iv. Physiological measures: Heart rate variability analysis will be performed on data recorded with a Polar RS800CX heart rate monitor using a H2 chest sensor. Polar monitors have been tested against standard clinical ECG equipment and found to be suitable for heart rate variability analysis (Vanderlei et al 2008). In the morning of the study day, the participants will place the Polar heart rate sensor on their chest using the elastic strap. They will be instructed how to use the heart rate monitor watch Polar RS800CX to record the heart rate and press the mark recording button at specific times during each procedure. At the end of each study day, the data will be downloaded from the heart rate monitor watch and 3 time intervals of 2 minutes will be identified: at the beginning of the condition - time 0, at the beginning of the procedure - time 15, at the end of the condition - time 45. Time intervals will be screened for artefacts and used for heart rate variability analysis using standalone analysis software (Niskanen et al 2004). Variables selected for analysis will include heart rate per minute, time domain measures (RMSSD, NN50) and frequency domain measures (nLF at 0.05-0.15Hz, nHF at 0.15-0.04Hz, LF/HF).