C, 41% relative humidity at a metabolic heat production of 10 W.kg-1. The non-sleep deprived HST was followed by the sleep deprivation (SDHST) trial for all participants, during the follicular phase of the menstrual cycle. Physiological and perceptual measures were recorded at 5 min intervals during the HSTs. On the cessation of the HSTs, heat illness symptom index (HISI) was completed.Results: HISI scores increased after sleep deprivation by 28±16 vs. 20±16 (P=0.01). Peak (39.40±0.35°C vs. 39.35±0.33°C) and change in rectal temperature (1.91±0.21 vs. 1.93±0.34°C), and whole body sweat rate (1.08±0.31 vs. 1.15±0.36 L.h-1) did not differ (P>0.05) between tests. No difference was observed in peak, nor rise in; heart rate, mean skin temperature, perceived exertion or thermal sensation during the HSTs.Conclusion: 24 hrs sleep deprivation increased perceptual symptoms associated with heat-related illness, however, no thermoregulatory alterations were observed.