Tiredness is a frequent complaint in the labour sphere, but difficult to analyse. Works that sought to produce an explanatory theory of tiredness ran up indeed against the subjective and multidimensional dimension of the phenomenon. From there comes the will of many specialists in tiredness at work to concentrate their studies on what would constitute the psychosocial dimension of tiredness in the industrialized and tertiarized economies : boredom, monotony and mental suffering. If these researches made it possible to widen our comprehension of the question of tiredness at work, they remain marked by an implicit representation of a human being equipped with invariant psychological and social needs. Thus they do not make it possible to account for the variations in the meaning of the various difficulties the workers meet, nor in the imposition of the feeling of discomfort which might result. A more constructivist approach, holding into account the collective representations, the stakes and the specific balance of power to each professional environment, shows that the categories used to think tiredness at work and the experience of this tiredness by the employees are not easily dissociable. The examples of the musculo-skeletal disorders in the industrial companies and of the burn out of the hospital nurses make it possible to illustrate the complexity of the interactions between specific forms of expression of tiredness and particular contexts of work.