A device, a system and methods to extract pulse wave features and select a combination of these features for calculating and determining the level of fatigue and discriminating between different sources of fatigue in a subject. The different sources of fatigue are physical fatigue, mental fatigue, lack of oxygen fatigue, sleep trouble fatigue, stress fatigue or a combination thereof. The device and its methods is to be used primarily for personal diagnosis and home use but can also be used by therapists, trainers and physicians to help diagnose patients and follow patient's progress. The system is designed to accurately obtain, measure, register and interpret the pulse to determine the level of energy or level of fatigue of a subject. By collecting pulse wave features, selecting those that are most significant and developing algorithms, the device and its method calculates levels of fatigue and discriminates between different sources of fatigue.
