1) In the same way as the employer is warned not to steal the wages of a worker nor to delay them, the worker himself is warned not to waste time when working for the employer and to ensure that he works with all of his strength. (Kitzur Shulchan Aruch 185)
2) It follows that a worker is not allowed to work at night and then hire himself out during the day (when he is weak because of his night’s work) nor is he permitted to starve or afflict himself as this weakness his body and he will be unable to perform the work of the employer in the correct way. (ibid)