West, DJ, Cook, CJ, Beaven, MC, and Kilduff, LP. The influence of the time of day on core temperature and lower body power output in elite rugby union sevens players. J Strength Cond Res 28(6): 1524–1528, 2014—Core temperature typically displays a low circadian in the morning before peaking later in the day, and these changes occur within small physiological ranges. Body temperature plays an important role in physical performance, and some athletes may be required to train and compete in both the morning and evening. However, the influence of the circadian change in body temperature and its influence on physical performance in elite athletes are unclear. This study examined the effects of the time of day on core temperature and lower body power output in elite rugby union sevens players. Sixteen elite rugby union sevens players completed morning (in am) countermovement jump and core temperature (Tcore) measurement, which were then repeated later the same day (in pm). Countermovement jump was processed for peak power output (PPO). Data were analyzed using paired samples t-test and Pearson's product moment correlation and are presented in mean ± SD. Tcore significantly increased from am to pm (am, 36.92 ± 0.23 vs. pm, 37.18 ± 0.18° C; P < 0.001) with PPO significantly increasing from am to pm in all 16 players (am, 5248 ± 366 vs. pm, 5413 ± 361 W; P < 0.001). The delta change in Tcore (0.26 ± 0.13° C) and PPO (164 ± 78 W) was significantly related (r = 0.781; P < 0.001). In conclusion, small circadian changes in core temperature can influence physical performance in elite athletes. Coaches should seek to use strategies, which may raise morning body temperature to offset the circadian low in the morning.