Rise and Shine

Everyone knows how hard it is to wake up in the morning. One Siyum HaShas video in fact focused on the coffee served at the Daf. Many Daf Yomi shiurim take place early in the morning, and coffee provides a welcome boost. Difficulty getting up is not a recent phenomenon. Indeed, the Gemara records it was an issue over 2,000 years ago.
Rav Zilberstein (Chashukei Chemed, Yoma 21a) said the phenomenon has halachic consequences. Although the laws of choshen mishpat are very complex, what follows is a simplified shailah and response only focusing on one particular halachic aspect.
There was an employer who hired an employee to work various hours. The exact hours were not specified at the time of the hiring, but it was understood that the hours may, at times, include a night shift. The night shift entailed working the hours of 11 p.m. until 3:30 a.m. The employer later changed his mind and wanted the employee to work the early morning shift that started at 3:30 a.m. It was understood all along that the employee’s hours may change, however, the early morning shift was never discussed. The employee refused to work during the early morning shift. May the employer insist that the employee take the early morning shift? Is the employee’s refusal grounds for dismissal? Rav Zilberstein said that based on the Gemara in Yoma (21a) it is not.