The invention relates to a device, a system for the device and a set of methods used to extract pulse wave features and select an optimal combination of these features for calculating and determining the level of fatigue and discriminating between different sources of fatigue in a subject, wherein said different sources of fatigues are selected among physical fatigue, mental fatigue, fatigue related to lack of oxygen, fatigue related to sleep troubles, fatigue related to stress or a combination thereof. The device and its methods is meant to be used primarily for personal health care diagnosis and home use but can also be used by therapists, trainers and physicians to help them diagnose their patients and follow their patient's progress. The system is designed as a means of accurately obtaining, measuring, registering and interpreting 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 the user's levels of fatigue and discriminates between different sources of fatigue of said subject.
