In this work, it is analyzed the sentence of National High Court in relation to thequalification that must be given to a change in the schedule of a large part of the workforce, from having a continuous workday to a split shift. The National High Court reviews the judicial doctrine and jurisprudence in relation to the legal institution of the substantial modification of working conditions to try to establish criteria that help when qualifying a change in working conditions of substantial or accidental, being one of them the damage caused to the workers.