A new study published in the European Journal of Applied Physiology aimed to define performance status by serial heat tolerance test HTT during structured natural heat adaptation HA and to evaluate surrogate markers of autonomic activation including heart rate variability HRV in relation to HA status. In this study 13 participants were assessed by HTT during short term heat acclimatization STHA day 2 6 and 9 and long term heat acclimatization LTHA day 23. It was observed that significant interactions existed for core temperature Tc log LF HF low frequency power high frequency power cortisol and nephrines two way analysis of variance ANOVA HTT by Day . Relative to LTHA Study measures performed after HTT POST results differed notably for Tc HR log RMSSD Root mean square of successive differences between NN intervals log LF HF cortisol and nephrines. POST differences in HRV were 8201 9.9 log RMSSD and 18.6 log LF HF . From the results it was concluded that early reductions in HR and cortisol characterized STHA. On the other hand LTHA showed diminished excitability by Tc HRV and nephrine measures. It was stated that measurement of HRV may have potential to aid real time assessment of readiness for activity in the heat.