Cognitive and emotional stress induce measurable cardiovascular responses including increases in heart rate (HR) and blood pressure. Revision, in preparation for exams, is a highly cognitively demanding process but it is less emotionally stressful than the taking of a written exam were performance on the day is of consequence. This study examined the cardiac autonomic nervous system (ANS) modulation in response to the stress induced by a scheduled University written exam compared to a revision session at home using non invasive heart rate variability (HRV). Following ethical approval, eight (2 female) healthy students undertaking a Masters Degree in Sports Science (age: 25.3.3±3.1years, height: 171.8±6.46cm, weight 68.8±14.5kg) gave informed consent to participate. At the first visit all the steps of the protocol were explained and participants were shown how to attach a Polar S810i HR monitor (Polar Electro OY, Kempele, Finland) for RR-interval recording. Participants were instructed to make two separate RR-interval recordings; at the start of a revision session at home and at the start of the written exam. The revision session was recorded at the same time of day as the scheduled exam (MSc in Sports Science). The HR data was analysed post hoc using Kubios HRV analysis software, (University of Kupio, Finland). Following the removal of any artefacts, the data were divided into 12, five min epochs. Results from all epochs were averaged. Three indices of vagal activity were calculated: the root mean square of successive differences (RMSSD), natural logarithm of high frequency power (frequency domain) (lnHF), and standard deviation of instantaneous beat to beat variability (SD1). Paired sample t-tests were used to compare the differences between the days, level of significance (p≤0.05). HR during exam was 9.7 beats per minute (bpm) higher compared with revision (82.6±9.0 bpm Vs 72.9±9.8 bpm) (p value: 0.002). There was a significant decrease in all three indices of vagal activity during exam: RMSSD, lnHF, SD1 (with p values: 0.01; 0.03; 0.017, respectively). These results suggest that there is a signficant reduction in cardiac vagal activity under conditions of emotional stress and cognitive challenge compared with the response to cognitive challenge alone. Moreover, this suggest that HRV can be used to assess changes in the ANS with mental stress.