The collection of photoplethysmography (PPG) signals from optical based sensor probe has been used extensively for the monitoring of heart rate (HR), respiration rate and oxygen saturation (SpO(2)). Prior research by the Optical Fibre Sensor Research Group has extended the information provided by PPG based devices from monitoring HR and SpO(2) to include total haemoglobin concentration (Hb). Research presented here demonstrates that PPG signals are severely influenced by the autonomic nervous system (ANS) variations induced by physiological events, such as yawning and coughing. Using a finger sensor probe with an embedded accelerometer, further investigation demonstrates that PPG based measurements including HR, SpO(2) and Hb are strongly degraded by ANS both during a) movement of body position and b) physiological variation.