GEN1332 Just as it is prohibited for an employer to cheat his workers, so are workers required to do their work as diligently as they are able, as did Yaakov, who served his father-in-law Lavan with all his might in spite of the poor treatment he received. [this verse]. Furthermore, they are prohibited from wasting time on the job Choshen Mishpat 337:20 which is a form of stealing Mesillas Yesharim, Ch. 11.  When the Sages greeted Abba Chilkiyah he did not even return their greeting because he was in the middle of a workday and did not want to waste even a moment from his employment. Taanis 23b.  A worker is not allowed to deprive himself of food or sleep (e.g. by taking on an additional job at night or fasting) if this will impair his work efficiency during the day. Choshen Mishpat 337:19  This applies to all employees, but in particular to teachers of Torah, who must consider themselves employed by the Master of the Universe. Yoreh Deah 245:17, Rema and Beur HaGra 30. JOURNEY 349